Forums

Laser locking (control loops with two feedback paths.)

Started by George Herold January 17, 2013
Phil Hobbs wrote:
> > Joerg wrote: >> John Larkin wrote: >>> On Sat, 19 Jan 2013 11:32:36 -0800 (PST), George Herold <gherold@teachspin.com> >>> wrote: >>> >>>> On Jan 19, 12:34 pm, John Larkin >>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>> On Fri, 18 Jan 2013 21:26:59 -0800 (PST), George Herold <gher...@teachspin.com> >>>>> wrote: >>>>> >>>>> >>>>> >>>>> >>>>> >>>>>> On Jan 18, 4:36 pm, Joerg <inva...@invalid.invalid> wrote: >>>>>>> George Herold wrote: >>>>>>>> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: >>>>>>>>> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: >>>>>>>>>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: >>>>>>>>>>>> Too many variables. >>>>>>>>>>>> What are the characteristics of the modulation you get from the piezo >>>>>>>>>>>> vs. modulating the laser current? >>>>>>>>>>> As long as the change is small they are both approximately linear. >>>>>>>>>>> Modulating the current also changes the amplitude... but I actually take >>>>>>>>>>> the difference of two photodiode signals to get the error signal... so >>>>>>>>>>> to first order the amplitude change caused by current modulation >>>>>>>>>>> shouldn't be that much of an issue. >>>>>>>>>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo and >>>>>>>>>>> the current.) >>>>>>>>>>>> Why does your piezo loop tend to oscillate at around 3kHz? >>>>>>>>>>> Well back in the dim past I did a back of the envelope calculation and >>>>>>>>>>> figured this was the self resonant frequency of the piezo stack and the >>>>>>>>>>> piece of Aluminum that it is pushing around. (Ratio of mass of aluminum >>>>>>>>>>> vs mass of piezo to the one half power times the unloaded SRF of the >>>>>>>>>>> piezo.) >>>>>>>>>>> The Piezo is part number AE0203D04F made by Tokin and a rather long link >>>>>>>>>>> to a data sheet, >>>>>>>>>>> http://store.bravoelectro.com/redirect.php? >>>>>>>>>> action=url&goto=www.bravoelectro.com%2Fpdf% >>>>>>>>>> 2Fpiezoelectric_actuators.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm >>>>>>>>>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is correct.. >>>>>>>>>>> but about the right number came out the far side of the calculation. The >>>>>>>>>>> aluminum and grating are part of a flexure... I sorta wondered if the >>>>>>>>>>> spring constant is different too.... But I'm not sure how I get the >>>>>>>>>>> spring constant for either the piezo or the flexure, and the mass was >>>>>>>>>>> easy to measure. (I did try and do some measuments of the flexure >>>>>>>>>>> spring constant using the piezo as the sensor, very 'squishy' measuments >>>>>>>>>>> IIRC) >>>>>>>>>>>> Why can't you just control the laser current? >>>>>>>>>>> Hmm... OK that's a good question. I'll have to try it! But for long >>>>>>>>>>> term DC drifts it's better to change the piezo (grating angle.) >>>>>>>>>>>> Do you want to have closed-loop control using the laser current, with >>>>>>>>>>>> increased loop bandwidth, or do you just want to push the laser around >>>>>>>>>>>> open loop at those high frequencies? >>>>>>>>>>> Oh for sure closed loop control with higher bandwidth. It'd be cool to >>>>>>>>>>> be able to really bang on the table and have the thing stay locked! >>>>>>>>>>> I think I've got a paper describing how someone else did this...(Carl >>>>>>>>>>> Weiman and Leo Hollberg?) it might be in here, (another long link... to >>>>>>>>>>> a RSI paper) >>>>>>>>>>> http://www.google.com/url? >>>>>>>>>> sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http% >>>>>>>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% >>>>>>>>>> 2F739.pdf&ei=NWH5ULCdFMfg0gHQioDICA&usg=AFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ4ypXg&&#65533; sig2=5lnQLOJC42fKCF_VCTMF8Q >>>>>>>>>>> But sometimes it's more fun to 'invent' your own method and then see >>>>>>>>>>> what someone else did. >>>>>>>>>> Here's one leading candidate in the list of things that I'd try, then: >>>>>>>>>> Make a block, call it "laser", with a frequency-steering signal in, and a >>>>>>>>>> frequency out. >>>>>>>>>> Inside of that block, take the frequency-steering signal and run it >>>>>>>>>> through matched low- and high-pass filters. Make the cutoff frequency >>>>>>>>>> lower than the piezo resonance. Take the low-pass filter, run it through >>>>>>>>>> a notch at the piezo resonance frequency, and feed it to the piezo. Take >>>>>>>>>> the high-pass filter, and feed it to the diode current. Jigger gains >>>>>>>>>> around so that your GHz/whatever (I'm not assuming digital or analog at >>>>>>>>>> this point -- GHz/volt, GHz/ADC count, whatever). >>>>>>>>>> If your laser current response stays flat up to a much higher frequency >>>>>>>>>> than the piezo does, then the overall response of your "laser" block >>>>>>>>>> should also be flat out that high, probably with a hiccup around the >>>>>>>>>> frequency where you transition from piezo to laser current, and possibly >>>>>>>>>> around the piezo resonance, too (although you can damp that one out by >>>>>>>>>> playing with your notch and the cutoff frequency). >>>>>>>>>> Now wrap a loop around that. Figure that when you knock on the table >>>>>>>>>> you'll see it in amplitude -- your cavity will be changing, and you'll be >>>>>>>>>> fixing it by changing the laser current, so you'll see it in amplitude. >>>>>>>>>> Have fun. >>>>>>>>>> -- >>>>>>>>>> My liberal friends think I'm a conservative kook. >>>>>>>>>> My conservative friends think I'm a liberal kook. >>>>>>>>>> Why am I not happy that they have found common ground? >>>>>>>>>> Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com-Hidequotedtext - >>>>>>>>>> - Show quoted text -- Hide quoted text - >>>>>>>>>> - Show quoted text - >>>>>>>>> Hi Tim, Thanks for that! I logged in to report that I tried locking >>>>>>>>> with just current modulation... one peice at a time so to speak. And >>>>>>>>> that worked fine, I could bang a bit more on the table. But the >>>>>>>>> current loop oscillates at ~20kHz when I crank up the gain. I don't >>>>>>>>> understand that at all! The current modulation electronics has a >>>>>>>>> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' >>>>>>>>> physics reason. Modulating the current changes the wavlength through >>>>>>>>> thermal effects. I have no idea what the thermal time of the laser >>>>>>>>> diode is. Would 50us be a reasonable time? (retorical question no >>>>>>>>> answer expected.) >>>>>>>>> I'm going to try measuring the current to frequency modulation >>>>>>>>> parameter as a function of frequency. Hey I might learn someting >>>>>>>>> today! >>>>>>>>> If I get around to closing the 'double loop', I may have more >>>>>>>>> questions.... >>>>>>>>> It's not clear to me where I should put the integrator. >>>>>>>>> Having friday fun, >>>>>>>>> George H.- Hide quoted text - >>>>>>>>> - Show quoted text - >>>>>>>> Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! >>>>>>> 20kHz? That's like molasses. Why so low? And it should not cause it to >>>>>>> oscillate. >>>>>>> -- >>>>>>> Regards, Joerg >>>>>>> http://www.analogconsultants.com/-Hide quoted text - >>>>>>> - Show quoted text - >>>>>> Ja Ja, The photodiode design is from 10+ years ago. I hadn't heard of >>>>>> Phil H. then, let alone read his book. >>>>>> I've got at least 3 projects now that can use a faster photodiode. >>>>>> Oh for the above you have to keep the intensity low in order to not >>>>>> saturate the atomic transistion. So a fairly large PD (0.25" diam), >>>>>> at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level signal) And >>>>>> only a 1 MHz opamp (opa124... it has a bad noise gain peak.) >>>>>> George H. >>>>> This uses an optical-feedback Phil circuit that was discussed some here. It has >>>>> orders of magnitide more bw than comparable low-noise TIAs. >>>>> >>>>> http://www.highlandtechnology.com/DSS/PH200DS.shtml >>>>> >>>>> I learned a lot working on this. Like, jfets have lots of gate leakage if the >>>>> drain voltage is high. That gotcha is in AoE but I missed it. >>>>> >>>>> -- >>>>> >>>>> John Larkin Highland Technology Incwww.highlandtechnology.com jlarkin at highlandtechnology dot com >>>>> >>>>> Precision electronic instrumentation >>>>> Picosecond-resolution Digital Delay and Pulse generators >>>>> Custom timing and laser controllers >>>>> Photonics and fiberoptic TTL data links >>>>> VME analog, thermocouple, LVDT, synchro, tachometer >>>>> Multichannel arbitrary waveform generators- Hide quoted text - >>>>> >>>>> - Show quoted text - >>>> Yeah, can you release the price for a PH200? >>>> (When I tried, your marketing people wanted my mothers maiden name >>>> and >>>> part of my SS# :^) >>> Really? We're not supposed to keep pricing a secret; people will find out >>> anyhow. We do like to keep a record of who downloads manuals or gets pricing, >>> but the motives are pretty benign. Our registration form is minimal, and we >>> never spam. >>> >>> $1986, qty 1. I think The Brat priced it at her birth year. >>> >> Now you'll have lots of folks banging on your door at Otis Street, >> wanting to see her :-) >> >>>> 1 MHz at 1uA is that 1Meg Ohm gain? >>> The transresistances are 10M and 100K on the two ranges. >>> >>>> For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain. >>> That's the low gain range. It's good there, but the real performance is on the >>> high gain range, where it gets over 1 MHz bandwidth at 10M equivalent and very >>> low noise, numbers like 100x better than most of the stuff out there. >>> >> Sometimes it's best not to have all the gain in the first (TIA) stage. >> Opamps are fairly cheap these days and the following ones don't have to >> be very fancy. Maybe something George could look at. >> > > > Yup. You don't want the first stage output > to be more than about half a volt at lowest > nominal photocurrent, because you stop gaining > SNR and can start getting squirrelly behaviour. >
500mV for the lowest signals? I usually try to keep it under 1Vpp for the highest.
> The one John's talking about uses fancy homemade > optocouplers, with their photodiodes wired in series. > The parlour trick is to avoid the low f_T of transistors > running at nanoamp I_C levels. Even the late lamented > BFG25A has an f_T of a few megahertz down there, and > that's not even counting C-B cutoff. >
"Late" as in clanging of the last order bell? Please say it ain't so, that this one won't bite the dust ... <sob, sniffle>
> Cheers > > Phil Hobbs > (Back on the ship)
Who let you back on and what's your excuse for being AWOL? :-) -- Regards, Joerg http://www.analogconsultants.com/

Joerg wrote:
> Phil Hobbs wrote: > >> Sometimes it's best not to have all the gain in the first (TIA) stage. > >> Opamps are fairly cheap these days and the following ones don't have to > >> be very fancy. Maybe something George could look at. > >> > > > > > > Yup. You don't want the first stage output > > to be more than about half a volt at lowest > > nominal photocurrent, because you stop gaining > > SNR and can start getting squirrelly behaviour. > > > > 500mV for the lowest signals? I usually try to keep it under 1Vpp for > the highest.
Not lowest-lowest, but lowest nominal. In a normal resistive-FB TIA, shot noise equals Johnson noise at 50 mV, so you lose 1dB if the output is 200 mV, 0.25 dB at 400 mV. Normally the other parts of the system are way more expensive than the TIA, so from a cost-benefit POV you want the TIA to be unobtrusive.
> > > > The one John's talking about uses fancy homemade > > optocouplers, with their photodiodes wired in series. > > The parlour trick is to avoid the low f_T of transistors > > running at nanoamp I_C levels. Even the late lamented > > BFG25A has an f_T of a few megahertz down there, and > > that's not even counting C-B cutoff. > > > > "Late" as in clanging of the last order bell? Please say it ain't so, > that this one won't bite the dust ... <sob, sniffle>
Alas, 'tis true. You can still get BFT25As, but they're not as good. Cheers Phil Hobbs
> > Who let you back on and what's your excuse for being AWOL? :-) >
I had a 6-hour pass, sir. Honest.
> -- > Regards, Joerg > > http://www.analogconsultants.com/
On Jan 19, 5:25=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Sat, 19 Jan 2013 11:32:36 -0800 (PST), George Herold <gher...@teachspi=
n.com>
> wrote: > > > > > > >On Jan 19, 12:34=A0pm, John Larkin > ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> On Fri, 18 Jan 2013 21:26:59 -0800 (PST), George Herold <gher...@teach=
spin.com>
> >> wrote: > > >> >On Jan 18, 4:36 pm, Joerg <inva...@invalid.invalid> wrote: > >> >> George Herold wrote: > >> >> > On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: > >> >> >> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: > > >> >> >>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: > >> >> >>>>> Too many variables. > >> >> >>>>> What are the characteristics of the modulation you get from t=
he piezo
> >> >> >>>>> vs. modulating the laser current? > >> >> >>>> As long as the change is small they are both approximately lin=
ear.
> >> >> >>>> Modulating the current also changes the amplitude... but I act=
ually take
> >> >> >>>> the difference of two photodiode signals to get the error sign=
al... so
> >> >> >>>> to first order the amplitude change caused by current modulati=
on
> >> >> >>>> shouldn't be that much of an issue. > >> >> >>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the=
piezo and
> >> >> >>>> the current.) > >> >> >>>>> Why does your piezo loop tend to oscillate at around 3kHz? > >> >> >>>> Well back in the dim past I did a back of the envelope calcula=
tion and
> >> >> >>>> figured this was the self resonant frequency of the piezo stac=
k and the
> >> >> >>>> piece of Aluminum that it is pushing around. (Ratio of mass of=
aluminum
> >> >> >>>> vs mass of piezo to the one half power times the unloaded SRF =
of the
> >> >> >>>> piezo.) > >> >> >>>> The Piezo is part number AE0203D04F made by Tokin and a rather=
long link
> >> >> >>>> to a data sheet, > >> >> >>>>http://store.bravoelectro.com/redirect.php? > >> >> >>> action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > >> >> >>> 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b=
0rq258qm
> >> >> >>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is co=
rrect..
> >> >> >>>> but about the right number came out the far side of the calcul=
ation. The
> >> >> >>>> aluminum and grating are part of a flexure... I sorta wondered=
if the
> >> >> >>>> spring constant is different too.... But I'm not sure how I ge=
t the
> >> >> >>>> spring constant for either the piezo or the flexure, and the m=
ass was
> >> >> >>>> easy to measure. (I did try and do some measuments of the flex=
ure
> >> >> >>>> spring constant using the piezo as the sensor, very 'squishy' =
measuments
> >> >> >>>> IIRC) > >> >> >>>>> Why can't you just control the laser current? > >> >> >>>> Hmm... OK that's a good question. I'll have to try it! But for=
long
> >> >> >>>> term DC drifts it's better to change the piezo (grating angle.=
)
> >> >> >>>>> Do you want to have closed-loop control using the laser curre=
nt, with
> >> >> >>>>> increased loop bandwidth, or do you just want to push the las=
er around
> >> >> >>>>> open loop at those high frequencies? > >> >> >>>> Oh for sure closed loop control with higher bandwidth. It'd be=
cool to
> >> >> >>>> be able to really bang on the table and have the thing stay lo=
cked!
> >> >> >>>> I think I've got a paper describing how someone else did this.=
..(Carl
> >> >> >>>> Weiman and Leo Hollberg?) it might be in here, (another long l=
ink... to
> >> >> >>>> a RSI paper) > >> >> >>>>http://www.google.com/url? > >> >> >>> sa=3Dt&rct=3Dj&q=3D&esrc=3Ds&frm=3D1&source=3Dweb&cd=3D1&cad=3D=
rja&ved=3D0CDIQFjAA&url=3Dhttp%
> >> >> >>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > >> >> >>> 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4oci=
l_Sq6Fm7JQ4ypXg&=AD=AD sig2=3D5lnQLOJC42fKCF_VCTMF8Q
> >> >> >>>> But sometimes it's more fun to 'invent' your own method and th=
en see
> >> >> >>>> what someone else did. > >> >> >>> Here's one leading candidate in the list of things that I'd try=
, then:
> >> >> >>> Make a block, call it "laser", with a frequency-steering signal=
in, and a
> >> >> >>> frequency out. > >> >> >>> Inside of that block, take the frequency-steering signal and ru=
n it
> >> >> >>> through matched low- and high-pass filters. Make the cutoff fre=
quency
> >> >> >>> lower than the piezo resonance. Take the low-pass filter, run i=
t through
> >> >> >>> a notch at the piezo resonance frequency, and feed it to the pi=
ezo. Take
> >> >> >>> the high-pass filter, and feed it to the diode current. Jigger =
gains
> >> >> >>> around so that your GHz/whatever (I'm not assuming digital or a=
nalog at
> >> >> >>> this point -- GHz/volt, GHz/ADC count, whatever). > >> >> >>> If your laser current response stays flat up to a much higher f=
requency
> >> >> >>> than the piezo does, then the overall response of your "laser" =
block
> >> >> >>> should also be flat out that high, probably with a hiccup aroun=
d the
> >> >> >>> frequency where you transition from piezo to laser current, and=
possibly
> >> >> >>> around the piezo resonance, too (although you can damp that one=
out by
> >> >> >>> playing with your notch and the cutoff frequency). > >> >> >>> Now wrap a loop around that. Figure that when you knock on the =
table
> >> >> >>> you'll see it in amplitude -- your cavity will be changing, and=
you'll be
> >> >> >>> fixing it by changing the laser current, so you'll see it in am=
plitude.
> >> >> >>> Have fun. > >> >> >>> -- > >> >> >>> My liberal friends think I'm a conservative kook. > >> >> >>> My conservative friends think I'm a liberal kook. > >> >> >>> Why am I not happy that they have found common ground? > >> >> >>> Tim Wescott, Communications, Control, Circuits & Softwarehttp:/=
/www.wescottdesign.com-Hidequotedtext-
> >> >> >>> - Show quoted text -- Hide quoted text - > >> >> >>> - Show quoted text - > >> >> >> Hi Tim, Thanks for that! I logged in to report that I tried lock=
ing
> >> >> >> with just current modulation... one peice at a time so to speak.=
And
> >> >> >> that worked fine, I could bang a bit more on the table. But the > >> >> >> current loop oscillates at ~20kHz when I crank up the gain. I do=
n't
> >> >> >> understand that at all! The current modulation electronics has a > >> >> >> bandwdith that's near 1 MHz, so the 20kHz might be for some 'rea=
l'
> >> >> >> physics reason. Modulating the current changes the wavlength thr=
ough
> >> >> >> thermal effects. I have no idea what the thermal time of the las=
er
> >> >> >> diode is. Would 50us be a reasonable time? (retorical question n=
o
> >> >> >> answer expected.) > >> >> >> I'm going to try measuring the current to frequency modulation > >> >> >> parameter as a function of frequency. Hey I might learn someting > >> >> >> today! > > >> >> >> If I get around to closing the 'double loop', I may have more > >> >> >> questions.... > >> >> >> It's not clear to me where I should put the integrator. > > >> >> >> Having friday fun, > > >> >> >> George H.- Hide quoted text - > > >> >> >> - Show quoted text - > > >> >> > Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! > > >> >> 20kHz? That's like molasses. Why so low? And it should not cause it=
to
> >> >> oscillate. > > >> >> -- > >> >> Regards, Joerg > > >> >>http://www.analogconsultants.com/-Hidequoted text - > > >> >> - Show quoted text - > > >> >Ja Ja, The photodiode design is from 10+ years ago. =A0I hadn't heard=
of
> >> >Phil H. then, let alone read his book. > > >> >I've got at least 3 projects now that can use a faster photodiode. > > >> >Oh for the above you have to keep the intensity low in order to not > >> >saturate the atomic transistion. =A0 So a fairly large PD (0.25" diam=
),
> >> >at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level signal) =A0A=
nd
> >> >only a 1 MHz opamp (opa124... it has a bad noise gain peak.) > > >> >George H. > > >> This uses an optical-feedback Phil circuit that was discussed some her=
e. It has
> >> orders of magnitide more bw than comparable low-noise TIAs. > > >>http://www.highlandtechnology.com/DSS/PH200DS.shtml > > >> I learned a lot working on this. Like, jfets have lots of gate leakage=
if the
> >> drain voltage is high. That gotcha is in AoE but I missed it. > > >> -- > > >> John Larkin =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Highland Technology Inc=
www.highlandtechnology.com=A0jlarkin at highlandtechnology dot com
> > >> Precision electronic instrumentation > >> Picosecond-resolution Digital Delay and Pulse generators > >> Custom timing and laser controllers > >> Photonics and fiberoptic TTL data links > >> VME =A0analog, thermocouple, LVDT, synchro, tachometer > >> Multichannel arbitrary waveform generators- Hide quoted text - > > >> - Show quoted text - > > >Yeah, can you release the price for a PH200? > >(When I tried, your marketing people wanted my mothers maiden name > >and > >part of my SS# =A0:^) > > Really?
OK not that bad, I can't recall the details. We're not supposed to keep pricing a secret; people will find out
> anyhow. We do like to keep a record of who downloads manuals or gets pric=
ing,
> but the motives are pretty benign. Our registration form is minimal, and =
we
> never spam. > > $1986, qty 1. I think The Brat priced it at her birth year. > > > > >1 MHz at 1uA is that 1Meg Ohm gain? > > The transresistances are 10M and 100K on the two ranges. > > > > >For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain. > > That's the low gain range. It's good there, but the real performance is o=
n the
> high gain range, where it gets over 1 MHz bandwidth at 10M equivalent and=
very
> low noise, numbers like 100x better than most of the stuff out there.
1Mhz at 10M Ohm that is impressive! I only need something 100 times worse, so there's some chance I can make it happen. George H.
> > -- > > John Larkin =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Highland Technology Incwww=
.highlandtechnology.com=A0 jlarkin at highlandtechnology dot com
> > Precision electronic instrumentation > Picosecond-resolution Digital Delay and Pulse generators > Custom timing and laser controllers > Photonics and fiberoptic TTL data links > VME =A0analog, thermocouple, LVDT, synchro, tachometer > Multichannel arbitrary waveform generators- Hide quoted text - > > - Show quoted text -
On Jan 19, 3:58=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Fri, 18 Jan 2013 21:35:01 -0800, George Herold wrote: > > On Jan 18, 5:04=A0pm, Tim Wescott <t...@seemywebsite.com> wrote: > >> On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote: > >> > Hi Tim, =A0Thanks for that! =A0I logged in to report that I tried lo=
cking
> >> > with just current modulation... one peice at a time so to speak. =A0=
And
> >> > that worked fine, I could bang a bit more on the table. =A0But the > >> > current loop oscillates at ~20kHz when I crank up the gain. =A0I don=
't
> >> > understand that at all! =A0The current modulation electronics has a > >> > bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' > >> > physics reason. Modulating the current changes the wavlength through > >> > thermal effects. =A0I have no idea what the thermal time of the lase=
r
> >> > diode is. =A0Would 50us be a reasonable time? (retorical question no > >> > answer expected.) I'm going to try measuring the current to frequenc=
y
> >> > modulation parameter as a function of frequency. =A0Hey I might lear=
n
> >> > someting today! > > >> > If I get around to closing the 'double loop', I may have more > >> > questions.... > >> > It's not clear to me where I should put the integrator. > > >> freq error -o-> prop. gain - + --> laser > >> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A > >> =A0 =A0 =A0 =A0 =A0 =A0 '-> integrator --' > > > But with two feed back paths should there be an integrator in each loop=
?
> > Only if you want things screwed up! =A0With an integrator in each loop > you'll have an uncontrollable, metastable mode -- said mode being the > difference between the two integrator states, and metastable because > it'll be integrating. =A0So what will happen is that your loop will hold > just fine until either the piezo or the current goes to the positive > rail, and the other one goes to the opposite rail. > > Besides, if you take my suggestion on this you'll have a DC-blocking > filter between the one "laser" input and the current -- only the piezo > will respond to the integrator at any rate.
Hi Tim, thanks again for the sage advice. So at the moment the gain vs freq looks like, ~1Hz L ^ | O |-- G | \ | \ g | \ a | \ i | \ ~1kHz n | \| +--------\---------> gain=3D1 LOG frequency So If I make a break point (LF to piezo and HF to current) at say 100 Hz do I still want the high frequency part of the gain to have a one pole roll-off... or maybe the gain can be flat (for a while.... I gotta roll off the HF eventually.) Not to worry, I=92ll play around and maybe learn something.
> > > Not to worry first I need a faster PD. > > I gathered that, it being the size of a dinner plate (well, for > cockroaches, perhaps). =A0I'm not on top of the physics of photodiodes, b=
ut
> I thought that -- given low enough impedances -- your bandwidth is more > limited by your amplifier than by your photodiode itself. =A0Am I all wet > here?
As far as the photodiode design goes, I don=92t know of any =91new=92 tricks*. But the old rule of thumb (for TIA=92s) is that the max. frequency is the geometric mean of the RC =91frequency=92 and the opamp GBW. For my pedestrian circuit, with unbiased photodiode C=3D~700pF, R=3D1Meg, GBW =3D1.5MHz,(opa124) for which I get an f(rc)~240Hz and sqrt(f(rc)*GBW)~19kHz.. (I thought the opa124 had a 1MHz GBW, but the number was then a bit off.) George H. *well there are ways to reduce the capacitance. reverse bias the PD, bootstrapping, and then Phil H.'s cascode between the PD and TIA. I've never tried the cascode.
> > >> If your loop is crapping out at 20kHz with your 20kHz photodiode, > >> chances are that even with a better photodiode in there you'll need > >> some derivative action to push much above 20-ish kHz: > > >> freq error -o-> prop. gain - + --> laser > >> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A > >> =A0 =A0 =A0 =A0 =A0 =A0 o-> integrator --+ > >> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A > >> =A0 =A0 =A0 =A0 =A0 =A0 '-> derivative --' > > >> If you're doing this in analog, or if you're sampling good and fast in > >> digital-land, you'll almost certainly want to band-limit the > >> derivative. > > > Oh all analog. > > -- > My liberal friends think I'm a conservative kook. > My conservative friends think I'm a liberal kook. > Why am I not happy that they have found common ground? > > Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wesco=
ttdesign.com- Hide quoted text -
> > - Show quoted text -
On Jan 20, 12:22=A0pm, Joerg <inva...@invalid.invalid> wrote:
> George Herold wrote: > > On Jan 18, 5:16 pm, Joerg <inva...@invalid.invalid> wrote: > >> Tim Wescott wrote: > >>> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote: > >>>> George Herold wrote: > >>>>> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: > >>>>>> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: > >>>>>>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: > >>>>>>>>> Too many variables. > >>>>>>>>> What are the characteristics of the modulation you get from the > >>>>>>>>> piezo vs. modulating the laser current? > >>>>>>>> As long as the change is small they are both approximately linea=
r.
> >>>>>>>> Modulating the current also changes the amplitude... but I actua=
lly
> >>>>>>>> take the difference of two photodiode signals to get the error > >>>>>>>> signal... so to first order the amplitude change caused by curre=
nt
> >>>>>>>> modulation shouldn't be that much of an issue. > >>>>>>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the p=
iezo
> >>>>>>>> and the current.) > >>>>>>>>> Why does your piezo loop tend to oscillate at around 3kHz? > >>>>>>>> Well back in the dim past I did a back of the envelope calculati=
on
> >>>>>>>> and figured this was the self resonant frequency of the piezo st=
ack
> >>>>>>>> and the piece of Aluminum that it is pushing around. =A0(Ratio o=
f mass
> >>>>>>>> of aluminum vs mass of piezo to the one half power times the > >>>>>>>> unloaded SRF of the piezo.) > >>>>>>>> The Piezo is part number AE0203D04F made by Tokin and a rather l=
ong
> >>>>>>>> link to a data sheet, > >>>>>>>>http://store.bravoelectro.com/redirect.php? > >>>>>>> action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > >>>>>>> 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b0r=
q258qm
> >>>>>>>> SRF ~ 261 kHz. =A0I have no idea if the simple mass scaling is > >>>>>>>> correct.. but about the right number came out the far side of th=
e
> >>>>>>>> calculation. The aluminum and grating are part of a flexure... I > >>>>>>>> sorta wondered if the spring constant is different too.... But I=
'm
> >>>>>>>> not sure how I get the spring constant for either the piezo or t=
he
> >>>>>>>> flexure, and the mass was easy to measure. =A0(I did try and do =
some
> >>>>>>>> measuments of the flexure spring constant using the piezo as the > >>>>>>>> sensor, very 'squishy' measuments IIRC) > >>>>>>>>> Why can't you just control the laser current? > >>>>>>>> Hmm... OK that's a good question. =A0I'll have to try it! But fo=
r long
> >>>>>>>> term DC drifts it's better to change the piezo (grating angle.) > >>>>>>>>> Do you want to have closed-loop control using the laser current=
,
> >>>>>>>>> with increased loop bandwidth, or do you just want to push the > >>>>>>>>> laser around open loop at those high frequencies? > >>>>>>>> Oh for sure closed loop control with higher bandwidth. It'd be c=
ool
> >>>>>>>> to be able to really bang on the table and have the thing stay > >>>>>>>> locked! I think I've got a paper describing how someone else did > >>>>>>>> this...(Carl Weiman and Leo Hollberg?) it might be in here, > >>>>>>>> (another long link... to a RSI paper) > >>>>>>>>http://www.google.com/url? > >>> sa=3Dt&rct=3Dj&q=3D&esrc=3Ds&frm=3D1&source=3Dweb&cd=3D1&cad=3Drja&ve=
d=3D0CDIQFjAA&url=3Dhttp%
> >>>>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > >>> 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4ocil_Sq6F=
m7JQ4ypXg&=AD=AD=AD=AD
> >>> sig2=3D5lnQLOJC42fKCF_VCTMF8Q > >>>>>>>> =A0 But sometimes it's more fun to 'invent' your own method and =
then
> >>>>>>>> =A0 see > >>>>>>>> what someone else did. > >>>>>>> Here's one leading candidate in the list of things that I'd try, > >>>>>>> then: Make a block, call it "laser", with a frequency-steering si=
gnal
> >>>>>>> in, and a frequency out. > >>>>>>> Inside of that block, take the frequency-steering signal and run =
it
> >>>>>>> through matched low- and high-pass filters. =A0Make the cutoff > >>>>>>> frequency lower than the piezo resonance. =A0Take the low-pass fi=
lter,
> >>>>>>> run it through a notch at the piezo resonance frequency, and feed=
it
> >>>>>>> to the piezo. =A0Take the high-pass filter, and feed it to the di=
ode
> >>>>>>> current. =A0Jigger gains around so that your GHz/whatever (I'm no=
t
> >>>>>>> assuming digital or analog at this point -- GHz/volt, GHz/ADC cou=
nt,
> >>>>>>> whatever). If your laser current response stays flat up to a much > >>>>>>> higher frequency than the piezo does, then the overall response o=
f
> >>>>>>> your "laser" block should also be flat out that high, probably wi=
th a
> >>>>>>> hiccup around the frequency where you transition from piezo to la=
ser
> >>>>>>> current, and possibly around the piezo resonance, too (although y=
ou
> >>>>>>> can damp that one out by playing with your notch and the cutoff > >>>>>>> frequency). Now wrap a loop around that. =A0Figure that when you =
knock
> >>>>>>> on the table you'll see it in amplitude -- your cavity will be > >>>>>>> changing, and you'll be fixing it by changing the laser current, =
so
> >>>>>>> you'll see it in amplitude. Have fun. > >>>>>>> -- > >>>>>>> My liberal friends think I'm a conservative kook. My conservative > >>>>>>> friends think I'm a liberal kook. Why am I not happy that they ha=
ve
> >>>>>>> found common ground? Tim Wescott, Communications, Control, Circui=
ts &
> >>>>>>> Softwarehttp://www.wescottdesign.com-Hidequotedtext - - Show quot=
ed
> >>>>>>> text -- Hide quoted text - - Show quoted text - > >>>>>> Hi Tim, =A0Thanks for that! =A0I logged in to report that I tried =
locking
> >>>>>> with just current modulation... one peice at a time so to speak. =
=A0And
> >>>>>> that worked fine, I could bang a bit more on the table. =A0But the > >>>>>> current loop oscillates at ~20kHz when I crank up the gain. =A0I d=
on't
> >>>>>> understand that at all! =A0The current modulation electronics has =
a
> >>>>>> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' > >>>>>> physics reason. =A0Modulating the current changes the wavlength th=
rough
> >>>>>> thermal effects. =A0I have no idea what the thermal time of the la=
ser
> >>>>>> diode is. =A0Would 50us be a reasonable time? (retorical question =
no
> >>>>>> answer expected.) > >>>>>> I'm going to try measuring the current to frequency modulation > >>>>>> parameter as a function of frequency. =A0Hey I might learn sometin=
g
> >>>>>> today! > >>>>>> If I get around to closing the 'double loop', I may have more > >>>>>> questions.... > >>>>>> It's not clear to me where I should put the integrator. > >>>>>> Having friday fun, > >>>>>> George H.- Hide quoted text - > >>>>>> - Show quoted text - > >>>>> Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! > >>>> 20kHz? That's like molasses. Why so low? And it should not cause it =
to
> >>>> oscillate. > >>> If he's already got substantial phase shift elsewhere, then the photo=
diode
> >>> rolling off would cause oscillation somewhere around 20kHz. > >>> The fact that it happens at _exactly_ 20kHz just means that, sans > >>> photodiode, he's got about 45 degrees of margin at 20kHz. > >> But where does all that phase margin fall through the cracks? Unless > >> everything rolls off fast, of course. 20kHz BW for the photodiode soun=
ds
> >> really low, unless it is one the size of a dinner plate. > > >> -- > >> Regards, Joerg > > >>http://www.analogconsultants.com/-Hide quoted text - > > >> - Show quoted text - > > > Grin, well not quite dinner plate size. ~6-7mm diam. > > That should be a lot more zippy than 20kHz if connected to a somewhat > reasonable TIA. Or did you give it a hefty dose of Ambien? :-)
Grin... I posted numbers for Tim. I can get a factor of 2 or so by reverse biasing the PD, and maybe a factor of 5-10 with a faster opamp. George H.
> > -- > Regards, Joerg > > http://www.analogconsultants.com/- Hide quoted text - > > - Show quoted text -
On Jan 20, 12:29=A0pm, Joerg <inva...@invalid.invalid> wrote:
> John Larkin wrote: > > On Sat, 19 Jan 2013 11:32:36 -0800 (PST), George Herold <gher...@teachs=
pin.com>
> > wrote: > > >> On Jan 19, 12:34 pm, John Larkin > >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >>> On Fri, 18 Jan 2013 21:26:59 -0800 (PST), George Herold <gher...@teac=
hspin.com>
> >>> wrote: > > >>>> On Jan 18, 4:36 pm, Joerg <inva...@invalid.invalid> wrote: > >>>>> George Herold wrote: > >>>>>> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: > >>>>>>> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: > >>>>>>>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: > >>>>>>>>>> Too many variables. > >>>>>>>>>> What are the characteristics of the modulation you get from th=
e piezo
> >>>>>>>>>> vs. modulating the laser current? > >>>>>>>>> As long as the change is small they are both approximately line=
ar.
> >>>>>>>>> Modulating the current also changes the amplitude... but I actu=
ally take
> >>>>>>>>> the difference of two photodiode signals to get the error signa=
l... so
> >>>>>>>>> to first order the amplitude change caused by current modulatio=
n
> >>>>>>>>> shouldn't be that much of an issue. > >>>>>>>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the =
piezo and
> >>>>>>>>> the current.) > >>>>>>>>>> Why does your piezo loop tend to oscillate at around 3kHz? > >>>>>>>>> Well back in the dim past I did a back of the envelope calculat=
ion and
> >>>>>>>>> figured this was the self resonant frequency of the piezo stack=
and the
> >>>>>>>>> piece of Aluminum that it is pushing around. (Ratio of mass of =
aluminum
> >>>>>>>>> vs mass of piezo to the one half power times the unloaded SRF o=
f the
> >>>>>>>>> piezo.) > >>>>>>>>> The Piezo is part number AE0203D04F made by Tokin and a rather =
long link
> >>>>>>>>> to a data sheet, > >>>>>>>>>http://store.bravoelectro.com/redirect.php? > >>>>>>>> action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > >>>>>>>> 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b0=
rq258qm
> >>>>>>>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is cor=
rect..
> >>>>>>>>> but about the right number came out the far side of the calcula=
tion. The
> >>>>>>>>> aluminum and grating are part of a flexure... I sorta wondered =
if the
> >>>>>>>>> spring constant is different too.... But I'm not sure how I get=
the
> >>>>>>>>> spring constant for either the piezo or the flexure, and the ma=
ss was
> >>>>>>>>> easy to measure. (I did try and do some measuments of the flexu=
re
> >>>>>>>>> spring constant using the piezo as the sensor, very 'squishy' m=
easuments
> >>>>>>>>> IIRC) > >>>>>>>>>> Why can't you just control the laser current? > >>>>>>>>> Hmm... OK that's a good question. I'll have to try it! But for =
long
> >>>>>>>>> term DC drifts it's better to change the piezo (grating angle.) > >>>>>>>>>> Do you want to have closed-loop control using the laser curren=
t, with
> >>>>>>>>>> increased loop bandwidth, or do you just want to push the lase=
r around
> >>>>>>>>>> open loop at those high frequencies? > >>>>>>>>> Oh for sure closed loop control with higher bandwidth. It'd be =
cool to
> >>>>>>>>> be able to really bang on the table and have the thing stay loc=
ked!
> >>>>>>>>> I think I've got a paper describing how someone else did this..=
.(Carl
> >>>>>>>>> Weiman and Leo Hollberg?) it might be in here, (another long li=
nk... to
> >>>>>>>>> a RSI paper) > >>>>>>>>>http://www.google.com/url? > >>>>>>>> sa=3Dt&rct=3Dj&q=3D&esrc=3Ds&frm=3D1&source=3Dweb&cd=3D1&cad=3Dr=
ja&ved=3D0CDIQFjAA&url=3Dhttp%
> >>>>>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > >>>>>>>> 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4ocil=
_Sq6Fm7JQ4ypXg&=AD=AD sig2=3D5lnQLOJC42fKCF_VCTMF8Q
> >>>>>>>>> But sometimes it's more fun to 'invent' your own method and the=
n see
> >>>>>>>>> what someone else did. > >>>>>>>> Here's one leading candidate in the list of things that I'd try,=
then:
> >>>>>>>> Make a block, call it "laser", with a frequency-steering signal =
in, and a
> >>>>>>>> frequency out. > >>>>>>>> Inside of that block, take the frequency-steering signal and run=
it
> >>>>>>>> through matched low- and high-pass filters. Make the cutoff freq=
uency
> >>>>>>>> lower than the piezo resonance. Take the low-pass filter, run it=
through
> >>>>>>>> a notch at the piezo resonance frequency, and feed it to the pie=
zo. Take
> >>>>>>>> the high-pass filter, and feed it to the diode current. Jigger g=
ains
> >>>>>>>> around so that your GHz/whatever (I'm not assuming digital or an=
alog at
> >>>>>>>> this point -- GHz/volt, GHz/ADC count, whatever). > >>>>>>>> If your laser current response stays flat up to a much higher fr=
equency
> >>>>>>>> than the piezo does, then the overall response of your "laser" b=
lock
> >>>>>>>> should also be flat out that high, probably with a hiccup around=
the
> >>>>>>>> frequency where you transition from piezo to laser current, and =
possibly
> >>>>>>>> around the piezo resonance, too (although you can damp that one =
out by
> >>>>>>>> playing with your notch and the cutoff frequency). > >>>>>>>> Now wrap a loop around that. Figure that when you knock on the t=
able
> >>>>>>>> you'll see it in amplitude -- your cavity will be changing, and =
you'll be
> >>>>>>>> fixing it by changing the laser current, so you'll see it in amp=
litude.
> >>>>>>>> Have fun. > >>>>>>>> -- > >>>>>>>> My liberal friends think I'm a conservative kook. > >>>>>>>> My conservative friends think I'm a liberal kook. > >>>>>>>> Why am I not happy that they have found common ground? > >>>>>>>> Tim Wescott, Communications, Control, Circuits & Softwarehttp://=
www.wescottdesign.com-Hidequotedtext-
> >>>>>>>> - Show quoted text -- Hide quoted text - > >>>>>>>> - Show quoted text - > >>>>>>> Hi Tim, Thanks for that! I logged in to report that I tried locki=
ng
> >>>>>>> with just current modulation... one peice at a time so to speak. =
And
> >>>>>>> that worked fine, I could bang a bit more on the table. But the > >>>>>>> current loop oscillates at ~20kHz when I crank up the gain. I don=
't
> >>>>>>> understand that at all! The current modulation electronics has a > >>>>>>> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real=
'
> >>>>>>> physics reason. Modulating the current changes the wavlength thro=
ugh
> >>>>>>> thermal effects. I have no idea what the thermal time of the lase=
r
> >>>>>>> diode is. Would 50us be a reasonable time? (retorical question no > >>>>>>> answer expected.) > >>>>>>> I'm going to try measuring the current to frequency modulation > >>>>>>> parameter as a function of frequency. Hey I might learn someting > >>>>>>> today! > >>>>>>> If I get around to closing the 'double loop', I may have more > >>>>>>> questions.... > >>>>>>> It's not clear to me where I should put the integrator. > >>>>>>> Having friday fun, > >>>>>>> George H.- Hide quoted text - > >>>>>>> - Show quoted text - > >>>>>> Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! > >>>>> 20kHz? That's like molasses. Why so low? And it should not cause it=
to
> >>>>> oscillate. > >>>>> -- > >>>>> Regards, Joerg > >>>>>http://www.analogconsultants.com/-Hidequoted text - > >>>>> - Show quoted text - > >>>> Ja Ja, The photodiode design is from 10+ years ago. =A0I hadn't hear=
d of
> >>>> Phil H. then, let alone read his book. > >>>> I've got at least 3 projects now that can use a faster photodiode. > >>>> Oh for the above you have to keep the intensity low in order to not > >>>> saturate the atomic transistion. =A0 So a fairly large PD (0.25" dia=
m),
> >>>> at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level signal) =A0=
And
> >>>> only a 1 MHz opamp (opa124... it has a bad noise gain peak.) > >>>> George H. > >>> This uses an optical-feedback Phil circuit that was discussed some he=
re. It has
> >>> orders of magnitide more bw than comparable low-noise TIAs. > > >>>http://www.highlandtechnology.com/DSS/PH200DS.shtml > > >>> I learned a lot working on this. Like, jfets have lots of gate leakag=
e if the
> >>> drain voltage is high. That gotcha is in AoE but I missed it. > > >>> -- > > >>> John Larkin =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Highland Technology In=
cwww.highlandtechnology.com=A0jlarkin at highlandtechnology dot com
> > >>> Precision electronic instrumentation > >>> Picosecond-resolution Digital Delay and Pulse generators > >>> Custom timing and laser controllers > >>> Photonics and fiberoptic TTL data links > >>> VME =A0analog, thermocouple, LVDT, synchro, tachometer > >>> Multichannel arbitrary waveform generators- Hide quoted text - > > >>> - Show quoted text - > >> Yeah, can you release the price for a PH200? > >> (When I tried, your marketing people wanted my mothers maiden name > >> and > >> part of my SS# =A0:^) > > > Really? We're not supposed to keep pricing a secret; people will find o=
ut
> > anyhow. We do like to keep a record of who downloads manuals or gets pr=
icing,
> > but the motives are pretty benign. Our registration form is minimal, an=
d we
> > never spam. > > > $1986, qty 1. I think The Brat priced it at her birth year. > > Now you'll have lots of folks banging on your door at Otis Street, > wanting to see her :-) > > > > >> 1 MHz at 1uA is that 1Meg Ohm gain? > > > The transresistances are 10M and 100K on the two ranges. > > >> For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain. > > > That's the low gain range. It's good there, but the real performance is=
on the
> > high gain range, where it gets over 1 MHz bandwidth at 10M equivalent a=
nd very
> > low noise, numbers like 100x better than most of the stuff out there. > > Sometimes it's best not to have all the gain in the first (TIA) stage. > Opamps are fairly cheap these days and the following ones don't have to > be very fancy. Maybe something George could look at. >
Hi Joerg, thanks for the advice. As Phil says later on, I've always tried to keep signals in the Volt range for noise reasons. (But perhaps I was too focused on noise?) Anyway I can sure reduce the TIA gain and dial in more later. (I start to worry about the bandwidth of the rest of the signal chain though...) George H.
> -- > Regards, Joerg > > http://www.analogconsultants.com/- Hide quoted text - > > - Show quoted text -
On Mon, 21 Jan 2013 07:17:14 -0800, George Herold wrote:

> On Jan 20, 12:29&nbsp;pm, Joerg <inva...@invalid.invalid> wrote: >> John Larkin wrote: >> > On Sat, 19 Jan 2013 11:32:36 -0800 (PST), George Herold >> > <gher...@teachspin.com> >> > wrote: >> >> >> On Jan 19, 12:34 pm, John Larkin >> >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Fri, 18 Jan 2013 21:26:59 -0800 (PST), George Herold >> >>> <gher...@teachspin.com> >> >>> wrote: >> >> >>>> On Jan 18, 4:36 pm, Joerg <inva...@invalid.invalid> wrote: >> >>>>> George Herold wrote: >> >>>>>> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: >> >>>>>>> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: >> >>>>>>>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: >> >>>>>>>>>> Too many variables. >> >>>>>>>>>> What are the characteristics of the modulation you get from >> >>>>>>>>>> the piezo vs. modulating the laser current? >> >>>>>>>>> As long as the change is small they are both approximately >> >>>>>>>>> linear. Modulating the current also changes the amplitude... >> >>>>>>>>> but I actually take the difference of two photodiode signals >> >>>>>>>>> to get the error signal... so to first order the amplitude >> >>>>>>>>> change caused by current modulation shouldn't be that much of >> >>>>>>>>> an issue. >> >>>>>>>>> (Hmm maybe I can generate freq vs 'voltage' scans for both >> >>>>>>>>> the piezo and the current.) >> >>>>>>>>>> Why does your piezo loop tend to oscillate at around 3kHz? >> >>>>>>>>> Well back in the dim past I did a back of the envelope >> >>>>>>>>> calculation and figured this was the self resonant frequency >> >>>>>>>>> of the piezo stack and the piece of Aluminum that it is >> >>>>>>>>> pushing around. (Ratio of mass of aluminum vs mass of piezo >> >>>>>>>>> to the one half power times the unloaded SRF of the piezo.) >> >>>>>>>>> The Piezo is part number AE0203D04F made by Tokin and a >> >>>>>>>>> rather long link to a data sheet, >> >>>>>>>>>http://store.bravoelectro.com/redirect.php? >> >>>>>>>> action=url&goto=www.bravoelectro.com%2Fpdf% >> >>>>>>>>
2Fpiezoelectric_actuators.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm
>> >>>>>>>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is >> >>>>>>>>> correct.. but about the right number came out the far side of >> >>>>>>>>> the calculation. The aluminum and grating are part of a >> >>>>>>>>> flexure... I sorta wondered if the spring constant is >> >>>>>>>>> different too.... But I'm not sure how I get the spring >> >>>>>>>>> constant for either the piezo or the flexure, and the mass >> >>>>>>>>> was easy to measure. (I did try and do some measuments of the >> >>>>>>>>> flexure spring constant using the piezo as the sensor, very >> >>>>>>>>> 'squishy' measuments IIRC) >> >>>>>>>>>> Why can't you just control the laser current? >> >>>>>>>>> Hmm... OK that's a good question. I'll have to try it! But >> >>>>>>>>> for long term DC drifts it's better to change the piezo >> >>>>>>>>> (grating angle.) >> >>>>>>>>>> Do you want to have closed-loop control using the laser >> >>>>>>>>>> current, with increased loop bandwidth, or do you just want >> >>>>>>>>>> to push the laser around open loop at those high >> >>>>>>>>>> frequencies? >> >>>>>>>>> Oh for sure closed loop control with higher bandwidth. It'd >> >>>>>>>>> be cool to be able to really bang on the table and have the >> >>>>>>>>> thing stay locked! >> >>>>>>>>> I think I've got a paper describing how someone else did >> >>>>>>>>> this...(Carl Weiman and Leo Hollberg?) it might be in here, >> >>>>>>>>> (another long link... to a RSI paper) >> >>>>>>>>>http://www.google.com/url? >> >>>>>>>>
sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%
>> >>>>>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% >> >>>>>>>>
2F739.pdf&ei=NWH5ULCdFMfg0gHQioDICA&usg=AFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ4ypXg&&shy;&shy;
>> >>>>>>>> sig2=5lnQLOJC42fKCF_VCTMF8Q >> >>>>>>>>> But sometimes it's more fun to 'invent' your own method and >> >>>>>>>>> then see what someone else did. >> >>>>>>>> Here's one leading candidate in the list of things that I'd >> >>>>>>>> try, then: >> >>>>>>>> Make a block, call it "laser", with a frequency-steering >> >>>>>>>> signal in, and a frequency out. >> >>>>>>>> Inside of that block, take the frequency-steering signal and >> >>>>>>>> run it through matched low- and high-pass filters. Make the >> >>>>>>>> cutoff frequency lower than the piezo resonance. Take the >> >>>>>>>> low-pass filter, run it through a notch at the piezo resonance >> >>>>>>>> frequency, and feed it to the piezo. Take the high-pass >> >>>>>>>> filter, and feed it to the diode current. Jigger gains around >> >>>>>>>> so that your GHz/whatever (I'm not assuming digital or analog >> >>>>>>>> at this point -- GHz/volt, GHz/ADC count, whatever). >> >>>>>>>> If your laser current response stays flat up to a much higher >> >>>>>>>> frequency than the piezo does, then the overall response of >> >>>>>>>> your "laser" block should also be flat out that high, probably >> >>>>>>>> with a hiccup around the frequency where you transition from >> >>>>>>>> piezo to laser current, and possibly around the piezo >> >>>>>>>> resonance, too (although you can damp that one out by playing >> >>>>>>>> with your notch and the cutoff frequency). >> >>>>>>>> Now wrap a loop around that. Figure that when you knock on the >> >>>>>>>> table you'll see it in amplitude -- your cavity will be >> >>>>>>>> changing, and you'll be fixing it by changing the laser >> >>>>>>>> current, so you'll see it in amplitude. >> >>>>>>>> Have fun. >> >>>>>>>> -- >> >>>>>>>> My liberal friends think I'm a conservative kook. >> >>>>>>>> My conservative friends think I'm a liberal kook. >> >>>>>>>> Why am I not happy that they have found common ground? >> >>>>>>>> Tim Wescott, Communications, Control, Circuits & >> >>>>>>>> Softwarehttp://www.wescottdesign.com-Hidequotedtext- >> >>>>>>>> - Show quoted text -- Hide quoted text - >> >>>>>>>> - Show quoted text - >> >>>>>>> Hi Tim, Thanks for that! I logged in to report that I tried >> >>>>>>> locking with just current modulation... one peice at a time so >> >>>>>>> to speak. And that worked fine, I could bang a bit more on the >> >>>>>>> table. But the current loop oscillates at ~20kHz when I crank >> >>>>>>> up the gain. I don't understand that at all! The current >> >>>>>>> modulation electronics has a bandwdith that's near 1 MHz, so >> >>>>>>> the 20kHz might be for some 'real' physics reason. Modulating >> >>>>>>> the current changes the wavlength through thermal effects. I >> >>>>>>> have no idea what the thermal time of the laser diode is. Would >> >>>>>>> 50us be a reasonable time? (retorical question no answer >> >>>>>>> expected.) >> >>>>>>> I'm going to try measuring the current to frequency modulation >> >>>>>>> parameter as a function of frequency. Hey I might learn >> >>>>>>> someting today! >> >>>>>>> If I get around to closing the 'double loop', I may have more >> >>>>>>> questions.... >> >>>>>>> It's not clear to me where I should put the integrator. >> >>>>>>> Having friday fun, >> >>>>>>> George H.- Hide quoted text - >> >>>>>>> - Show quoted text - >> >>>>>> Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! >> >>>>> 20kHz? That's like molasses. Why so low? And it should not cause >> >>>>> it to oscillate. >> >>>>> -- >> >>>>> Regards, Joerg >> >>>>>http://www.analogconsultants.com/-Hidequoted text - >> >>>>> - Show quoted text - >> >>>> Ja Ja, The photodiode design is from 10+ years ago. &nbsp;I hadn't >> >>>> heard of Phil H. then, let alone read his book. >> >>>> I've got at least 3 projects now that can use a faster photodiode. >> >>>> Oh for the above you have to keep the intensity low in order to >> >>>> not saturate the atomic transistion. &nbsp; So a fairly large PD (0.25" >> >>>> diam), at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level >> >>>> signal) &nbsp;And only a 1 MHz opamp (opa124... it has a bad noise gain >> >>>> peak.) >> >>>> George H. >> >>> This uses an optical-feedback Phil circuit that was discussed some >> >>> here. It has orders of magnitide more bw than comparable low-noise >> >>> TIAs. >> >> >>>http://www.highlandtechnology.com/DSS/PH200DS.shtml >> >> >>> I learned a lot working on this. Like, jfets have lots of gate >> >>> leakage if the drain voltage is high. That gotcha is in AoE but I >> >>> missed it. >> >> >>> -- >> >> >>> John Larkin &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Highland Technology >> >>> Incwww.highlandtechnology.com&nbsp;jlarkin at highlandtechnology dot com >> >> >>> Precision electronic instrumentation Picosecond-resolution Digital >> >>> Delay and Pulse generators Custom timing and laser controllers >> >>> Photonics and fiberoptic TTL data links VME &nbsp;analog, thermocouple, >> >>> LVDT, synchro, tachometer Multichannel arbitrary waveform >> >>> generators- Hide quoted text - >> >> >>> - Show quoted text - >> >> Yeah, can you release the price for a PH200? >> >> (When I tried, your marketing people wanted my mothers maiden name >> >> and part of my SS# &nbsp;:^) >> >> > Really? We're not supposed to keep pricing a secret; people will find >> > out anyhow. We do like to keep a record of who downloads manuals or >> > gets pricing, but the motives are pretty benign. Our registration >> > form is minimal, and we never spam. >> >> > $1986, qty 1. I think The Brat priced it at her birth year. >> >> Now you'll have lots of folks banging on your door at Otis Street, >> wanting to see her :-) >> >> >> >> >> 1 MHz at 1uA is that 1Meg Ohm gain? >> >> > The transresistances are 10M and 100K on the two ranges. >> >> >> For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain. >> >> > That's the low gain range. It's good there, but the real performance >> > is on the high gain range, where it gets over 1 MHz bandwidth at 10M >> > equivalent and very low noise, numbers like 100x better than most of >> > the stuff out there. >> >> Sometimes it's best not to have all the gain in the first (TIA) stage. >> Opamps are fairly cheap these days and the following ones don't have to >> be very fancy. Maybe something George could look at. >> >> > Hi Joerg, thanks for the advice. As Phil says later on, I've always > tried to keep signals in the Volt range for noise reasons. (But perhaps > I was too focused on noise?) Anyway I can sure reduce the TIA gain and > dial in more later. (I start to worry about the bandwidth of the rest > of the signal chain though...)
You always have to be careful about asking for too much gain from an op- amp. Those gain-bandwidth figures seem huge, until you burn up too much bandwidth for single-stage gain. The same rule applies with TIA amplifiers and capacitive sources -- only it's harder to pin down all the effects. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On Mon, 21 Jan 2013 06:50:28 -0800, George Herold wrote:

> On Jan 19, 3:58&nbsp;pm, Tim Wescott <t...@seemywebsite.com> wrote: >> On Fri, 18 Jan 2013 21:35:01 -0800, George Herold wrote: >> > On Jan 18, 5:04&nbsp;pm, Tim Wescott <t...@seemywebsite.com> wrote: >> >> On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote: >> >> > Hi Tim, &nbsp;Thanks for that! &nbsp;I logged in to report that I tried >> >> > locking with just current modulation... one peice at a time so to >> >> > speak. &nbsp;And that worked fine, I could bang a bit more on the >> >> > table. &nbsp;But the current loop oscillates at ~20kHz when I crank up >> >> > the gain. &nbsp;I don't understand that at all! &nbsp;The current modulation >> >> > electronics has a bandwdith that's near 1 MHz, so the 20kHz might >> >> > be for some 'real' physics reason. Modulating the current changes >> >> > the wavlength through thermal effects. &nbsp;I have no idea what the >> >> > thermal time of the laser diode is. &nbsp;Would 50us be a reasonable >> >> > time? (retorical question no answer expected.) I'm going to try >> >> > measuring the current to frequency modulation parameter as a >> >> > function of frequency. &nbsp;Hey I might learn someting today! >> >> >> > If I get around to closing the 'double loop', I may have more >> >> > questions.... >> >> > It's not clear to me where I should put the integrator. >> >> >> freq error -o-> prop. gain - + --> laser >> >> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;A '-> integrator --' >> >> > But with two feed back paths should there be an integrator in each >> > loop? >> >> Only if you want things screwed up! &nbsp;With an integrator in each loop >> you'll have an uncontrollable, metastable mode -- said mode being the >> difference between the two integrator states, and metastable because >> it'll be integrating. &nbsp;So what will happen is that your loop will hold >> just fine until either the piezo or the current goes to the positive >> rail, and the other one goes to the opposite rail. >> >> Besides, if you take my suggestion on this you'll have a DC-blocking >> filter between the one "laser" input and the current -- only the piezo >> will respond to the integrator at any rate. > > Hi Tim, thanks again for the sage advice. > > So at the moment the gain vs freq looks like, > > ~1Hz > L ^ | > O |-- > G | \ > | \ > g | \ > a | \ > i | \ ~1kHz > n | \| > +--------\---------> gain=1 > LOG frequency > > So If I make a break point (LF to piezo and HF to current) > at say 100 Hz do I still want the high frequency part of the gain to > have a one pole roll-off... or maybe the gain can be flat (for a > while.... I gotta roll off the HF eventually.) > > Not to worry, I&rsquo;ll play around and maybe learn something.
I'm not sure what you're showing me the gain _of_ here. It's a really good idea to have the open-loop gain be descending at around 20dB/decade with 60 to 120 degrees of lag at the loop closure point. Too little slope on the gain curve gives you these weird broad low peaks (or troughs, depending on the phase). Going back to my original suggestion, you want a block that you can call "laser" which _internally_ has the high & lowpass filters, and the feed to the diode and piezo, and the feedback from the PD. Whatever the response of that block is, it should be well-behaved. I've been assuming that its frequency response would be fairly constant up to some cut-off frequency, at which point it would start dropping by 20 or 40 dB/decade. Then you'd wrap that with your controller to get a nice descending characteristic, with a gain=1 point as high as you can push it.
>> > Not to worry first I need a faster PD. >> >> I gathered that, it being the size of a dinner plate (well, for >> cockroaches, perhaps). &nbsp;I'm not on top of the physics of photodiodes, >> but I thought that -- given low enough impedances -- your bandwidth is >> more limited by your amplifier than by your photodiode itself. &nbsp;Am I >> all wet here? > > As far as the photodiode design goes, I don&rsquo;t know of any &lsquo;new&rsquo; tricks*. > But the old rule of thumb (for TIA&rsquo;s) is that the max. frequency is the > geometric mean of the RC &lsquo;frequency&rsquo; and the opamp GBW. For my > pedestrian circuit, with unbiased photodiode C=~700pF, R=1Meg, GBW > =1.5MHz,(opa124) for which I get an f(rc)~240Hz and > sqrt(f(rc)*GBW)~19kHz.. (I thought the opa124 had a 1MHz GBW, but the > number was then a bit off.) > > George H. > > *well there are ways to reduce the capacitance. > reverse bias the PD, bootstrapping, and then Phil H.'s cascode between > the PD and TIA. I've never tried the cascode.
You're not bothering to bias your photodiode?!?!?! In a world of easy-to- get 10MHz or better op-amps you're using 1.5MHz?!?!?!?! Um -- I think there's some circuit improvements that you could make, there. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
George Herold wrote:
> On Jan 20, 12:22 pm, Joerg <inva...@invalid.invalid> wrote: >> George Herold wrote: >>> On Jan 18, 5:16 pm, Joerg <inva...@invalid.invalid> wrote: >>>> Tim Wescott wrote: >>>>> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote:
[...]
>>>>>> 20kHz? That's like molasses. Why so low? And it should not cause it to >>>>>> oscillate. >>>>> If he's already got substantial phase shift elsewhere, then the photodiode >>>>> rolling off would cause oscillation somewhere around 20kHz. >>>>> The fact that it happens at _exactly_ 20kHz just means that, sans >>>>> photodiode, he's got about 45 degrees of margin at 20kHz. >>>> But where does all that phase margin fall through the cracks? Unless >>>> everything rolls off fast, of course. 20kHz BW for the photodiode sounds >>>> really low, unless it is one the size of a dinner plate. >>>> -- >>>> Regards, Joerg >>>> http://www.analogconsultants.com/-Hide quoted text - >>>> - Show quoted text - >>> Grin, well not quite dinner plate size. ~6-7mm diam. >> That should be a lot more zippy than 20kHz if connected to a somewhat >> reasonable TIA. Or did you give it a hefty dose of Ambien? :-) > > Grin... I posted numbers for Tim. I can get a factor of 2 or so by > reverse biasing the PD, and maybe a factor of 5-10 with a faster > opamp. >
Then it's time to flick the little switch on the Weller and do that :-) Reverse bias is easy if there is no compelling reason why you need to be photovoltaic with the PD. Just keep the abs max limit in view. With the opamp also keep in mind its slew rate. GBW means nothing if it can't swing its tail around fast enough. -- Regards, Joerg http://www.analogconsultants.com/
On Jan 21, 11:07=A0am, Tim Wescott <t...@seemywebsite.please> wrote:
> On Mon, 21 Jan 2013 06:50:28 -0800, George Herold wrote: > > On Jan 19, 3:58=A0pm, Tim Wescott <t...@seemywebsite.com> wrote: > >> On Fri, 18 Jan 2013 21:35:01 -0800, George Herold wrote: > >> > On Jan 18, 5:04=A0pm, Tim Wescott <t...@seemywebsite.com> wrote: > >> >> On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote: > >> >> > Hi Tim, =A0Thanks for that! =A0I logged in to report that I tried > >> >> > locking with just current modulation... one peice at a time so to > >> >> > speak. =A0And that worked fine, I could bang a bit more on the > >> >> > table. =A0But the current loop oscillates at ~20kHz when I crank =
up
> >> >> > the gain. =A0I don't understand that at all! =A0The current modul=
ation
> >> >> > electronics has a bandwdith that's near 1 MHz, so the 20kHz might > >> >> > be for some 'real' physics reason. Modulating the current changes > >> >> > the wavlength through thermal effects. =A0I have no idea what the > >> >> > thermal time of the laser diode is. =A0Would 50us be a reasonable > >> >> > time? (retorical question no answer expected.) I'm going to try > >> >> > measuring the current to frequency modulation parameter as a > >> >> > function of frequency. =A0Hey I might learn someting today! > > >> >> > If I get around to closing the 'double loop', I may have more > >> >> > questions.... > >> >> > It's not clear to me where I should put the integrator. > > >> >> freq error -o-> prop. gain - + --> laser > >> >> =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A '-> inte=
grator --'
> > >> > But with two feed back paths should there be an integrator in each > >> > loop? > > >> Only if you want things screwed up! =A0With an integrator in each loop > >> you'll have an uncontrollable, metastable mode -- said mode being the > >> difference between the two integrator states, and metastable because > >> it'll be integrating. =A0So what will happen is that your loop will ho=
ld
> >> just fine until either the piezo or the current goes to the positive > >> rail, and the other one goes to the opposite rail. > > >> Besides, if you take my suggestion on this you'll have a DC-blocking > >> filter between the one "laser" input and the current -- only the piezo > >> will respond to the integrator at any rate. > > > Hi Tim, =A0thanks again for the sage advice. > > > So at the moment the gain vs freq looks like, > > > =A0 =A0 =A0~1Hz > > L ^ =A0| > > O |-- > > G | =A0\ > > =A0 | =A0 \ > > g | =A0 =A0\ > > a | =A0 =A0 \ > > i | =A0 =A0 =A0\ ~1kHz > > n | =A0 =A0 =A0 \| > > =A0 +--------\---------> gain=3D1 > > =A0 =A0 =A0 LOG frequency > > > So If I make a break point (LF to piezo and HF to current) > > at say 100 Hz do I still want the high frequency part of the gain to > > have a one pole roll-off... or maybe the gain can be flat (for a > > while.... I gotta roll off the HF eventually.) > > > Not to worry, I=92ll play around and maybe learn something. >
> I'm not sure what you're showing me the gain _of_ here.
I hope it's the closed loop gain of the piezo feedback circuit.
> > It's a really good idea to have the open-loop gain be descending at > around 20dB/decade with 60 to 120 degrees of lag at the loop closure > point. =A0Too little slope on the gain curve gives you these weird broad > low peaks (or troughs, depending on the phase). > > Going back to my original suggestion, you want a block that you can call > "laser" which _internally_ has the high & lowpass filters, and the feed > to the diode and piezo, and the feedback from the PD. =A0Whatever the > response of that block is, it should be well-behaved. =A0I've been assumi=
ng
> that its frequency response would be fairly constant up to some cut-off > frequency, at which point it would start dropping by 20 or 40 dB/decade. > Then you'd wrap that with your controller to get a nice descending > characteristic, with a gain=3D1 point as high as you can push it. > > > > > > >> > Not to worry first I need a faster PD. > > >> I gathered that, it being the size of a dinner plate (well, for > >> cockroaches, perhaps). =A0I'm not on top of the physics of photodiodes=
,
> >> but I thought that -- given low enough impedances -- your bandwidth is > >> more limited by your amplifier than by your photodiode itself. =A0Am I > >> all wet here? > > > As far as the photodiode design goes, I don=92t know of any =91new=92 t=
ricks*.
> > =A0But the old rule of thumb (for TIA=92s) is that the max. frequency i=
s the
> > geometric mean of the RC =91frequency=92 and the opamp GBW. =A0For my > > pedestrian circuit, with unbiased photodiode C=3D~700pF, R=3D1Meg, GBW > > =3D1.5MHz,(opa124) for which I get an f(rc)~240Hz and > > sqrt(f(rc)*GBW)~19kHz.. =A0(I thought the opa124 had a 1MHz GBW, but th=
e
> > number was then a bit off.) > > > George H. > > > *well there are ways to reduce the capacitance. > > reverse bias the PD, bootstrapping, and then Phil H.'s cascode between > > the PD and TIA. =A0I've never tried the cascode. > > You're not bothering to bias your photodiode?!?!?! =A0In a world of easy-=
to-
> get 10MHz or better op-amps you're using 1.5MHz?!?!?!?! > > Um -- I think there's some circuit improvements that you could make, > there.
Hangs head in shame, kicks dirt. "Gee Tim, there's no need to rub it in". But seriously, I did this in 2001, when it seems like, I harldy knew anything electronics-wise. There are several hundred of these "slow as molasses" detectors out there. I can't write up some side locking technique that uses modified detectors. But maybe I can sell them all newer faster detectors? Anyway again thanks, George H.
> > -- > Tim Wescott > Control system and signal processing consultingwww.wescottdesign.com- Hid=
e quoted text -
> > - Show quoted text -- Hide quoted text - > > - Show quoted text -