Forums

Laser locking (control loops with two feedback paths.)

Started by George Herold January 17, 2013
On Fri, 18 Jan 2013 14:16:57 -0800, Joerg 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 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-Hide quoted text - - 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. >> >> 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.
And besides, it's not just the photodiode, it's the amplifier... I dunno -- it's not my circuit. But George would be well-served by figuring that out. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On Jan 18, 4:36=A0pm, 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 pie=
zo
> >>>>> 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 a=
nd
> >>>> figured this was the self resonant frequency of the piezo stack and =
the
> >>>> piece of Aluminum that it is pushing around. =A0(Ratio of mass of al=
uminum
> >>>> 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=3Dcgek9fio38jfi297es1j5g8b0rq258=
qm
> >>>> SRF ~ 261 kHz. =A0I have no idea if the simple mass scaling is corre=
ct..
> >>>> 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 th=
e
> >>>> 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 wa=
s
> >>>> easy to measure. =A0(I did try and do some measuments of the flexure > >>>> spring constant using the piezo as the sensor, very 'squishy' measum=
ents
> >>>> IIRC) > >>>>> Why can't you just control the laser current? > >>>> Hmm... OK that's a good question. =A0I'll have to try it! But for lo=
ng
> >>>> term DC drifts it's better to change the piezo (grating angle.) > >>>>> Do you want to have closed-loop control using the laser current, wi=
th
> >>>>> increased loop bandwidth, or do you just want to push the laser aro=
und
> >>>>> 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...(Car=
l
> >>>> Weiman and Leo Hollberg?) it might be in here, =A0(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=ADsig2=3D5lnQLOJC42fKCF_VCTMF8Q
> >>>> =A0 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, a=
nd 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 freque=
ncy
> >>> lower than the piezo resonance. =A0Take the low-pass filter, run it t=
hrough
> >>> a notch at the piezo resonance frequency, and feed it to the piezo. =
=A0Take
> >>> the high-pass filter, and feed it to the diode current. =A0Jigger gai=
ns
> >>> 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 frequen=
cy
> >>> 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 possi=
bly
> >>> around the piezo resonance, too (although you can damp that one out b=
y
> >>> playing with your notch and the cutoff frequency). > >>> Now wrap a loop around that. =A0Figure that when you knock on the tab=
le
> >>> you'll see it in amplitude -- your cavity will be changing, and you'l=
l be
> >>> fixing it by changing the laser current, so you'll see it in amplitud=
e.
> >>> 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.w=
escottdesign.com-Hidequoted text -
> >>> - Show quoted text -- Hide quoted text - > >>> - Show quoted text - > >> Hi Tim, =A0Thanks for that! =A0I logged in to report that I tried lock=
ing
> >> with just current modulation... one peice at a time so to speak. =A0An=
d
> >> 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. =A0Modulating the current changes the wavlength throug=
h
> >> 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. > > >> 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.
On Jan 18, 5:16=A0pm, 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 linear. > >>>>>> Modulating the current also changes the amplitude... but I actuall=
y
> >>>>>> 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 pie=
zo
> >>>>>> 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 stac=
k
> >>>>>> and the piece of Aluminum that it is pushing around. =A0(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 lon=
g
> >>>>>> link to a data sheet, > >>>>>>http://store.bravoelectro.com/redirect.php? > >>>>> action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > >>>>> 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b0rq2=
58qm
> >>>>>> 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 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. =A0(I did try and do so=
me
> >>>>>> 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 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 coo=
l
> >>>>>> 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&ved=
=3D0CDIQFjAA&url=3Dhttp%
> >>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > > > 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7=
JQ4ypXg&=AD=AD=AD
> > sig2=3D5lnQLOJC42fKCF_VCTMF8Q > >>>>>> =A0 But sometimes it's more fun to 'invent' your own method and th=
en
> >>>>>> =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 sign=
al
> >>>>> 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 filt=
er,
> >>>>> run it through a notch at the piezo resonance frequency, and feed i=
t
> >>>>> to the piezo. =A0Take the high-pass filter, and feed it to the diod=
e
> >>>>> current. =A0Jigger 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 lase=
r
> >>>>> 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. =A0Figure that when you kn=
ock
> >>>>> 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-Hidequoted text - - Show quote=
d
> >>>>> text -- Hide quoted text - - Show quoted text - > >>>> 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. =A0Modulating the current changes the wavlength thro=
ugh
> >>>> 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 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. > > >>>> 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 photodi=
ode
> > 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. George H.
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 locki=
ng
> > 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 curr=
ent
> > loop oscillates at ~20kHz when I crank up the gain. =A0I don't understa=
nd
> > 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 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 paramete=
r
> > 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 > =A0 =A0 =A0 =A0 =A0 =A0 '-> integrator --' >
But with two feed back paths should there be an integrator in each loop? Not to worry first I need a faster PD.
> 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 Fri, 18 Jan 2013 21:26:59 -0800 (PST), George Herold <gherold@teachspin.com>
wrote:

>On Jan 18, 4:36&#2013266080;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. &#2013266080;(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. &#2013266080;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. &#2013266080;(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. &#2013266080;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, &#2013266080;(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&&#2013266093;&#2013266093;&#2013266093;sig2=5lnQLOJC42fKCF_VCTMF8Q >> >>>> &#2013266080; 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. &#2013266080;Make the cutoff frequency >> >>> lower than the piezo resonance. &#2013266080;Take the low-pass filter, run it through >> >>> a notch at the piezo resonance frequency, and feed it to the piezo. &#2013266080;Take >> >>> the high-pass filter, and feed it to the diode current. &#2013266080;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. &#2013266080;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-Hidequoted text - >> >>> - Show quoted text -- Hide quoted text - >> >>> - Show quoted text - >> >> Hi Tim, &#2013266080;Thanks for that! &#2013266080;I logged in to report that I tried locking >> >> with just current modulation... one peice at a time so to speak. &#2013266080;And >> >> that worked fine, I could bang a bit more on the table. &#2013266080;But the >> >> current loop oscillates at ~20kHz when I crank up the gain. &#2013266080;I don't >> >> understand that at all! &#2013266080;The current modulation electronics has a >> >> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' >> >> physics reason. &#2013266080;Modulating the current changes the wavlength through >> >> thermal effects. &#2013266080;I have no idea what the thermal time of the laser >> >> diode is. &#2013266080;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. &#2013266080;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 Inc www.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
On Fri, 18 Jan 2013 16:04:20 -0600, Tim Wescott <tim@seemywebsite.com> wrote:

>On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote: > >> 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. > >freq error -o-> prop. gain - + --> laser > | A > '-> integrator --' > >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 > | A > o-> integrator --+ > | A > '-> 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.
Interestingly, the analog controls guys tend to do
>error -o-> prop. gain------+-----1------>|
| | | | +----int----->|sum-------- | | | | +----der----->| because it's easier for people to tune. -- John Larkin Highland Technology Inc www.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
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...@teachspi=
n.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 actual=
ly 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 pi=
ezo 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 calculatio=
n and
> >> >>>> figured this was the self resonant frequency of the piezo stack a=
nd the
> >> >>>> piece of Aluminum that it is pushing around. (Ratio of mass of al=
uminum
> >> >>>> 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 lo=
ng link
> >> >>>> to a data sheet, > >> >>>>http://store.bravoelectro.com/redirect.php? > >> >>> action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > >> >>> 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b0rq=
258qm
> >> >>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is corre=
ct..
> >> >>>> but about the right number came out the far side of the calculati=
on. 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 t=
he
> >> >>>> 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' mea=
suments
> >> >>>> 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 lo=
ng
> >> >>>> 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 co=
ol to
> >> >>>> be able to really bang on the table and have the thing stay locke=
d!
> >> >>>> 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=
&ved=3D0CDIQFjAA&url=3Dhttp%
> >> >>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > >> >>> 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4ocil_S=
q6Fm7JQ4ypXg&=AD sig2=3D5lnQLOJC42fKCF_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, t=
hen:
> >> >>> 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 i=
t
> >> >>> through matched low- and high-pass filters. Make the cutoff freque=
ncy
> >> >>> lower than the piezo resonance. Take the low-pass filter, run it t=
hrough
> >> >>> 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 gai=
ns
> >> >>> around so that your GHz/whatever (I'm not assuming digital or anal=
og at
> >> >>> this point -- GHz/volt, GHz/ADC count, whatever). > >> >>> If your laser current response stays flat up to a much higher freq=
uency
> >> >>> than the piezo does, then the overall response of your "laser" blo=
ck
> >> >>> should also be flat out that high, probably with a hiccup around t=
he
> >> >>> frequency where you transition from piezo to laser current, and po=
ssibly
> >> >>> around the piezo resonance, too (although you can damp that one ou=
t by
> >> >>> playing with your notch and the cutoff frequency). > >> >>> Now wrap a loop around that. Figure that when you knock on the tab=
le
> >> >>> you'll see it in amplitude -- your cavity will be changing, and yo=
u'll be
> >> >>> fixing it by changing the laser current, so you'll see it in ampli=
tude.
> >> >>> 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://ww=
w.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. An=
d
> >> >> 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 throug=
h
> >> >> 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. =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) =A0And > >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 =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 -
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# :^) 1 MHz at 1uA is that 1Meg Ohm gain? For one project (Rb magnetometer) I'd like ~1MHz at 100kohm gain. Something the about the same would work for this laser locking. Going from 10kHz to 1MHz is only a factor of 10^4 in Cap*GBW... :^) George H.
On Jan 19, 12:39=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 18 Jan 2013 16:04:20 -0600, Tim Wescott <t...@seemywebsite.com> w=
rote:
> >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 lock=
ing
> >> with just current modulation... one peice at a time so to speak. =A0An=
d
> >> that worked fine, I could bang a bit more on the table. =A0But the cur=
rent
> >> loop oscillates at ~20kHz when I crank up the gain. =A0I don't underst=
and
> >> that at all! =A0The current modulation electronics has a bandwdith tha=
t'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 50u=
s be
> >> a reasonable time? (retorical question no answer expected.) > >> I'm going to try measuring the current to frequency modulation paramet=
er
> >> 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 > > =A0 =A0 =A0 =A0 =A0 =A0'-> integrator --' > > >If your loop is crapping out at 20kHz with your 20kHz photodiode, chance=
s
> >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 =A0o-> 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. > > Interestingly, the analog controls guys tend to do > > >error -o-> prop. gain------+-----1------>| > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0=
=A0 =A0 |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0=
=A0 =A0 |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 +----int----->|su=
m--------
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0=
=A0 =A0 |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0=
=A0 =A0 |
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 +----der----->| > > because it's easier for people to tune.
That's how I've always done it. But now (I think, according to Phelan) >error -o->-+- prop. gain---+----gain?--->| | | | | | | | +----int----->|sum-------- | | | | +-----------------Neg FB----->| Hmm Phelan's at work... I might have screwed that up. (Is there gain in JL's gain of one path?) 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 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 >> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; '-> integrator --' >> >> > But with two feed back paths should there be an integrator in each loop?
Only if you want things screwed up! 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. 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.
> Not to worry first I need a faster PD.
I gathered that, it being the size of a dinner plate (well, for cockroaches, perhaps). 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. Am I all wet here?
>> 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 >> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;A >> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; o-> integrator --+ >> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;A >> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; '-> 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 & Software http://www.wescottdesign.com
On Sat, 19 Jan 2013 09:39:23 -0800, John Larkin wrote:

> On Fri, 18 Jan 2013 16:04:20 -0600, Tim Wescott <tim@seemywebsite.com> > wrote: > >>On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote: >> >>> 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. >> >>freq error -o-> prop. gain - + --> laser >> | A >> '-> integrator --' >> >>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 >> | A >> o-> integrator --+ >> | A >> '-> 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. > > Interestingly, the analog controls guys tend to do > >>error -o-> prop. gain------+-----1------>| > | | > | | > +----int----->|sum-------- > | | > | | > +----der----->| > > > because it's easier for people to tune.
That depends on how the people in question are doing the tuning. The topology that I showed is easier to apply the "seat of the pants" method where you get derivative tuned in, then proportional, then integral. The one you favor is easier if you do a Bode plot, tune the integrator zero, then tune the derivative zero, then bring the gain up. By the time you get to the point where you're fussing with the last few dB and degrees of margin, they're about equal. -- 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 & Software http://www.wescottdesign.com