Forums

Laser locking (control loops with two feedback paths.)

Started by George Herold January 17, 2013
On Sat, 19 Jan 2013 15:00:41 -0600, Tim Wescott <tim@seemywebsite.com> wrote:

>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.
There are some ancient PID tuning procedures from the pneumatics days, that assume the config I drew. I heard the story that PID was actually discovered accidentally as a consequence of a leak in a bellows or something. Equation 4 here https://controls.engin.umich.edu/wiki/index.php/PIDIntro is equivalent to a controller gain of Kp*(1+integral). https://controls.engin.umich.edu/wiki/index.php/PIDTuningClassical Fresh EEs tend to sum P+I+D, which I did at first, until an old marine engineer pointed out the advantages of P * (1+I+D) -- 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 Sat, 19 Jan 2013 11:43:52 -0800 (PST), George Herold <gherold@teachspin.com>
wrote:

>On Jan 19, 12:39&#2013266080;pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Fri, 18 Jan 2013 16:04:20 -0600, Tim Wescott <t...@seemywebsite.com> wrote: >> >On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote: >> >> >> 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. >> >> 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. >> >> >freq error -o-> prop. gain - + --> laser >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;| &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;A >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;'-> 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 >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;| &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;A >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;o-> integrator --+ >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;| &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;A >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;'-> 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------>| >> >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; +----int----->|sum-------- >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; +----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?)
The gain of 1 is - computes furiously - 1! -- 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 Sat, 19 Jan 2013 11:32:36 -0800 (PST), George Herold <gherold@teachspin.com>
wrote:

>On Jan 19, 12:34&#2013266080;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&&#2013266093; 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. &#2013266080;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. &#2013266080; So a fairly large PD (0.25" diam), >> >at zero bias, and 1 M Ohm of gain. (for a 3-5 volt level signal) &#2013266080;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 &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;Highland Technology Incwww.highlandtechnology.com&#2013266080; 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 &#2013266080;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.
> >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. -- 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 Sat, 19 Jan 2013 13:54:27 -0800, John Larkin wrote:

> On Sat, 19 Jan 2013 15:00:41 -0600, Tim Wescott <tim@seemywebsite.com> > wrote: > >>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. > > There are some ancient PID tuning procedures from the pneumatics days, > that assume the config I drew. I heard the story that PID was actually > discovered accidentally as a consequence of a leak in a bellows or > something.
From "On Governors", James Clerk Maxwell, 1868: "But if the part acted on by centrifugal force, instead of acting directly on the machine, sets in motion a contrivance which continually increases the resistance as long as the velocity is above its normal value, and reverses its action when the velocity is below that value, the governor will bring the velocity to the same normal value whatever variation (within the working limits of the machine) be made in the driving-power or the resistance." Or, translated into modern English, integrators. He follows that a bit later with: "The first and third cases are evidently inconsistent with the stability of the motion; and the second and fourth alone are admissible in a good governor. This condition is mathematically equivalent to the condition that all the possible roots, and all the possible parts of the impossible roots, of a certain equation shall be negative." Or, in modern control-ese, that all of the poles are in the left half plane. It goes on. And on. This is the seminal English-language paper on stability and control (Tchebychev wrote one in German or Russian). Most of the elements that we all deal with to make stable, well-behaved loops are in there, in some embryonic form. Only the math that makes it easy (like the Laplace transform) is missing. Maxwell did it all The Hard Way -- and he made it work. He also systematized electromagnetics, unified electricity with magnetism (before vectors had been invented, even), and did serious work in optics (which was not understood to be an electromagnetic phenomenon at the time because E&M didn't _exist_ yet). -- 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

George Herold wrote:
> 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>=
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 c=
urrent
> > >> loop oscillates at ~20kHz when I crank up the gain. =A0I don't under=
stand
> > >> that at all! =A0The current modulation electronics has a bandwdith t=
hat'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 5=
0us be
> > >> a reasonable time? (retorical question no answer expected.) > > >> I'm going to try measuring the current to frequency modulation param=
eter
> > >> 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, chan=
ces
> > >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 derivativ=
e.
> > > > 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----->|=
sum--------
> > =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?) >
What I usually do is to make both I and T loops more or less integrating, with the T loop dominating at very low freq, and make sure the I loop rails safely. Then just run them together. The T loop will keep the I loop centred, and nobody has any excess phase down at 0.1 Hz or wherever the low frequency cross is. Cheers Phil Hobbs (Via Google Groups, from the Carnival Miracle at Port Canaveral-- heading for the beach bar. Having a daughter in the travel industry means we can't afford not to go. )
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 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&&#2013266093;&#2013266093;&#2013266093; >>> 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-Hidequoted 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. >> >> -- >> 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? :-) -- Regards, Joerg http://www.analogconsultants.com/
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&&#2013266093; 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. -- Regards, Joerg http://www.analogconsultants.com/
Tim Wescott wrote:

[...]


> From "On Governors", James Clerk Maxwell, 1868: > > "But if the part acted on by centrifugal force, instead of acting > directly on the machine, sets in motion a contrivance which continually > increases the resistance as long as the velocity is above its normal > value, and reverses its action when the velocity is below that value, the > governor will bring the velocity to the same normal value whatever > variation (within the working limits of the machine) be made in the > driving-power or the resistance." > > Or, translated into modern English, integrators. > > He follows that a bit later with: > > "The first and third cases are evidently inconsistent with the stability > of the motion; and the second and fourth alone are admissible in a good > governor. ...
Unfortunately not with many governors voted into office these days. -- SCNR, Joerg http://www.analogconsultants.com/
On Sun, 20 Jan 2013 09:36:14 -0800, Joerg wrote:

> Tim Wescott wrote: > > [...] > > >> From "On Governors", James Clerk Maxwell, 1868: >> >> "But if the part acted on by centrifugal force, instead of acting >> directly on the machine, sets in motion a contrivance which continually >> increases the resistance as long as the velocity is above its normal >> value, and reverses its action when the velocity is below that value, >> the governor will bring the velocity to the same normal value whatever >> variation (within the working limits of the machine) be made in the >> driving-power or the resistance." >> >> Or, translated into modern English, integrators. >> >> He follows that a bit later with: >> >> "The first and third cases are evidently inconsistent with the >> stability of the motion; and the second and fourth alone are admissible >> in a good governor. ... > > > Unfortunately not with many governors voted into office these days.
A farm hand was driving down the road with a load of manure, going fast. He got pulled over by a state trooper. "Good morning young man -- fine day to be driving farm products, isn't it" "Yes sir" "You were going rather fast there -- is there a governor on this truck?" "No sir -- that's _real_ bull shit you smell!" -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com

Joerg wrote:
> John Larkin wrote: > > On Sat, 19 Jan 2013 11:32:36 -0800 (PST), George Herold <gherold@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&=EF=BF=BD 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/-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) An=
d
> >>>> 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 Highland Technology Incwww.highlandtechn=
ology.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 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. >
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. 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. Cheers Phil Hobbs (Back on the ship)