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Laser locking (control loops with two feedback paths.)

Started by George Herold January 17, 2013
On Jan 21, 2:01=A0pm, Joerg <inva...@invalid.invalid> wrote:
> George Herold wrote: > > On Jan 20, 12:22 pm, Joerg <inva...@invalid.invalid> wrote: > >> George Herold wrote: > >>> On Jan 18, 5:16 pm, Joerg <inva...@invalid.invalid> wrote: > >>>> Tim Wescott wrote: > >>>>> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote: > > [...] > > > > > > >>>>>> 20kHz? That's like molasses. Why so low? And it should not cause i=
t to
> >>>>>> oscillate. > >>>>> If he's already got substantial phase shift elsewhere, then the pho=
todiode
> >>>>> 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 so=
unds
> >>>> really low, unless it is one the size of a dinner plate. > >>>> -- > >>>> Regards, Joerg > >>>>http://www.analogconsultants.com/-Hidequoted text - > >>>> - Show quoted text - > >>> Grin, well not quite dinner plate size. ~6-7mm diam. > >> That should be a lot more zippy than 20kHz if connected to a somewhat > >> reasonable TIA. Or did you give it a hefty dose of Ambien? :-) > > > Grin... I posted numbers for Tim. =A0I can get a factor of 2 or so by > > reverse biasing the PD, and maybe a factor of 5-10 with a faster > > opamp. > > Then it's time to flick the little switch on the Weller and do that :-) > > Reverse bias is easy if there is no compelling reason why you need to be > photovoltaic with the PD. Just keep the abs max limit in view. With the > opamp also keep in mind its slew rate. GBW means nothing if it can't > swing its tail around fast enough.
Grin, yup, that's fine for me. Not so easy for most customers. I figure I should put a cap multiplier on the bias supply too. I don't want all the power rail noise getting into the photodiode. I've got a 'screaming' fast FET opamp in a drawer somewhere... I can try that too. (ADA4817.. looking at the spec sheet this may be more than I can handle!) George H.
> > -- > Regards, Joerg > > http://www.analogconsultants.com/- Hide quoted text - > > - Show quoted text -
George Herold wrote:
> On Jan 21, 2:01 pm, Joerg <inva...@invalid.invalid> wrote: >> George Herold wrote: >>> On Jan 20, 12:22 pm, Joerg <inva...@invalid.invalid> wrote: >>>> George Herold wrote: >>>>> On Jan 18, 5:16 pm, Joerg <inva...@invalid.invalid> wrote: >>>>>> Tim Wescott wrote: >>>>>>> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote: >> [...] >> >> >> >> >> >>>>>>>> 20kHz? That's like molasses. Why so low? And it should not cause it to >>>>>>>> oscillate. >>>>>>> If he's already got substantial phase shift elsewhere, then the photodiode >>>>>>> rolling off would cause oscillation somewhere around 20kHz. >>>>>>> The fact that it happens at _exactly_ 20kHz just means that, sans >>>>>>> photodiode, he's got about 45 degrees of margin at 20kHz. >>>>>> But where does all that phase margin fall through the cracks? Unless >>>>>> everything rolls off fast, of course. 20kHz BW for the photodiode sounds >>>>>> really low, unless it is one the size of a dinner plate. >>>>>> -- >>>>>> Regards, Joerg >>>>>> http://www.analogconsultants.com/-Hidequoted text - >>>>>> - Show quoted text - >>>>> Grin, well not quite dinner plate size. ~6-7mm diam. >>>> That should be a lot more zippy than 20kHz if connected to a somewhat >>>> reasonable TIA. Or did you give it a hefty dose of Ambien? :-) >>> Grin... I posted numbers for Tim. I can get a factor of 2 or so by >>> reverse biasing the PD, and maybe a factor of 5-10 with a faster >>> opamp. >> Then it's time to flick the little switch on the Weller and do that :-) >> >> Reverse bias is easy if there is no compelling reason why you need to be >> photovoltaic with the PD. Just keep the abs max limit in view. With the >> opamp also keep in mind its slew rate. GBW means nothing if it can't >> swing its tail around fast enough. > > Grin, yup, that's fine for me. Not so easy for most customers. >
Oh, I thought you were the R&D guy for this project.
> I figure I should put a cap multiplier on the bias supply too. I > don't want all the power rail noise getting into the photodiode. >
No worries, usually an RC filter is fine.
> I've got a 'screaming' fast FET opamp in a drawer somewhere... I can > try that too. (ADA4817.. looking at the spec sheet this may be more > than I can handle!) >
Usually just a fast enough low-noise amp would work. IIRC I used an LM833 on a similar design, on account of it's nicely low 1/f knee. Good stability around DC was important to the client and that one hit the spot quite well. -- Regards, Joerg http://www.analogconsultants.com/
On Mon, 21 Jan 2013 13:07:42 -0800, George Herold wrote:

> Hangs head in shame, kicks dirt. "Gee Tim, there's no need to rub it > in". > > But seriously, I did this in 2001, when it seems like, I harldy knew > anything electronics-wise. There are several hundred of these "slow as > molasses" detectors out there. I can't write up some side locking > technique that uses modified detectors. But maybe I can sell them all > newer faster detectors? >
Sorry, I didn't realize you were working on improvements to fielded products. You may be able to shove a lead-lag network in there that kicks in at around 20kHz that'll boost your bandwidth up a little bit. And hey -- 20kHz is better than 2, or whatever you get with your piezo, right? -- 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 Jan 21, 5:21=A0pm, Joerg <inva...@invalid.invalid> wrote:
> George Herold wrote: > > On Jan 21, 2:01 pm, Joerg <inva...@invalid.invalid> wrote: > >> George Herold wrote: > >>> On Jan 20, 12:22 pm, Joerg <inva...@invalid.invalid> wrote: > >>>> George Herold wrote: > >>>>> On Jan 18, 5:16 pm, Joerg <inva...@invalid.invalid> wrote: > >>>>>> Tim Wescott wrote: > >>>>>>> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote: > >> [...] > > >>>>>>>> 20kHz? That's like molasses. Why so low? And it should not cause=
it to
> >>>>>>>> oscillate. > >>>>>>> If he's already got substantial phase shift elsewhere, then the p=
hotodiode
> >>>>>>> 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? Unle=
ss
> >>>>>> 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/-Hidequotedtext - > >>>>>> - 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 somewha=
t
> >>>> reasonable TIA. Or did you give it a hefty dose of Ambien? :-) > >>> Grin... I posted numbers for Tim. =A0I can get a factor of 2 or so by > >>> reverse biasing the PD, and maybe a factor of 5-10 with a faster > >>> opamp. > >> Then it's time to flick the little switch on the Weller and do that :-=
)
> > >> Reverse bias is easy if there is no compelling reason why you need to =
be
> >> photovoltaic with the PD. Just keep the abs max limit in view. With th=
e
> >> opamp also keep in mind its slew rate. GBW means nothing if it can't > >> swing its tail around fast enough. > > > Grin, =A0yup, that's fine for me. =A0Not so easy for most customers. > > Oh, I thought you were the R&D guy for this project. > > > I figure I should put a cap multiplier on the bias supply too. =A0I > > don't want all the power rail noise getting into the photodiode. > > No worries, usually an RC filter is fine. > > > I've got a 'screaming' fast FET opamp in a drawer somewhere... I can > > try that too. =A0(ADA4817.. looking at the spec sheet this may be more > > than I can handle!) > > Usually just a fast enough low-noise amp would work. IIRC I used an > LM833 on a similar design, on account of it's nicely low 1/f knee. Good > stability around DC was important to the client and that one hit the > spot quite well. > > -- > Regards, Joerg > > http://www.analogconsultants.com/- Hide quoted text - > > - Show quoted text -
I don't know the lm833. 300nA 0f bias current doesn't look good. ... 0.5pA/rtHz of current noise times 1 meg ohm.... 500nV!.. ouch. George H.
George Herold wrote:
> On Jan 21, 5:21 pm, Joerg <inva...@invalid.invalid> wrote: >> George Herold wrote: >>> On Jan 21, 2:01 pm, Joerg <inva...@invalid.invalid> wrote: >>>> George Herold wrote: >>>>> On Jan 20, 12:22 pm, Joerg <inva...@invalid.invalid> wrote: >>>>>> George Herold wrote: >>>>>>> On Jan 18, 5:16 pm, Joerg <inva...@invalid.invalid> wrote: >>>>>>>> Tim Wescott wrote: >>>>>>>>> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote: >>>> [...] >>>>>>>>>> 20kHz? That's like molasses. Why so low? And it should not cause it to >>>>>>>>>> oscillate. >>>>>>>>> If he's already got substantial phase shift elsewhere, then the photodiode >>>>>>>>> rolling off would cause oscillation somewhere around 20kHz. >>>>>>>>> The fact that it happens at _exactly_ 20kHz just means that, sans >>>>>>>>> photodiode, he's got about 45 degrees of margin at 20kHz. >>>>>>>> But where does all that phase margin fall through the cracks? Unless >>>>>>>> everything rolls off fast, of course. 20kHz BW for the photodiode sounds >>>>>>>> really low, unless it is one the size of a dinner plate. >>>>>>>> -- >>>>>>>> Regards, Joerg >>>>>>>> http://www.analogconsultants.com/-Hidequotedtext - >>>>>>>> - Show quoted text - >>>>>>> Grin, well not quite dinner plate size. ~6-7mm diam. >>>>>> That should be a lot more zippy than 20kHz if connected to a somewhat >>>>>> reasonable TIA. Or did you give it a hefty dose of Ambien? :-) >>>>> Grin... I posted numbers for Tim. I can get a factor of 2 or so by >>>>> reverse biasing the PD, and maybe a factor of 5-10 with a faster >>>>> opamp. >>>> Then it's time to flick the little switch on the Weller and do that :-) >>>> Reverse bias is easy if there is no compelling reason why you need to be >>>> photovoltaic with the PD. Just keep the abs max limit in view. With the >>>> opamp also keep in mind its slew rate. GBW means nothing if it can't >>>> swing its tail around fast enough. >>> Grin, yup, that's fine for me. Not so easy for most customers. >> Oh, I thought you were the R&D guy for this project. >> >>> I figure I should put a cap multiplier on the bias supply too. I >>> don't want all the power rail noise getting into the photodiode. >> No worries, usually an RC filter is fine. >> >>> I've got a 'screaming' fast FET opamp in a drawer somewhere... I can >>> try that too. (ADA4817.. looking at the spec sheet this may be more >>> than I can handle!) >> Usually just a fast enough low-noise amp would work. IIRC I used an >> LM833 on a similar design, on account of it's nicely low 1/f knee. Good >> stability around DC was important to the client and that one hit the >> spot quite well. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/- Hide quoted text - >> >> - Show quoted text - > > I don't know the lm833. 300nA 0f bias current doesn't look good. > ... 0.5pA/rtHz of current noise times 1 meg ohm.... 500nV!.. ouch. >
Ok, if you absolutely must have tons of gain in that one stage then you'll have to get something more expensive. I used much lower gain and had that one in there for the low frequency portion, because of the 1/f knee. Old rule for me is that if the 1/f knee isn't at least visible in a graph then chances are the marketeers didn't like it and banned it from the datasheet :-) -- Regards, Joerg http://www.analogconsultants.com/