Reply by Phil Hobbs September 26, 20132013-09-26
On 9/26/2013 10:47 AM, George Herold wrote:
> On Wednesday, September 25, 2013 10:13:03 PM UTC-4, Spehro Pefhany wrote: >> On Tue, 24 Sep 2013 18:02:21 -0700 (PDT), the renowned George Herold >> >> <gherold@teachspin.com> wrote: >> >> >> >>> >> >>> I was going to say, I thought the problem with the diode/balanced mixer might be the dynamic range. >>> >>> But I was interested in Vlad's suggestion of analog switches and balanced mixers. >>> (Is that how srs etal do it?) >>> >>> George H. >> The SRS lock-in I have access to uses a DSP. I have some schematics of >> >> older ones but have not attempted to decipher that part of them. >> > Hi Spehro, I was talking about the 200 MHz lockin. > http://www.thinksrs.com/downloads/PDFs/Manuals/SR844m.pdf > > Scanning down (pages 181 and 219) it appears they use an AD831 as the mixer. > > George H.
Way back in the day, I used to really like Mini-Circuits' RPD-1 phase detector. Lovely and quiet, low and stable offset voltage (~0.1 mV or so out of a +- 1V range, iirc). They make a superficially similar one in a smaller package, the MPD-1. Cheers Phil Hobbs -- Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by Phil Hobbs September 26, 20132013-09-26
On 9/24/2013 3:01 PM, John Larkin wrote:
> On Tue, 24 Sep 2013 14:06:39 -0400, Spehro Pefhany > <speffSNIP@interlogDOTyou.knowwhat> wrote: > >> On Tue, 24 Sep 2013 06:38:27 -0700, John Larkin >> <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>>> >>>> 1mV + 1V - (-1mV + 1V) = 2mV signal >>>> >>>> If the 2nd one is * 0.99, then I'll have >>>> >>>> 1mV + 1V - (-0.99mV + 0.99V) = 1.99mV signal + 10mV error >>> >>> Well, I don't understand that. I was figuring that each detector >>> might have about the same charge-injection inspired zero-signal >>> offset vs temperature, which we'd subtract. And they would >>> produce opposite-polarity signals, which we'd also subtract. >>> Their detected noise signals are opposite polarity, so, like the >>> signal, "add" when subtracted. So the result is twice the gain of >>> a single detector for both signal and noise, but we get to cancel >>> any offset. Something like that. >> >> Okay, I think I know what you're talking about. Two detectors >> operating on antiphase input (and the same LO) allows you to >> subtract out artifacts due to the detectors, to the extent that >> they are well matched. Subtraction done after demodulation, of >> course. >> >> Presumably with Gilbert cell demods we'd get ~sqrt(2) worse since >> they're already as well matched as possible. >> > > Or the same signal, but flip the LO phase. I'm assuming that you're > groveling for nanovolts and this sort of effort is worth it. > > Hmmm, you could also flip the LO phase, say, once a second, digitize > the PSD output, and subtract the samples taken during opposite clock > phases. That's sort of one lock-in inside another lock-in. Any DC > offset or other constant error, in the PSD or ADC, gets removed.
That was done in the demod that Mullard made for Hanbury Brown's intensity interferometer. See <http://electrooptical.net/hanbury/The_Intensity_Interferometer-Hanbury_Brown.pdf> P. 103 ff. They managed to get the drift down to below the shot noise level in a 12-hour measurement. (P. 109) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by Tim Wescott September 26, 20132013-09-26
On Thu, 26 Sep 2013 08:44:30 -0700, John Larkin wrote:

> On Thu, 26 Sep 2013 11:25:42 -0400, Spehro Pefhany > <speffSNIP@interlogDOTyou.knowwhat> wrote: > >>On Thu, 26 Sep 2013 07:47:46 -0700 (PDT), George Herold >><gherold@teachspin.com> wrote: >> >> >>> >>>Hi Spehro, I was talking about the 200 MHz lockin. >>>http://www.thinksrs.com/downloads/PDFs/Manuals/SR844m.pdf >>> >>>Scanning down (pages 181 and 219) it appears they use an AD831 as the >>>mixer. >>> >>>George H. >> >>Thanks for that link. I suppose they're using two to get quadrature >>outputs. >> >>As usual with RF parts, very little information on DC performance of the >>AD831, but based on the huge offset voltage (typical 15mV maximum >>+/-40mV), I'd guess it's uglier than sin. >> >> > Use my phase-flipping trick!
Let us all chime in, in n-part harmony: Use my <insert name here> trick! -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by John Larkin September 26, 20132013-09-26
On Thu, 26 Sep 2013 11:25:42 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Thu, 26 Sep 2013 07:47:46 -0700 (PDT), George Herold ><gherold@teachspin.com> wrote: > >> >>> >>Hi Spehro, I was talking about the 200 MHz lockin. >>http://www.thinksrs.com/downloads/PDFs/Manuals/SR844m.pdf >> >>Scanning down (pages 181 and 219) it appears they use an AD831 as the mixer. >> >>George H. > >Thanks for that link. I suppose they're using two to get quadrature >outputs. > >As usual with RF parts, very little information on DC performance of >the AD831, but based on the huge offset voltage (typical 15mV maximum >+/-40mV), I'd guess it's uglier than sin. >
Use my phase-flipping trick! -- 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
Reply by Spehro Pefhany September 26, 20132013-09-26
On Thu, 26 Sep 2013 07:47:46 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

> >> >Hi Spehro, I was talking about the 200 MHz lockin. >http://www.thinksrs.com/downloads/PDFs/Manuals/SR844m.pdf > >Scanning down (pages 181 and 219) it appears they use an AD831 as the mixer. > >George H.
Thanks for that link. I suppose they're using two to get quadrature outputs. As usual with RF parts, very little information on DC performance of the AD831, but based on the huge offset voltage (typical 15mV maximum +/-40mV), I'd guess it's uglier than sin.
Reply by George Herold September 26, 20132013-09-26
On Wednesday, September 25, 2013 10:13:03 PM UTC-4, Spehro Pefhany wrote:
> On Tue, 24 Sep 2013 18:02:21 -0700 (PDT), the renowned George Herold > > <gherold@teachspin.com> wrote: > > > > > > > >I was going to say, I thought the problem with the diode/balanced mixer might be the dynamic range. > > > >But I was interested in Vlad's suggestion of analog switches and balanced mixers. > >(Is that how srs etal do it?) > > > >George H. > The SRS lock-in I have access to uses a DSP. I have some schematics of > > older ones but have not attempted to decipher that part of them. >
Hi Spehro, I was talking about the 200 MHz lockin. http://www.thinksrs.com/downloads/PDFs/Manuals/SR844m.pdf Scanning down (pages 181 and 219) it appears they use an AD831 as the mixer. George H.
> > > > > Best regards, > > Spehro Pefhany > > -- > > "it's the network..." "The Journey is the reward" > > speff@interlog.com Info for manufacturers: http://www.trexon.com > > Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by Spehro Pefhany September 25, 20132013-09-25
On Tue, 24 Sep 2013 18:02:21 -0700 (PDT), the renowned George Herold
<gherold@teachspin.com> wrote:

> >I was going to say, I thought the problem with the diode/balanced mixer might be the dynamic range. > >But I was interested in Vlad's suggestion of analog switches and balanced mixers. >(Is that how srs etal do it?) > >George H.
The SRS lock-in I have access to uses a DSP. I have some schematics of older ones but have not attempted to decipher that part of them. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Reply by George Herold September 24, 20132013-09-24
On Tuesday, September 24, 2013 4:25:55 PM UTC-4, Spehro Pefhany wrote:
> On Tue, 24 Sep 2013 13:58:58 -0500, Tim Wescott > > <tim@seemywebsite.really> wrote: > > > > >On Mon, 23 Sep 2013 06:05:47 -0400, Spehro Pefhany wrote: > > > > > >> What's a good approach for use at 1MHz, give or take 2:1? > > >> > > >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert > > >> Cell balanced demodulator with LO >> Vt) has input referred drift and > > >> offset typically in the ~1uV/K and a couple mV. I'd like both to be > > >> better by at least an order of magnitude, and preferably with some gain > >> like the Gilbert cell things, which have 15-20dB of gain. > > >> > >> Analog switches have negligible offset- how much trouble will all that > > >> charge injection (~4pC for a good one) cause at 1-2MHz? > > > >I know it doesn't have the gain you want, but have you considered a diode- > > >ring mixer? With care taken to keep the LO where the LO belongs, and > >maybe a preamp to get the overall gain you want, you may be able to do as > >well or better than a Gilbert Cell mixer. > > > >Mini-Circuits may even have something you can just slap in... > > I'm using Mini-Circuits SRA-8 mixer on another board. I assumed the DC > stability might not be very good because of the diodes, but have not > really analyzed it properly.
I was going to say, I thought the problem with the diode/balanced mixer might be the dynamic range. But I was interested in Vlad's suggestion of analog switches and balanced mixers. (Is that how srs etal do it?) George H.
Reply by Tim Wescott September 24, 20132013-09-24
On Tue, 24 Sep 2013 16:25:55 -0400, Spehro Pefhany wrote:

> On Tue, 24 Sep 2013 13:58:58 -0500, Tim Wescott > <tim@seemywebsite.really> wrote: > >>On Mon, 23 Sep 2013 06:05:47 -0400, Spehro Pefhany wrote: >> >>> What's a good approach for use at 1MHz, give or take 2:1? >>> >>> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >>> Cell balanced demodulator with LO >> Vt) has input referred drift and >>> offset typically in the ~1uV/K and a couple mV. I'd like both to be >>> better by at least an order of magnitude, and preferably with some >>> gain like the Gilbert cell things, which have 15-20dB of gain. >>> >>> Analog switches have negligible offset- how much trouble will all that >>> charge injection (~4pC for a good one) cause at 1-2MHz? >> >>I know it doesn't have the gain you want, but have you considered a >>diode- >>ring mixer? With care taken to keep the LO where the LO belongs, and >>maybe a preamp to get the overall gain you want, you may be able to do >>as well or better than a Gilbert Cell mixer. >> >>Mini-Circuits may even have something you can just slap in... > > I'm using Mini-Circuits SRA-8 mixer on another board. I assumed the DC > stability might not be very good because of the diodes, but have not > really analyzed it properly.
I'm not sure if it'd work or not, I'm just throwing it out as a possibility to consider. I do know that -- by repute, at least -- the DC offset issues with ring- diode DBMs is due to the LO getting into the signal. I'm not sure if they are, or can be made, inherently better than Gilbert-cell mixers or not. Come to think of it, whatever you use, if you can periodically cut off the signal to the mixer you can measure the DC offset in "quiet" mode and null it out automagically, then go back to your regularly scheduled programming. If I did that I'd have it going on all the time, which means that (a) my intended signal would be getting well chopped up, and I'd have to deal with it, (b) I'd need to capture the DC bias well and often, and (c), I'd have to pay close attention to settling. But it might work. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by Spehro Pefhany September 24, 20132013-09-24
On Tue, 24 Sep 2013 13:58:58 -0500, Tim Wescott
<tim@seemywebsite.really> wrote:

>On Mon, 23 Sep 2013 06:05:47 -0400, Spehro Pefhany wrote: > >> What's a good approach for use at 1MHz, give or take 2:1? >> >> Things like the AD630 are slow, and my usualy go-to approach (Gilbert >> Cell balanced demodulator with LO >> Vt) has input referred drift and >> offset typically in the ~1uV/K and a couple mV. I'd like both to be >> better by at least an order of magnitude, and preferably with some gain >> like the Gilbert cell things, which have 15-20dB of gain. >> >> Analog switches have negligible offset- how much trouble will all that >> charge injection (~4pC for a good one) cause at 1-2MHz? > >I know it doesn't have the gain you want, but have you considered a diode- >ring mixer? With care taken to keep the LO where the LO belongs, and >maybe a preamp to get the overall gain you want, you may be able to do as >well or better than a Gilbert Cell mixer. > >Mini-Circuits may even have something you can just slap in...
I'm using Mini-Circuits SRA-8 mixer on another board. I assumed the DC stability might not be very good because of the diodes, but have not really analyzed it properly.