Precision synchronous demodulator

Multiplying performance up in the megahertz is generally very disappointing.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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ElectroOptical Innovations LLC 
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There are papers on such schemes using CMOS switches for cancellation. Also schemes regarding controlling the DV/DT of the clocks. You would probably want to go fully differential on the clock as well if you want to be anal about this.

I just saw analog have several rated for ~10MHz multiplying BW and parallel in out so they should be real easy to drive. But I never tried them so I'm not sure what that means in the real world

-Lasse

Of course you have to let the sidebands through the BPF.

That's the kind of thing I'm worried about. Op-amps like to act as detectors all on their own sometimes.

I don't think so.. but please tell me if I missed something. If I have a 1mV signal and 1V of offset (LF noise), the average will be:

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

Kind of a lot when you're looking for a microvolt. Are they good for anything below 100's of MHz? Apparently they still work nicely when it's a bit chilly (eg. sub-4K). How about a cheapish scope probe for probing crystal oscillator nodes?

I doubt it would take kindly to swapping the reference polarity at

1MHz-- if it's in digital form anyway, might as well do it digitally.

I figured you had a good reason not to go digital, but it needed sayin'

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Den mandag den 23. september 2013 22.23.49 UTC+2 skrev Spehro Pefhany:

afaikt something like this can do it:

-Lasse

Would it be completely silly to apply the input signal to the reference and drive the DAC inputs to +/- Vref? Unfortunately, the gain is only flat to about 100kHz for the -Vref code.

Den mandag den 23. september 2013 23.31.17 UTC+2 skrev Spehro Pefhany:

That was my thought

where do you see that?

-Lasse

Figure 28 on page 13:

I sure would not have expected the bandwidth to be code-dependent.

Best regards, Spehro Pefhany

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Den tirsdag den 24. september 2013 00.51.35 UTC+2 skrev Spehro Pefhany:

to me that look more like an unipolar case illustrating of the limited "isolation" at higher frequencies

-Lasse

They always are--that's why MDACs are so disappointing for signal processing. Too many copies of the switch capacitance, and too small a voltage divider ratio for the intended path to swamp it out at codes near 0.

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 

hobbs at electrooptical dot net 
http://electrooptical.net

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.

--

John Larkin                  Highland Technology Inc 
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Precision electronic instrumentation 
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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.

Den tirsdag den 24. september 2013 01.03.18 UTC+2 skrev Lasse Langwadt Christensen:

I see it now, I should have read the datasheet ;)

I somehow thought it could actually do bipolar, but they use an opamp to do it in a "switch hitter"

-Lasse

The devil sez:

You know that by the time you get all these analog gew-gaws on there, you could've just ADC'd it and FPGA'd it and it'd work...

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

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...

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

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.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

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.