Precision synchronous demodulator

[Spehro Pefhany]

Den mandag den 23. september 2013 12.05.47 UTC+2 skrev Spehro Pefhany:

I wonder how well a multiplying dac would work?

-Lasse

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.

 

[Lasse Langwadt Christensen]

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

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.

afaikt something like this can do it:

http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf


-Lasse

 

[Spehro Pefhany]

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

afaikt something like this can do it:

http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf


-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.

 

[Lasse Langwadt Christensen]

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

Would it be completely silly to apply the input signal to the
reference and drive the DAC inputs to +/- Vref?

That was my thought

Unfortunately, the
gain is only flat to about 100kHz for the -Vref code.

where do you see that?

-Lasse

 

[Spehro Pefhany]

where do you see that?

-Lasse

Figure 28 on page 13:
http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf

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



Best regards,
Spehro Pefhany

 

[Lasse Langwadt Christensen]

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

Figure 28 on page 13:

http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf



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

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

-Lasse

 

[Phil Hobbs]

Figure 28 on page 13:
http://www.analog.com/static/imported-files/data_sheets/AD5424_5433_5445.pdf

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



Best regards,
Spehro Pefhany

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

 

[Spehro Pefhany]

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.

 

[Lasse Langwadt Christensen]

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

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

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

 

[Spehro Pefhany]

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.

 

[George Herold]

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.

 

[Spehro Pefhany]

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

 

[George Herold]

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.

 

[Spehro Pefhany]

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.

 

[Phil Hobbs]

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

 

[Phil Hobbs]

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