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Precision synchronous demodulator

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 to
Tim Wescott
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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 to
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.

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 to
Spehro Pefhany

Hi Spehro, I was talking about the 200 MHz lockin.

formatting link

Scanning down (pages 181 and 219) it appears they use an AD831 as the mixer.

George H.

Reply to
George Herold

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 to
Spehro Pefhany

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 to
John Larkin

Let us all chime in, in n-part harmony:

Use my trick!

--

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

That was done in the demod that Mullard made for Hanbury Brown's intensity interferometer. See

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 to
Phil Hobbs

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 to
Phil Hobbs

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