High speed sample and hold circuit

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pberry
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High speed sample and hold circuit

Post by pberry » Fri Aug 27, 2004 9:37 pm

Is this where everyone went to? I posted this to the poptronix site and the posting is growing cobwebs.<p>Can someone help on a sample and hold circuit? I have a ~10 MHz (+/-10kHz or so -- maybe closer to 10MHz if it needs to be for the circuit I'm asking for) TTL square wave to trigger a S/H circuit that will be sampling an exact 10MHz sine wave (GPS locked). I want to read the sine wave to detect the frequency difference between the trigger wave and the sine wave down to the single Hz and better over hours. The output of the S/H will be a sine wave of the frequency difference. (I don't think a PLL will work for this, but I'm not familiar enough with PLL to know for sure). All of the S/H chips that I see are too slow (I think). I thought about using an A/D back to back with a D/A chip, but the ones I found are also too slow. The A/D chips seem to sample in the uS range. This seems to be way to slow and would integrate a big part of the sampled wave. Not only would I like to read down to the Hz, but also to the fraction of a Hz over several hours, which I could do If I have a good sample of the sine wave. I have a 50MHz DSO that must have a better acquisition time than the circuits I've found, so I figure that the circuit I want is relatively simple. Am I missing something here (a few brain cells perhaps)? Thanks for any suggestions. Paul.

rshayes
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Re: High speed sample and hold circuit

Post by rshayes » Sat Aug 28, 2004 1:26 am

Calibration labs used to set up their frequency standards by feeding their local frequency standard and the WWV time signal into a phase detector and recording the phase diference with a chart recorder over a period of hours or days. This averaged out differences in propagation time through the atmosphere and allowed the local standard to be set to within a fraction of a cycle per day.<p>Feeding your two signals to a balanced modulator will give the sum and difference frequencies. A low pass filter will eliminate the sum (around 20 MHz) and leave the diference (less than 10 KHz). Actually, if the two frequencies are identical, a balanced modulator can be considered a phase detector.<p>An exclusive OR gate can also be used as a phase detector. The phase range is limited to plus or minus 90 degrees in this case.<p>Analog Devices (www.analog.com) and possibly Texas Instruments (www.ti.com) make A/D converters for digitizing video signals. These can work at sample rates abouve 10 MHz, but are usually limited to 8 or 10 bit accuracy. These usually use a track and hold type of input, with aperture times in the tens of nanoseconds or better. Video D/A converters are usually faster, in the 20 to 100 MHz range.<p>Comlinear used to make a very fast sample and hold. It used about 4 watts, was quite expensive, and came in an oddball package. I think it was discontinued after Nation Semiconductor bought the company, since it doesn't appear on the National web site.

pberry
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Joined: Fri Aug 27, 2004 1:01 am
Location: Sacramento, CA
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Re: High speed sample and hold circuit

Post by pberry » Sat Aug 28, 2004 10:27 pm

Thanks for the help and direction!<p>Phase detector, balanced modulator -- I have some more homework. I can see that applying a logic gate would be useful. I filtered out the QFP packages from the Digikey search, so that eliminated the video A/Ds. Yes it is fast enough, buy holy cow - 100 pins? I'll keep looking for a simpler part in the video range.<p>Paul

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