Another great post from Bob (as usual). Bob's advice is exactly correct. Unfortunately, the HP 11848 phase noise test set (part of the HP 3048 PN measurement system) is poorly designed, and does not break out the low gain non clipped signal. I modified mine to bring out this signal and it was MUCH easier to use. Follow Bob's advice if you are DIYing.
Rick N6RK On 7/4/2022 9:12 AM, Bob kb8tq via time-nuts wrote:
Hi If you are running a high gain op-amp to buffer things into a sound card *and* using the same op-amp output to drive the EFC, then you will have problems. Simple answer is to use a couple of op amps. Buffer the mixer with something low noise. Get the output of the mixer up to the point it almost saturates the op amp. Just how much gain that is depends a lot on your parts and power supplies. Running +/- 18V supplies into this op amp is no at all unusual. Since this output is linear, you have the full range of the beat note present. Nothing has been lost (yet). One path off this device goes to the high gain stage to the sound card. If the beat note is present, you will have clipping there. The other path goes to whatever you do to run the EFC. There are *many* approaches that could be used. One of many is a variable gain / variable roll off amp to “set” the PLL corner. That is followed by a simple summing amp to tune out the DC offset on the EFC.
Bob
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