Erik, I'd really recommend that you use a real, "solid" ground reference
on the instrumentation side, with +/- large (12-20 V) supplies, as
others have suggested.
Your most recent setup diagram indicates that you're relying on the
"differential" input of the audio PC card etc analyzer to allow for the
"floating" common of the analysis circuit. Do you know what the
common-mode rejection characteristics are? A true differential input
would have two coax lines entering a symmetric differential to
single-ended conversion stage at the front end. I doubt that the PC card
actually has this, but maybe some form of DC/LF isolation from the local
input common to chassis ground.
The PC likely has lots of SMPS noise in common-mode form, which probably
can be ignored for audio (the SMPS frequencies are almost always quite
far above audio). As long as the interference signals aren't too big to
upset the LNA operation by say, rectification in various junctions
(especially the front end), it should be OK. You will also have in-band
line frequency and harmonics present in the common-mode signal, but
these should be easier to deal with by virtue of whatever LF CMRR the
sound card does have at lower frequencies.
Now consider the analysis circuit environment, where you have apparently
zero intentional bypassing capacitance from the floating measurement
common to chassis/earth ground. Here, the only bypass caps effectively
are C1 at the REF buffer's input (which will only aggravate the
situation), and the small capacitance between the ports of the mixers. I
believe you have some bypassing at points in the other portion of the
circuit - the PLL for the reference - but I don't know what that looks
like now. So, just looking at this section, I'd say you need some
serious bypassing to ground, for the RF signals from the mixers, and the
common-mode signals in and out of the audio analyzer, DUT, and REF.
I recall there were some recent discussions about rail-splitting and
such, but I didn't look closely. I thought surely someone would have
mentioned the simple way to rail-split with an opamp, into a large
capacitive load, but maybe not.
Without resorting to a more desirable ground-referenced, +/- supply
scenario, you can add significant bypass capacitance from the signal
common to ground, with slight change to the buffer circuit.
1. Add a resistor between the opamp's output and the load, which is
signal common. The current demand appears small, so maybe around a
couple to few hundred ohms should do.
2. Add a resistor in series with the (sense line) inverting input. This
can be in the many k ohms range, depending the opamp's bias current.
3. Add a small capacitor between the opamp's output and inverting input
to stabilize it.
4. Add the bypass cap.
This setup just isolates the opamp from the capacitive load, with the
LF/DC regulated by the opamp, and the HF shunted by the bypass cap.
I'm guessing that once you get good bypassing here, the LNA will work
much better, and you should see the difference with the lower noise
opamp. The reason is that any opamp has limited CMRR, so improving the
bypassing makes the "CM" part smaller. This is also another reason to
operate opamp inputs at or near ground. Actually, the best CM
improvement can be provided by running in inverting mode, so both inputs
are always at ground. Non-inverting modes require the inputs to move,
depending on the signal. In your LNA, the CM input signal range is not
too bad, due to the high gain. The trick is to keep the overall CM - the
operating common level wrt ground and the power supplies - constant and
noise-free.
Regarding microphonics, since you mentioned tapping the housing, it
sounds like you have "canned it up," which is a good thing. Assuming the
REF and DUT are external, so not involved, the audible is coming from
the analysis circuit only, right? That's not too surprising since it's a
high gain system. It could be related to individual component
microphonics, but I'd guess it's an RF effect. The whole thing is awash
in the 2f signal and harmonics from the mixer, and to a lesser extent
the DUT frequency signal that leaks through, so mechanical dimension
changes or movements in the can, board, wiring etc, can change the EM
pattern inside, giving tiny, noticeable phase shifts - after all, that's
what it's for.
Ed
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