Super advice Ed, this is really really good advice.
Erik this is sage advice. especially CMR at high frequencies...
Oh and now LED lights overhead your bench which are driven at 5-50kHz
are are next new coupling of noise into your open bench circuits !!!
Glen.
(RF engineer)
On 13/07/2022 7:09 am, ed breya via time-nuts wrote:
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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