[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[ed breya]

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