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

[glenlist]

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
>