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

[Bob kb8tq]

Hi

If you have any ceramic capacitors in the mix, they are often microphonic. 
The X7R versions are typically the best “high C” types. NPO’s normally are
completely non-microphonic. Other non-ceramic caps should be ok, but 
who knows. 

Roughly speaking, 1 nV / Hz should be low enough to not matter. Since all
these specs are “typical” one never knows quite what this or that part may
be doing. You *should* see a drop putting in a 1 nV in place of a 5 nV.

Bob

> On Jul 12, 2022, at 7:53 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> I'm struggling with the noise floor.
> First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with 1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz was still -140dBc/Hz
> The setup was simplified to this schematic: http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG
> The REF_buffer creates a virtual ground, the Audio_LNA amplifies into the differential audio output .
> Why did the lower noise opamp not make a difference?
> Also the setup is acting like a nice microphone. Tapping the housing is clearly audible. Which component may be causing the microphony?
> 
>