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

[Lux, Jim]

On 7/12/22 8:53 AM, Erik Kaashoek via time-nuts 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

What's the noise contribution of the resistors? V = sqrt(4*k*T*R*B)

51 ohms  is sqrt (4 * 1.38E-23 * 300 * 51) = 0.9 nV/sqrt(Hz)  100 ohms 
is ~1.4 nV/sqrt(Hz)

What kind of op amp? what's the current noise vs the voltage noise?  - 
you might low voltage noise, but high current noise, and that current 
noise across the input impedance can turn into surprisingly high voltage 
noise at the output.


> 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?

My guess would be a parasitic capacitance between circuit and housing. 
changing the distance changes the C.


>
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe send an email to time-nuts-leave at lists.febo.com
>