[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[Dan Kemppainen]

Erik,

Just a thought. The 7805 in your schematic appears to be a big part of 
the signal chain. I've run into issues with 78xx series regulators being 
noisy. Spent a few weeks chasing down noise issues in some equipment and 
the 78xx regulator was a part of the problem. Replaced it with LT3042 as 
part of the solution.

Also note, not all 7805's are created equal. Different vendors may be 
better/worse than others.

Good luck!

Dan



On 7/12/2022 9:45 PM, time-nuts-request at lists.febo.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?