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

[Lux, Jim]

On 7/13/22 5:24 AM, Dan Kemppainen via time-nuts wrote:
> 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


+1 for the 200 mA LT3042 (and the higher current 0.5A LT3045). There are 
now negative voltage versions, too, LT3094, also 0.5A.

Noise from 10Hz to 100kHz is around 0.8 microvolts.  At 10 kHz 2 
nV/sqrt(Hz).  But the real thing these things are good at is PSRR at 
higher frequencies.  >70dB at 1 MHz

They have enable inputs, can be paralleled willy-nilly, etc


>
>
>
> 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?
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