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

[Richard Karlquist]

More excellent comments from Bob, this time about microphonic
capacitors. 

It is interesting to note that the LT3042/LT3045 data sheets  

recommend a Murata GJ831CR61E106KE83L ceramic capacitor because it is: 

1.  Low microphonic 

and  

2.  Has a relatively small loss of capacitance due to DC voltage being
applied 

The amount of capacitance needed is prohibitively high to consider NPO. 

This part is listed as "Not Recommended for New Design" at Digikey, but 

they have over 24,000 in stock.  See URL: 

https://www.eetimes.com/murata-low-profile-capacitors-reduce-acoustic-noise/#:~:text=Murata's%20GJ8%20series%20of%20multi,consumer%20and%20industrial%20electronics%20applications.


I see that on the Eval PCB's they use Samsung CL31A106MBHNNNE.  Digikey
has over 1 million of these in stock. 

No info about how good they are. 

--

Rick Karlquist
N6RK 

On 2022-07-12 09:17, Bob kb8tq via time-nuts wrote:

> 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?
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