[time-nuts] re low noise regulators

[Bruce Griffiths]

SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> the tempco effect of the series resistor may not be a factor at all for  
> typical OCXO's.
>  
> If we take a standard 1% resistor such as the Panasonic ERO-S2PHF2R00  
> (available on Digikey) with 100ppm/C tempco (that's a fairly bad tempco),  then we 
> get:
>  
>    0.32V * 100ppm = 32 microvolts per Degree C change (actually  it's 
> +/-50ppm, so this example is worse than what we would see in real  life).
>  
> If we now take the Supply-Voltage-versus-Frequency characteristic of  a 
> typical standard single oven OCXO  of 1E-08, at +/-5% variation for  a 12V part we 
> get:
>  
>    1E-08 for 1.2V change, then we get: 8.33E-09/V * 32E-06V =  2.66E-013 
> change per degree C.
>  
> For this particular OCXO with a stability of 1E-09 per degree C, the  change 
> in frequency due to the change in the 2 Ohm resistor value is about 3700x  
> less than the temperature stability of the OCXO itself.
>  
> In other words the effect of the resistor is  inconsequential.
>  
> For a good double-oven OCXO this is even less of a factor, for  exampe the 
> MTI double oven we use only has 8.3E-11 per volt change -  two orders of 
> magnitude less sensitivity than the example above.
>  
> The capacitors' microphonic sensitivity may actuallyy be more of a factor  
> here.
>  
>   
>> It still helps to have as low a noise as possible before using  such
>> brute force filtering to get that extra few dB of noise  reduction.
>>     
>
> It's not just a few dB, it's -3dB at 17Hz already, and that drops  at ~-20dB 
> per decade. At 170Hz it could theoretically give -23dB already,  or in other 
> words if the noise floor was entirely due to the supply voltage  noise, then 
> the difference would be between say -140dBc/Hz and about  -160dBc/Hz at 170Hz - 
> this could be a very significant improvement.
>  
>   
I usually regard anything less than about 20dB as a few dB.
> You are absolutely right, it is very difficult to get low noise  <10Hz.
>  
> bye,
> Said
>
>   
Said

I should have been more explicit:
If the OCXO has a single supply shared by both the OCXO and the oven
then since the oven current varies with ambient temperature the voltage
drop across the resistor will also vary which in turn will affect the
OCXO frequency.
If the Oven and oscillator/buffer supplies are separate the tempco of
voltage drop across the filter isnt a problem.
However some care is still required as the leakage current of an
electrolytic can be quite noisy.

Bruce