[time-nuts] Low noise powersupplies

Bob Stewart bob at evoria.net
Thu Aug 7 18:09:02 EDT 2014

What about a PC sound card?

 From: Alexander Pummer <alexpcs at ieee.org>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
Sent: Thursday, August 7, 2014 5:06 PM
Subject: Re: [time-nuts] Low noise powersupplies

to measure a power supply noise, better to say the noise spectrum,  you 
would need a very large non polarized capacitor and spectrum analyzer, 
The input of the spectrum analyzer does not like DC, and has low 
impedance. Since spectrum analyzer's input impedance is usually 50 ohm, 
for to be able to see the noise at low frequency you need  C = 1/( 2 x 
3.14 x 50 ohm x f Hz ) capacitor, and you would need a DC level limiter 
to prevent blowing the input of the spectrum analyzer during the charge 
up of that capacitor. If you could get a hold of an old HP 1Meg to 50ohm 
buffer amplifier you would need much lover capacitance or if  the buffer 
has AC input capability with low enough corner frequency like  the 
Tektronix P6201 FET probe, you would not need any capacitor. And that 
would make your life much nicer since capacitors could generate noise to..
Charles Wenzel  in his circuit collection files ha very nice good 
working noise reduction circuits.

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