people who designing low noise PLLs solved that problem a while ego go
to Charles Wenzels circuit collections he made a very low noise from DC
to a few hundred kHz amplifier just to amplify the phase noise, here is:
http://www.techlib.com/files/lowamp.pdf
On 8/7/2014 5:05 PM, Bob Camp wrote:
Hi
A low noise chopper stabilized op amp can make a pretty good pre-amp to put in
front of a low frequency spectrum analyzer. Something in the 20 to 30 db gain
is adequate for most analyzers. That will get you down to a level that’s well
below the noise floor on any OCXO I have ever seen.
Here’s a way to look at it:
The input resistors on the OCXO have KBT noise (they are real resistors). They
also are in the “many thousands of ohms” range. If you short the EFC (zero
noise in) you still have resistor voltage noise modulating the EFC. All you
need to do is to get down to the KTB level in a few thousand ohm resistor.
Yes 1/F noise does get into the mix. That’s why you want the chopper.
Bob
On Aug 7, 2014, at 6:32 PM, Alex Pummer <[email protected]> wrote:
the PC sound card has limited bandwidth bellow 20Hz and above 20kHz, is nothing and
also it is not so "noise less" like a spectrum analyzer which was made to
analyze spectrum and the sound card self is in a relative noisy environment in the PC
On 8/7/2014 3:09 PM, Bob Stewart wrote:
What about a PC sound card?
________________________________
From: Alexander Pummer <[email protected]>
To: Discussion of precise time and frequency measurement <[email protected]>
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.
73
Alex
KJ6UHN
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