[time-nuts] Phase noise measurement (was - no subject)

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Aug 24 20:03:11 UTC 2010

Having a simple method of determining the preamp frequency response can 
be a useful diagnostic tool during development, particularly if one uses 
componets like super capacitors in the amplifier signal path.

If one doesnt have a suitable offset source handy the mixer ports can be 
driven in near quadrature by the same signal and the dc output as a 
function of the relative phase shift between the 2 mixer inputs can be used.

However neither method calibrates the phase noise frequency response of 
the system.
Adding RF noise to one of the mixer inputs can be used to measure the 
frequency response of the system.
If the RF noise source is uncalibrated but stable then it can be used to 
measure the relative frequency response.
The results of a dc (or beat frequency) measurement of the gain can then 
be used to correct the results to obtain a calibrated frequency response.

If one is using a capacitive or other non conventional mixer IF port 
termination, then knowing the relative frequency response can be vital.


Bob Camp wrote:
> Hi
> I've always calibrated my phase noise setups to the phase slope of the mixer
> I'm using. It does involve switching gains, but it's a direct system
> calibration. Beat note is 360 degrees, so this chunk is x degrees and you
> got y mv over that chunk. Check the slope on the other side of the beat note
> to make sure it's the same. Do some math and you have a radian to volt
> transfer function.
> If you are sorting junk box OCXO's it's a pretty good way to do it. The only
> added steps are an independent measurement of the switched gain / gain
> flatness and a short circuit input check to estimate the noise floor. Both
> are an initial setup / one time only sort of thing with most amps.
> Bob
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Bruce Griffiths
> Sent: Tuesday, August 24, 2010 3:25 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Phase noise measurement (was - no subject)
> Bob Camp wrote:
>> Hi
> <  CHOP>
> ....
> Being able to calibrate the preamp + sound card frequency response using
> the thermal noise of a resistor is convenient.
> This is more difficult to achieve with a bipolar input stage as the
> amplifier input current noise is significant.
> Bruce

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