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.
Bruce
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-boun...@febo.com [mailto:time-nuts-boun...@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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