[time-nuts] Phase noise measurement (was - no subject)
lists at rtty.us
Tue Aug 24 20:19:33 UTC 2010
For a one time frequency response check, a directional coupler and a signal generator do a pretty good job of creating a useful test tone.
On Aug 24, 2010, at 4:03 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 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:
>> 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.
>> -----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:
>> < 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.
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