[time-nuts] 35601A as stand-in for 11848A ?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Jan 5 21:26:50 EST 2008

BriMDavis at aol.com wrote:
>  I need to measure things that are lower close in than the 11729/8662a
> combo noise floor; i.e. OCXO, direct synthesis from OCXO, amp/divider
> residual noise floors, down to 1-10 Hz offsets.
First you need a suitable spectrum analyser (filter characteristics are
critical for accurate measurements at low frequency offsets - much
easier to implement in software), or preferably at such low offsets a
pair of high resolution ADCs with suitable software.
A PC audio card is useful (with suitable low noise preamps) for
frequency offsets in the range  [20Hz, 20kHz].
Since a stereo card has 2 channels cross correlation techniques can be
For low offset frequencies (< ~50kHz) the phase detector noise floor is
significantly reduced by capacitively terminating the IF port and using
a high resistance dc load.
Both mixer ports should also be saturated.
A calibration source with a known phase noise spectrum is also required
as the phase detector gain is dependent on temperature, RF port input
frequency etc.
> thanks,
> Brian

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