[time-nuts] 10 MHz phase comparison using Bruel & Kjaer type 2971 phase meter

Luca Dal Passo iw2lje at gmail.com
Sat Jan 30 15:49:51 EST 2016

Hi All,
    while i was looking for an hp k34-59991A phase comparator on ebay in
order to make some long term measurements of frequency drift of a dut
against a reference (by logging the analog output of the meter) i found and
bought a Bruel & Kjaer type 2971 phase meter instead of the hp.
It is a very nice unit, with digital and analog readout, high resolution
and high precision.
The drawback is that it is a very low frequency device, specified for the
range 2Hz to 200 kHz.
So, to compare two 5 or 10 MHz signals i'm thinking to use two simple
double balanced mixer, some low pass filtering on the IF outs of the
mixers, and a common local oscillator of about 5,1 MHz or 10,1 MHz to
convert signals into the range of the instrument.
The actual frequency of the local oscillator does not metter, because it is
the same for the two input signal, and theoretically i could use even a
simple free oscillator, provided only the frequency falls in the correct
region regardless of phase noise and stability because their effects are
subtracted cancelling themselves. Anyway i think to use a simple ambient
temperature xtal oscillator for that.
What are your opinions? What i'm forghetting to consider? What kind of
drawback i have to expect?
The other question is about the availability of user/service manual (or at
least schematics) of the  Bruel & Kjaer 2971: i'm not able to find anything
on the net.
Many thanks!

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