[time-nuts] Seeking advice: Is this the right way to check very short term (below 1s) stability?

[Erik Kaashoek]

As the collection of frequency sources and counters in my home lab is 
growing I'd like to understand the performance of the frequency sources.
Two different GPSDO do help to check long term stability.
But the Rubidium frequency standard I have (Accubeat AR60A) is fairly 
unknown and seemingly not of good reputation, more specifically its 
(very) short term stability is doubted.
So how best to check very short term  (below 1s) frequency stability. 
The frequency counters available loose resolution quickly when the gate 
time is reduced below 1 second and high performance phase noise 
measurement equipment is not available so google helped with a search 
for alternative measurement methods.
What I found was a method using two frequency sources, one of the two 
being  a VCO, a mixer and some filters and amplifiers.
By weak locking (large time constant)Â  the VCO source using the mixer as 
phase detector to the other source, the output of the mixer's IF port 
should carry a voltage real time proportionally to the phase difference 
and by filtering and amplifying it should be possible to check for 
variations in the 1ms-1s range.
Maybe even a scope can see the variations.
When you know the amplification and the full range voltage you can even 
do an absolute measurement.
Would this method work?
Any specific concerns to take note of when doing the measurement?
Removing the DC component (or locking the VCO such that there is no DC 
component) will be crucial I guess but given the slow speed of the loop 
even an ADC->computer->DAC->VCO setup can work.
Any suggestion is welcome.
Erik.