[time-nuts] is there a cheap and simple way to measure OCXOs?

[Jim Lux]

On 5/30/12 8:11 AM, John Miles wrote:
>> Unfortunately, although i have a reasonable park of measurement
> instruments,
>> none of them are in the precision range that i'd need to characterize
>> the 8663's.
>>
>> I have three oscilloscopes (analog 20MHz, digital 200MHz and 1GHz),
>> a cheap handheld frequency counter and a couple of multimeters of
>> various quality. I also have a FE-5680A at hand as frequency reference.
>>
>> Any ideas what i could do to measure phase noise and frequency without
>> buying a lot of stuff? Soldering is not a problem though.
>
> Cheap, yes... cheap and simple, no.
>
> Since you have multiple copies of the oscillator in question, the cheapest
> way to measure PN would be to use a couple of isolation amplifiers, a mixer,
> and an FFT analyzer of some sort, rigged together in a quadrature-PLL
> arrangement.  You might have a look at the app notes on www.wenzel.com as
> well as the other references on PN measurement near the bottom of the page
> at www.ke5fx.com/stability.htm .
>

How close in frequency are these things?  tenths of Hz?

If you measure the low pass filtered output of a mixer and time zero 
crossings, you'll get some combination of the noises of the two 
oscillators. If you have 3, you can do all the pairs.

John's point about isolation amps is well taken, but inexpensive, 
reasonably low noise amps for this frequency range should be available.

You could probably use your digital scope to capture samples of the 
mixer output and then FFT them offline (or use the sound card on your 
PC, but the low frequency response is pretty bad on sound cards).