[time-nuts] Phase noise principle and measuring confusion

[Bob kb8tq]

Hi

Phase is what drives an ADEV measurement ( frequency from phase ….). Phase noise the noise
at a specific offset from a carrier, often normalized to a 1 Hz bandwidth. Phase noise is a different
measure than “phase”. ( = they are not the same thing ).

Indeed a single mixer can be used as the heart of a measurement system for both quantities. You
do configure it differently depending on what you are after. Same basic hardware, different setups
to get different results. 

If you are looking at phase noise you put the mixer in quadrature lock ( just like the HP 3048) and 
use some sort of audio spectrum analyzer to figure out what the numbers are. You need a servo 
amp to keep things in lock.

If you are after phase, you offset the oscillators and get an audio tone out of the mixer ( a beat 
note …). The lower the tone frequency the greater the amplification ( and the more mixer noise
to deal with…..). For phase you need a low noise limiter to turn the audio into something your 
counter can deal with. 

Does it make sense to build both setups into the same box? Maybe it does. It depends a lot on 
just how much that mixer hits your “playing around” budget. 

Bob

> On Apr 7, 2020, at 7:42 PM, Perry Sandeen via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Learned Gentleman,
> I've read several articles on phase noise but I'm lost.
> I need a *Ding-Dong* school explanation of what it is, why it's important and how one goes about measuring it.
> <snip> Bob wrote:
> Phase measurement of my GPSDO
> 
> Thequick way to do this is with a single mixer. Take something like anold 10811 and use the coarse tune to set it high in frequency by 5 to10 Hz.
> 
> 
> 
> 
> Thenfeed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone.That tone is the *difference* between the 10811 and your device undertest. 
> 
> 
> 
> 
> Ifthe DUT moves 1 Hz, the audio tone changes by 1 Hz.
> 
> 
> 
> 
> Ifyou measured the 10 MHz on the DUT, that 1 Hz would be a very smallshift ( 0.1 ppm ). At 10 Hz it’s a 10% change. You have ‘amplified’ the change in frequency by the ratio of 10 MHz to 10 Hz ( so amillion X increase ).
> 
> 
> 
> *IF*you could tack that on to the ADEV plot of your 5335 ( no, it’s notthat simple) your 7x10^-10 at 1 second would become more 7x10^-16 at1 second.
> 
> 
> OK, this seems to me that this is measuring frequency difference.
> 
> wrote:
> 
> 
> 
> 
> Thereason its not quite that simple is that the input circuit on thecounter really does not handle a 10 Hz audio tone as well as ithandles a 10 MHz RF signal. Instead of getting 9 digits a second, youprobably will get three *good* digits a second and another 6 digitsof noise.
> 
> OK, then would using a 3336A synthesizer work by using the *good* 10811 or other 10 MHz as an external reference and provide, say, a 10 MHz + 100 Hz or 10 MHz + 1KHz work with the 5335? (I have two). Or am I missing something (or a lot)?
> Regards,
> Perrier
> 
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