[time-nuts] Question about precise frequency / phase measurement

[WarrenS]

Wolfgang asked

>Does anybody know a possibility to get a resolution < 1 mHz ? (in 1 second)
> The goal is look for frequency deviations caused by external influences ...

A silly question to ask time nuts. :)
How good do you really want it to be?
1 mHz out of 10 MHz in one second is only 1 part in 1e-10 and needs a resolution of  0.1 ns
 
For a high end example showing external influences causing small freq variation, see the swinging OSC test  at
http://www.thegleam.com/ke5fx/tpll/swing.gif 
This has a resolution of ~0.01 mHz (1e-12) at ~100 Hz update rate, which is about 10K better than what you have asked for. 


Many of the high end suggestions you are getting is how to do it 100 plus times better than what you've asked for.
Yes, plain old HC parts and some care can get you resolution and repeatability below 0.1 ns when averaging for one second.

For something pretty simple, see Bruce's XOR Linear Phase detector page at,  
http://www.ko4bb.com/~bruce/LinearPhaseComparators.html
I made a version of that using 74AHCxx parts that gives ~1 mHz freq difference resolution at 100 Hz update rate.

For really high end, simple, low cost, with no digital parts,  there is a 2.0 version of the TPLL with resolution of 1e-14/sec.
That is capable of near 1mHz resolution with an update rate of 10K/sec.
Information on TPLL version 1 is at  http://www.ke5fx.com/tpll.htm

ws

********************
snip

I want to monitor the frequency deviation continuously (that means: i 
don't want to look at a scope ;) and log the data several times per second. 
The goal is not to make a  'quality test' of the oscillator,
but to look for frequency deviations which are caused by external 
influences of various kind.

The question is, if 74HCxx parts would be good enough to get < 1 mHz 
resolution for a 10 MHz frequency with an update rate of < 1 sec.

Regards,
   Wolfgang

****************

Hello @all,

my name is Wolfgang and i'm new to the list.  :)

I browsed through the list archive, but i didn't find the infos i need, 
so i decided to join the list and to ask the experts directly.  :)

I want to measure the frequency difference between a 10 MHz OCXO and a 
10 MHz Rubidium.
I think that's what many people here have done many times... but i don't 
want to use expensive
equipment like time interval counters with picosecond resolution etc. I 
would prefer a cheap and
easy solution. I also would like to have an update rate of more than 1 
measurement per second, or even more.

My first approach was to use a simple XOR phase comparator. I tried a 
74HCT86 and a 74HCT4046.
It works, but it's very noisy, so i don't get better than about 10 mHz 
frequency resolution.
If i look at the lowpass-filtered output i don't see a nice sine or 
triangular wave, but it looks more
than a triangular wave with round tops and some bumps between them. 
Another problem is that the
difference frequency gets very low when the frequencies are very close, 
so it's not enough to look
only for zero crossings of the difference signal.

Does anybody know a possibility to get a resolution < 1 mHz ?

Best regards,
   Wolfgang