Re: [Discuss-gnuradio] Using DSP for precise zero crossing measurement?

[John Ackermann N8UR]

Johnathan Corgan wrote:
> John Ackermann N8UR wrote:
>
> > The zero crossings are then used to calculate the time interval (or
> > phase) between the two waves.  I need to capture each pair of zero
> > crossings, and can't average over multiple intervals.
>
> If it is the phase difference that you are ultimately looking for, there
> are alternatives to measuring the zero crossing difference in the time
> domain.
>
> How precisely are the two sine waves matched in frequency?  How hard
> would it be to come up with reference waveform representing "zero"
> phase? How often do you need to measure the difference?

Good questions, and adding a bit more background may help.

What I am trying to do is measure the short term frequency stability 
(Allan Deviation) of a device under test compared to a reference.  The 
two signals are at nominally the same frequency, though in actuality 
they will be a small fraction of a Hz off from each other.

The signals start out at HF -- typically 10MHz -- but are each mixed in 
the analog domain with a common local oscillator that's offset by about 
1Hz (ie, LO is 10.000 001Hz) to yield a pair of 1 Hz difference 
frequencies.  Using a common local oscillator signal makes any noise on 
the LO common mode to the other two signals, and thereby reduces its 
impact.

The result is that those 1Hz signals carry the same noise components as 
the 10MHz signals did.  This improves the measurement resolution ) by 
the ratio of original to output frequency -- as much as seven decades 
(using a divider would divide the noise, too, so that wouldn't gain 
anything).

The goal is to measure the stability in terms of the time difference 
between the two 1Hz outputs; more correctly, we measure the delta of the 
time difference second-to-second to determine the stability.  It's 
possible to measure stability to parts in 10e-13 for one second using 
this method.

This "dual mixer time difference" technique is the standard one used for 
frequency stability measurement over short (0.1 to perhaps 1000 second) 
intervals.  The problem is that the trigger jitter in a normal time 
interval counter masks most of the resolution that's gained by the 
process; finding the zero crossing to 100ns accuracy against a 1Hz, 1 
volt p-p signal means the trigger jitter must be down in the nanovolts, 
and that's hard to accomplish.

Most systems currently use a series of limiting amplifiers to increase 
the slew rate and reduce the trigger jitter, but those amplifiers 
contribute their own noise and other complications like thermal 
instability.  My hope is that DSP might yield a better way to accurately 
determine the zerio crossing of a very slow slew rate signal.

Thanks!

John