[time-nuts] FMT on October 13

[Didier Juges]

I just wanted to point out that with reciprocal counters, you can get
resolution much better than the 1Hz/s you would get with conventional
frequency counters, even though the actual accuracy of the measurement may
be way off.

The original question seemed to imply that with a short transmission time,
you could not guarantee a frequency accuracy of 1e-6 Hz, which you probably
can't anyhow, but the limit is not the resolution of the instrument or the
measurement method. 

I do not know how far off calibration my HP 5370s are, but the 20pS
resolution is at best only usable under some circumstances that I have not
isolated yet, due to jitter.

When measuring a 3.5 MHz signal (@1dBm) from my HP 8657B through 1 meter of
good coax cable (with counter and generator phase locked to the Thunderbolt
GPSDO) in Frequency mode with a 1s gate time, the resolution is 1e-5Hz, with
about 1e-3Hz p-p variation. When measuring over 1 period with 10,000 periods
sample size, the resolution is only 1e-1Hz with a standard deviation of ~400
Hz (or about 0.1%). Of course, over the air, it will be much worse due to
noise, let alone propagation, fading and multipath.

Didier KO4BB

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Tom Van Baak
> Sent: Wednesday, September 26, 2007 1:45 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] FMT on October 13
> 
> > I guess it depends on signal to noise ratio. With 
> reciprocal counters, 
> > you only need one period to measure as acurately as you 
> need, but to 
> > have good acuracy, you need very good S/N, as there is no 
> filtering possible.
> > 
> > For example, the HP 5370 can measure a single period of a 
> signal with 
> > a resolution of 20pS (excluding noise and trigger 
> imperfections), so 
> > excluding these errors, the HP 5370 could measure a single 
> period of a 
> > ~3.5 MHz signal with 7 x10-5 precision (if I have not goofed the 
> > calculations....) More periods improve the resolution 
> proportionately 
> > to the quare root. Accuracy is another matter.
> > 
> > Didier KO4BB
> 
> The jitter on a single period is likely very, very high, 
> especially if it comes over the air. That's why one usually 
> measures over a duration of thousands or even millions of 
> periods (effectively called the gate time).
> 
> The HP 53132A makes something like 200,000 measurements per 
> second. As a result, for a certain range of frequencies, it 
> claims 12 digits/sec of resolution (vs. HP 5370 ~11 digits/sec).
> 
> /tvb
> 
> 
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