[time-nuts] FMT

[Mike Suhar]

John and I use a similar technique.  I am running Windows so I use DL4YHF's
Spectrum Lab FFT software.  Spectrum Lab will output each line of the
waterfall display to a tab delimited text file.  Using the function
peak_f(f1,f2) I output the frequency of the peak within F1 and F2 to the
file.  I also output the peak of the signal generator's carrier in a band
above or below the band of the desired carrier. The difference between the
two is used to calculate the final carrier frequency.  You just have to
remember to be in the same sideband USB or LSB and on the same side of the
signal generator carrier for each measurement in order not to confuse adding
or subtracting the delta.  At the end of the test the output file contains
several lines of data. I import the file into a spreadsheet.  The
spreadsheet calculates the delta and averages all the readings.  I may have
several hundred lines of output.  

As for the audio sample rate and FFT size I have used several the past
couple of years.  I am sure they are optimal but they work.  I am currently
running the audio sample rate at 11025 with the FFT size of 32768 with a
decimate factor of 2.  Bin size is around 168 millihertz.  This helps cut
down on any CW that may be near the desired carrier as I won't see those
signals as they are too fast to fill the bin. 

For this year's FMT I will use my HP 3586C with its internal tracking
generator looped back into its own antenna port (via a step attenuator).
This will serve as the signal generator used in the past.  A  Z3801 feeds
the external timebase of the 3586C.  

It has been interesting to measure the carriers of local AM broadcast and
shortwave stations.  Our local stations are a few hertz off but WLW (700
KHz) is dead on. I wonder what they are using for frequency control?

Mike

  

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Ackermann N8UR
Sent: Friday, November 10, 2006 6:34 PM
To: Hal Murray
Cc: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FMT

Hal Murray said the following on 11/10/2006 04:52 PM:
>> I think one of the reasons this method works so well is that the FFT
>> effectively averages the signal over some time, and I use a tool in
>> the  software to derive an average across all the FFT results.  That
>> smooths  out the instantaneous variations that make real-time
>> measurement such a  challenge. 
> 
> What size FFT are you using?  What sort of averaging are you doing?
> 
> What is the bandwidth of the signal you are looking at?  How does that 
> compare to the bin size of your FFT?
> 
> If you are recording the raw data and then post processing the signal, I'd

> expect you could FFT the whole thing.  It has to fit in memory, but that 
> doesn't look like a problem.  I think that would get the bin size down to
1/N 
> Hz if you had N seconds of data.  (But I'm not a DSP wizard.)
> 
> If you do that, there is only one sample so there is nothing to average in

> the time dimension.  If the signal is wide enough to end up in several
bins, 
> you could average in the frequency dimension.

The signal is a CW wave.

I use a Linux spectrum analysis program called Baudline.  It allows me
to do a couple of neat things.  First is that it can downconvert to
improve resolution by decimating and frequency shifting, so I can work
with a spectrum maybe 100 Hz wide.  Then I run a fairly deep FFT -- I
think I used 8192 bins last year.  You end up with resolution in the
milliHertz, but of course after decimating a bunch and using the deep
FFt, it takes 10 or more seconds to fill the bins.

I end up with a waterfall display showing the reference and unknown
signals over the length of the test.  Baudline has another neat tool,
which allows you to do an average over length of the waterfall.  So, I
end up with a single trace that represents the average over the length
of the test.  Baudline also has a "delta" feature that will calculate
the delta frequency between two signals, so that give me the final
output number that I work against the frequency of the known signal to
get the result.

John

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