[time-nuts] FMT

John Ackermann N8UR jra at febo.com
Fri Nov 10 16:13:08 EST 2006


Hi Normand --

Your method is similar to mine, except that I use a much closer beat 
note (usually less than 100 Hz) and use spectrum analyzer software and a 
sound card to measure the beat note (in the form of the delta between 
two traces on the waterfall display).

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.

John
----

Normand Martel wrote:
> Personnaly, i use a self-developed technique to
> remotely measure a station's frequency:
> 
> I use a precision OXCO controlled RF signal generator
> to inject an unmodulated (CW) signal (via a
> directional coupler) signal 1000 Hz below the actual
> station's frequency (example, to monitor CHU at 7335
> kHz, i inject a 7334 kHz signal into the coupler). I
> then adjust the generator's level to obtain a
> comfortable 1000 Hz from my receiver (in AM mode
> preferably, but it even works in FM... do not use SSB
> or CW modes, since the receiver's BFO will interfere).
> Finally, i measure the  1 kHz beat's frequency with
> precision (for that, i use an synthesized audio
> generator with a ramp (sawtooth) output on an o'scope
> in a X-Y function (X = ramp, Y = beat).
> 
> I prefer to use a ramp rather than a sine signal,
> since the ramp closely resembles a classic temporal
> sweep in a scope. This way, it becomes very easy to
> see if the generator's frequency is above or below the
> beat's frequency, which is much harder with a sine X
> input.
> 
> One other way is to use the scope in classic mode with
> the audio synthetizer (preferably in square wave, but
> sine would also do the job) feeding the scope's
> external trigger.
> 
> However, on distant HF signals, it becomes very hard
> to precisely measure the station's frequency due to
> the signal's fading which has important effects on the
> signal's phase. This phase unstability originates from
> the constantly changing RF signal's path due to the
> naturally unstable ionosphere's condition.
> 
> The receiver does NOT need to be a precision unit (you
> could even use a VFO controlled radio), since the beat
> comes from the heterodyning between the station's and
> the generator's signals.
> 
> 73 de Normand Martel VE2UM
> Montreal, Qc. Canada.
> 
> --- John Ackermann N8UR <jra at febo.com> wrote:
> 
>> Hi Colin --
>>
>> Actually, the transmitters used for the FMT seem to
>> be very stable and 
>> as far as I've been able to observe (during each of
>> the 4 FMTs since 
>> they restarted the event) don't drift by a
>> noticeable amount during the 
>> test.
>>
>> I'm actually more concerned about the ARRL's
>> measurement setup than I am 
>> about the transmitter stability.  At least through
>> last year, they 
>> measured the frequency off-air by hooking the
>> counter to an outside 
>> antenna through a bandpass filter, rather than
>> tapping off the output of 
>> the transmitters.  With multiple KW signals floating
>> around the 
>> vicinity, there's lots of opportunity for counter
>> confusion.  Some of us 
>> believe that ARRL's frequency measurement of the
>> 160M signal last year 
>> was about 0.4 Hz off, and I suspect the measurement
>> setup caused that.
>>
>> John
>> ----
>>
>> Colin Bradley wrote:
>>> I just finished several email exchanges with Joe
>> Carcia, station manager for W1AW, about the
>> operation of the station. I had hoped that the
>> regular daily bulletins broadcast by W1AW would be
>> tightly controlled in frequency, which would allow
>> me to get some practice measuring them. He informed
>> me that they use two IC-756Pro II¢s and one Orion I
>> for the transmissions. These radios do not permit
>> the use of external standards for frequency control.
>> Neither do the Harris 3200¢s. All of these radios
>> use TCXO¢s for frequency control. This setup will be
>> the same used for the FMT on the 15th.  They will
>> monitor frequency with a counter hooked to their
>> Z3801. 
>>>  
>>> It¢s hard to believe, with a 100-watt amplifier in
>> the same case, that these radios don¢t drift several
>> cycles during a long transmission. For that reason I
>> would encourage persons making measurements to do so
>> during the specified time for each frequency in
>> question. I think it would be very hard to measure
>> the frequency to 1 cycle or less with the frequency
>> control they use. The West Coast station that will
>> broadcast a 40-meter test signal which will, most
>> likely, be more accurate. That station will be using
>> a Heathkit DX-60 into a 400-watt amp. Frequency
>> control is from a HP-107BR into a HP-5100
>> synthesizer. While old, this equipment will probably
>> be up to the job. The oscillator is set against GPS
>> and the whole setup will be independently monitored
>> by another station a mile away with a Cesium
>> standard. 
>>> Colin Bradley
>>>
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