[time-nuts] FMT

[Normand Martel]

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
> > 
> > _______________________________________________
> > time-nuts mailing list
> > time-nuts at febo.com
> >
>
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> 
> 
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
>
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts



 
____________________________________________________________________________________
Do you Yahoo!?
Everyone is raving about the all-new Yahoo! Mail beta.
http://new.mail.yahoo.com