[Fwd: LF: Re: 136Khz signals]

Andre' Kesteloot akestelo@bellatlantic.net
Wed, 11 Nov 1998 23:16:31 -0500

Steve Olney wrote:

> G'day All,
> I think my previous post querying information about characteristics of
> 136KHz signals has caused some confusion. I guess I should have been more
> specific.
> The 136KHz signals I was referring to were the ones mentioned by Mike ZL4OL
> from RTTY stations.  They should have accuracies better than +/-10ppm. I
> wasn't referring to amateur signals.
> Perhaps the reference to stringent requirements for VFO accuracy mentioned
> in Andre's post to Alberto refers to receiver tuning accuracy.  It is
> correct to say that getting an accuracy on receiver frequency readout of
> +/-10ppm is not common, but it is not unusual.  My FRG-100 receiver has a
> spec. of +/-10ppm from 0-50C.  It also has an option for a TCXO which
> +/-2ppm over about the same temperature range.
> In actual fact, I do use a rubidium standard to check my frequency
> calibrations, but fortunately I didn't pay for it!  Our national TV
> broadcaster (ABC TV) and one of the commercial stations (Channel 9) here in
> Oz are using rubidium standards for generating the synchronising signals
> (horizontal and vertical).  I tap into an old BW TV set to access the
> vertical retrace pulse and use it as a calibration signal.
> The two calibrations I do are:
> 1. Sampling frequency of the Soundblaster Card used in acquiring the audio
> signal to be analysed. By feeding in a suitable level vertical retrace
> signal and looking for the 20th harmonic of 50Hz (1000Hz) I found that the
> sampling frequency at 11025Hz is 6.35ppm low (11024.93Hz). I made the
> adjustment for this error in my FFT software. This correction gives a
> reading accuracy of audio signals with the current maximum FFT points of
> 8,388,608 of a little over +/-1ppm (+/-0.0013Hz).
> 2. Receiver frequency accuracy.  Here I again use the vertical retrace
> signal, but look for RF harmonics in the 150KHz range. Looking for the
> 3000th harmonic of 50Hz (150KHz) I found that my FRG-100 actual receive
> frequency is 0.0735Hz lower than the displayed frequency again with an
> accuracy of +/-0.0013Hz (translates to about +/-0.01ppm @ 150KHz).
> Note in step 2 that I don't actually tune the receiver to the frequency I
> am looking for but 1Khz below and use the heterodyne tone.
> Being able to view signals with such resolution allows you to do some
> interesting measurements.  You can see tones buried in the noise some 25dB
> lower than Spectrogram and you can actually see the carrier frequency shift
> of some NDBs under modulation keying (kind of a micro-chirp - about 0.1Hz
> shift).
> Once you know the carrier frequency accurately, it is possible to
> positively identify a signal as although four or five stations might be
> listed as 136Khz, each one is likely to be slightly removed from that
> frequency.
> So to end this very long post, if anyone has the ability to measure (or has
> access to information on) the carrier frequencies and/or mark/space
> frequencies of the 136KHz RTTY stations mentioned previously by others, I
> would be very grateful.
> Regards
> Steve VK2ZTO
> -------------------------------------------------------------------------
> -       "Your point of view always depends on
> -               where you are standing"
> -
> -                    Steve Olney
> -                  North Richmond
> -                     Australia
> -
> -      email:  ollaneg@zeta.org.au
> -      Web Page:  http://www.zeta.org.au/~ollaneg
> -------------------------------------------------------------------------