[Lf] DCF spurious analysis]

[Andre' Kesteloot]

MarkusVester at aol.com wrote:

> Hi John, hi all,
>
> G4CNN wrote
> >  If you are sure that it is DCF49/42 please explain how you think the spur
> >  frequencies might be derived. I would like to learn a little more.
> >  Happy New Year, John, G4CNN
>
> Yes John, I'm quite certain about the source of the problem. A while ago, I
> also  had a very interesting talk with Gamal Soegiono, who knows a lot more
> about these things than I do and lives much closer to Frankfurt. Yesterday I
> made some more detailed measurements of the relevant frequencies and repeated
> the attenuator tests to make sure the IM was not produced by the rx.
>
> The two transmitters DCF42 and DCF49 are colocated at the same site
> (Mainflingen), using seperate antennas. The intermodulation is caused by the
> significant coupling between antennas and the nonlinearity in the tx output
> stage, in spite of the selectivity of the antenna matching circuits.
>
> DCF 42 is used to transmit DGPS data and has a fairly complex spectrum,
> looking like a USB signal with a reduced carrier. This carrier (the "pilot"
> as it aids the synchronisation of the decoders) is at
>  fp = 122.500 kHz, -15dBc.
> The information is sent at a data rate of
>  fd = 1187.5 Hz
> in a 2.3 kHz wide noise-like spectrum around
>  fp+fd = 123.6875 kHz ("0 dBc")
> in PSK (or is it MSK?). This contains most of the transmitted energy.
>
> The signal is accompanied by a number of undesired sidebands. There is a
> significant line at
>  fp-fd = 121.3125 kHz, -48dBc,
> caused either a second harmonic in the pilot generator or (more likely) by
> self-intermodulation of the data spectrum. This line again is accompanied by
> a set of satellites at
>  (fp-fd) +- (fd/26) = (fp-fd) +- 45.67Hz,  -58 dBc
> and smaller ones
>  (fp-fd)+- n * (fd/104) = (fp-fd) +- n * 11.42 Hz,  up to -63 dBc.
> These are apparently caused by 26- and 104-bit periodicities in the data. In
> the main spectrum you can actually find repeating frames of 88 ms duration.
> The next harmonic to the pilot is also visible here at
>  (fp-2*fd) = 120.125, -67dBc.
>
> DCF49 is stronger, but simpler, and used for telecontrol of switched-tariff
> electricity meters and street lights. Most of the time, it transmits its idle
> frequency of
>  f(DCF49) = 128.930 kHz,
> accompanied only by the Luxembourg effect of several broadcasting stations
> (Deutschlandfunk on LF, Voice of Russia on MF). However, at least every 12
> seconds it is modulated by bursts of 200 bd, +340 Hz shift FSK. Like DBF39,
> the modulation is phase-continuous, so the the carrier does not return to its
> old phase after a data packet, which sets a 0.1Hz lower limit on the usable
> detection bandwidth for DX-experiments a la VK2ZTO.
>
> The intermodulation spectrum is now easy to explain by the third-order scheme
>  f(IMP) = 2* f(DCF49) - f(DCF42).
> All of these carry the DCF49 bursts at +680 Hz shift. The strongest
> components are the broadband data around
>  2*f(DCF49) - (fp+fd) = 134.1725 kHz, +41 dBuV/m = 38 W EMRP
> and the pilot image
>  2*f(DCF49) - fp = 135.360 kHz, +21 dBuV/m = 400 mW EMRP,
> comparable to a good amateur station and thus perhaps a useful DX-indicator.
>
> Some products in our band are
>  2*f(DCF49) - (fp-fd) = 136.5475 kHz, +1 dBuV/m = 4 mW ,
> which was also noted by Alan 'NYK a while ago,
>  2*f(DCF49) - (fp-fd) +- (fd/26) = 136.5018 kHz (!), -9 dBuV/m = 0.4 mW,
>  2*f(DCF49) - (fp-fd) +- n(fd/104), eg. 136.4904 kHz, -15 dBuV/m = 0.1 mW.
>
> The bad news of course is 136501.8 Hz, spot on VE1ZZ's frequency. There are
> only two things which could help us down here:
> - Either one of the telecom technicians reads this and decides to readjust
> the traps at the tx location to get a few dB better decoupling,
> - or Jack pulls his crystal a bit to qsy somewhere between .496 and .499 kHz.
>
> Which of course brings up the matter of Loran lines. At my qth, I do see some
> in the upper part of the band, but below 137 kHz the noise appears to be
> higher, and they are quite faint if visible at all. Looking around in the
> neighbourhood of 100.0 kHz revealed three sets with periodicies of 5.5, 6.7
> and 7.5 Hz.
>
> Ok, that was a lot of stuff, hopefully interesting to some.
>
> 73 es all the best for the New Year
> Markus DF6NM