Re: LF: Re: WRC-2012 proposal for 'near' 500kHz band(s)

[Andy Talbot <andy.g4jnt@gmail.com>]

Or help me programme in a lower divider ratio into the DDS

 

'JNT

 

 

On 2 March 2011 16:20, M0FMT <m0fmt@yahoo.co.uk> wrote:

 

Wonderfull! and maybe you can help me grind all my crystals lower in frequency as well Jim.
 

73 es GL Pete M0FMT IO91UX

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From: James Moritz <james.moritz@btopenworld.com>
To: rsgb_lf_group@blacksheep.org
Sent: Wed, 2 March, 2011 15:43:25
Subject: LF: Re: WRC-2012 proposal for 'near' 500kHz band(s)

Dear David, LF Group,

Having poked around inside quite a few radios over the years, I think 465/470kHz IFs were common in the days of discrete transistors and tuned IF transformers, but are mostly ancient history now. Most of the more modern broadcast radios use either 455kHz or 450kHz - the former being the default centre frequency for ceramic filters, the latter especially for radios with PLL synthesisers (it works out quite nicely with a 4.500MHz reference crystal and 9kHz or 10kHz channel spacings). At least in theory, the more modern cheap broadcast radios should have better IF rejection, since these single-chip receivers have double balanced mixers, rather than the old single-ended, self-oscillating, discrete designs.

The trouble with trying to avoid broadcast RX IF frequencies is that there is now a wide range to be avoided. With f0 possibly of 450, 455, 465 or 470kHz, +/- tolerance, and a bandwidth typically around 9kHz, a big chunk of frequencies from say 440 - 480kHz would have to be excluded on these grounds. It would be a great waste of spectrum to say these frequencies must be off-limits for all eternity for the sake of the inadequacies of cheap receivers used by an ever-decreasing number of listeners. Anyway, these frequencies always have been used by high power maritime telegraphy, NDBs, etc.

Another consideration is that the proposed maritime "super-Navtex" that might be centerd on 500kHz will inevitably have big powerful transmitters, using OFDM modulation with several kHz bandwidth. Although the out-of band emissions for this type of modulation is in principle very small, there will inevitably be some "spectral re-growth" due to imperfections in transmitter linearity. The high power levels compared to amateur signals, and wide coverage areas will mean this would be a major problem for amateurs if we have to operate on adjacent frequencies (not unlike the problems with utilities on 136k). There would be no way of convincing the maritime users they should take remedial action - they would just reply that their transmitters were fully within spec, which they would be. So if the maritime proposals go ahead, a large spacing between any amateur band and 500kHz would be a good thing. I volunteer to help struggling experienced non-appliance operators to wind a few more turns on their VFO coils if this happens ;-)

Cheers, Jim Moritz
73 de M0BMU