[Lowfer] Re: curious signal (KFI in the Sky)

[dsp]

Greetings Dale,

You can see some really scary things looking at AM broadcast carriers.  For another interest and project I've been looking at *lots* of them.  (Yes, there are even weirder and more arcane pursuits than LFing.)  One would have thought that as mature as AM broadcast is and the sci-fi level of technology in general it would be entirely possible for them to generate stable carriers on their assigned frequencies.

Right.

Many are solid and on frequency, but many just aren't.  Depending on the transmitter's age and quality, the carriers originate with anything from crystals 'au naturel', ovened crystals or synthesisers.  These latter are similarly referred to any of raw crystals, TCXOs, ovens or in very, very rare cases an external source;  their lock-up characteristics vary from rock-solid PLLs, through long time-constant frequency comparison/correction, to the accidental, depending on design, maintenance and ageing.  It is common to see seasonal changes, diurnal variations as the transmitter room gets hot and cold, and periodic effects such as you've found.

I fell upon this by accident a few years ago when tuning into the Beeb World Service from Antigua on 5975kHz, with a radio off which was hanging 'Spectran' after a session looking for EU on 136kHz.  Before I switched the radio to AM, 'Spectran' drew Auntie's carrier.  Which was, um, moving . . . ?  In a sorta-parabolic curve of some 10Hz width and 40-odd seconds period.  (Dex'd've been proud.)  I contacted Merlin who politely dealt with whom they must have regarded as a total nutter, but shortly thereafter fixed or replaced the synthesiser.

My guess is that what you are seeing from KFI is an oven clonking on and off, with multiple thermal coefficients of various bits of the crystal, oven etc. making the woodles.  As conceptually simple as ovens are, they are prone to imperfect operation as they age and the on/off points change;  crystals not specifically designed for the elevated temperature can well have *worse* tempco's up there than at normal room-temperature-ish.  A combination of the two could easily result in something like KFI.  Either that or it's the neatest, most symmetrical ever-so-slightly-exactly-not-quite stabilised synthesiser.

It's quite good fun 'Argo'ing an AM channel;  apart from the AM-stereo stations whose carriers disappear in a phase-modulated mush, the above synthesiser horrors and assorted wifties make it easy to recognise 'old friends'.  (Yes, OK, the boy needs a life.)

        Cheers,

                Steve        W3EEE

dsp@hifidelity.com 

(ante scriptum:  This is the eleventeenth time I've tried to send this over the last day or two  -  I'm away on business and fighting webmails.