[Lf] another approach to LF receivers

[Andre' Kesteloot]

Talbot Andrew wrote:

> I think most modern transceivers derive all their internal frequencies
> from one master oscillator.   The IC746 uses a 30 MHz device which at
> extra cost can be replaced by a small 1-2ppm ovenned oscillator.  This
> has excellent stability in practice, and after about 30 minutes warm up
> stays within 0.1ppm if room temperature does not change violently.
> Exact frequency can be adjusted from a preset on the rear panel.   I
> haven't done it yet, but this could be easily phase locked to a
> frequency standard.
>
> Alternatively, the conversion LO for an LF transverter can be derived
> from this internal signal, thus cancelling out drift at the HF
> intermediate freq.  Net drift is then the 1ppm (or whatever) but now
> only at the LF frequency.  Getting respectable......  However, in
> practice for LF I use a RA1792 Rx.  Again all freqs are derived from one
> master - an external 5 MHz source with 10^-10 stability.
>
> For the narrow bandwidth Rx, if you want to start from scratch what
> about this as an outline scheme ?
>
> Start off with a DDS derived LO locked to a standard oscillator, and
> downconvert the LF band directly to 32765 Hz.  Make up a ladder filter
> using watch crystals - I have a first cut design for a five crystal
> device with a bandwidth of 1.1 Hz.  Subsequently convert the output down
> to 1 - 2 Hz  'baseband', again with a locked LO.   A/D convert directly
> at a sampling rate of, say, 10 Hz to 16 bit accuracy, again with the
> sampling clock locked to the frequency standard and feed the output to a
> PC via the serial port or even the parallel port.   At this data rate
> soundcards are irrelevant and any 16 bit language running in DOS can do
> some very advanced signal processing.
> If I hadn't got so many other projects and a perfectly good Rx I'd have
> a go myself !
>
> Andy G4JNT