On Sun, 5 Oct 2014, gareth wrote:

Being somewhat of a polymath (just spent all day fence judging
at a horse trial) I find that I have a string of ideas faster than I
could ever implement them (rather unkindly described in one area
as vapourware), but I think it to be useful to punt them for a wider
discussion.

Musing upon the Huff-and-Puff technique, I wondered if there
was a better way to improve the frequency stability of older RXs,
because the Huff-and-Puff necessarily brings about a punctuated
frequency span (eg, multiples of 32 Hz), and this is what I came up with ...

It's worth pointing out that the "Huff and Puff" seems to have been done
originally without a frequency counter. The circuitry is similar, but
it's just done with counters and gates, no inherent need for the latches
and decoders and readouts. I'm pretty sure that came first, unless the
concept came from two different places about the same.

So you could take any old receiver, so long as it had a decent dial, and
add some stability.

it was later that the same concept appeared (or so I remember) as an
extension of a frequency counter. That makes sense, since there was a
transition to frequency counters, and the "lock" circuit was just a bit
more.

Using the ubiquitous timers to be found en masse in most micros
that seem to sell for only a few pence / cents these days, implement
a frequency counter to measure the local oscillator. Then, when
the user presses a Lock button (yet to be provided) the same micro
can program an si570 to generate the same frequency indefinitely
and to switch the mixer stage from the original to this new oscillator.

As a standalone, the odd lock points are because of the way the counters
are arranged, I don't think it was anything to do with some design need
(other than simplicity). Once you are adapting a frequency counter, you
end up with decade type steps.

But (and I've never tried the circuitry) I don't think these tune the way
you are expecting. It takes time to count, so you aren't snapping between
steps like with an FM broadcast receiver with AFC or a synthesizer of the
type in the HRO-500. The notion of the circuit is to keep it locked, with
in the nearest step. So only if it drifts beyond the specified step does
it jump back (and "jump" is probably the wrong word here).

Michael