On Mon, 6 Oct 2014, rickman wrote:

On 10/5/2014 2:30 PM, 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
...

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.

Gareth really had me going on this one. I was actually thinking of building
this. But there is only one problem with the idea. To use the Si570 as the
VFO for a receiver it would need to output a sine wave. However the Si570
outputs square waves for digital circuitry. I don't think the mixer would
appreciate all the harmonics produced in a square wave would it?

Since a lot of mixers wnat to be driven hard, the square wave may not
matter.

About 1974, there was an article in Ham Radio for a phasing type direct
conversion SSB receiver, and he used ECL to generate the needed signal to
the mixers that were 90degrees apart. He didn't seem to find the square
wave an issue.

A bit later, someone did an article in Ham Radio wondering about this sort
of thing, and he tried square waves with a mosfet mixer, and a 1496 type
mixer, and I thought he found the outcome wasn't a problem with a square
wave.

If the input to the mixer isnt' well filtered, then I think you're going
to get all kinds of weird responses, since something can mix with a
harmonic of the oscillator and be converted down to the IF. But you'd
generally want good front end filtering, for other reasons.

Michael