On 10/6/2014 1:26 PM, Peter Able wrote:
On 06/10/2014 18:12, rickman wrote:
On 10/6/2014 12:38 PM, Peter Able wrote:
On 06/10/2014 16:43, rickman wrote:

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?

Then use a DDS.

There are lots of things you could do. I think Gareth got the idea
because the Si570 looked like it would be easy to use. But it just
doesn't fit the bill.

You posed a problem; I gave you an answer. BTW there cannot be an
easier item to lash to a microcontroller than a DDS

Thank you for your suggestion.


What frequency range would be of interest for tuning these older
receivers?


Up to 30MHz?

Once I read up on the Huff and Puff modification, I like the idea. It
doesn't change the receiver really, it just keeps it on the correct
frequency by using the VFO as a VCVFO.

Huff-and-Puff is fine - but its weak point is that the oscillator being
controlled must drift by less than the Huff-and-Puff step in the
Huff-and-Puff update period.

I have to say I did not look at this circuit long enough and thought I
understood it but in fact I was missing some of the crudeness to the
operation. There are a number of things that can be done to address
that particular limitation, the most obvious one is to use a longer
counter for the frequency measurement.

I still may not fully understand all the limitations but I'm not
suggesting this circuit should be used.


Is it common to have a voltage
control on the VFO in these receivers? Or is that a mod that would have
to be made?


No, most applications of Huff-and-Puff have included the varicap diode
as part of the mod. Not a big issue - unless you want to keep the
receiver totally authentic.

This is really quite an interesting idea. I've DDS'ed some classic
sets. The change in performance was startling - and I had no problem
getting used to driving the set via a 4 by 4 keypad - but I can readily
appreciate that this proposed combination of the operational advantage
of the newer technology with the older style of user interface has real
charm.

As was initially suggested, I think the original dial of the receiver
can still be used. Either the knob can be connected to something other
than the tuning capacitor or something like the Huff and Puff or better
a PLL can be used to stabilize the frequency.

I expect this has been done already by someone. Anyone seen a PLL used
to stabilize a VCO in a similar way as the Huff and Puff?

--

Rick