"Michael Black" wrote in message
news:alpine.LNX.2.02.1410052133310.11722@darkstar. example.org...

He wants a big Eddystone dial, likely connected to a Command Set or BC221
variable capacitor.

Partially right, but after recent posts from me on that matter, I
acquired an Eddystone EA12, which is very stable, because, as
its handbook claims, its thermal capacity is large.

However, just before the acquisition, I finally put money where my mouth
is and obtained some heavy grade aluminium plate to form the chassis
of my projected retro rig, and, anticipating oscillator drift wanted to
discuss various ways of circumventing that. I am fully aware of the
huff-and-puff and how it functions, BTW, but the huff-and-puff is
not absolutely stable to suit reception of some digital techniques.

Yes, I do want RX facilities that have the human-friendly big flywheel
knob and moving pointer, an ergonomic appraoch with its visual
feedback that has never been equalled in any rice box.

Think of the difference between using a DVM and an analogue meter
such as an Avometer where visual tweaking is more usable!

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?

On the other hand, the Huff-and-Puff approach can be improved by making
the reference frequency adjustable. Tune the receiver in the normal way
and when you press the button the MCU determines the appropriate
frequency to use for the PLL reference.

--

Rick

"rickman" wrote in message
...
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?


I don't know how it would work at RF, but as a square wave is made up of all
odd harmonics, if you use a low pass filter after it you should get a good
sine wave.

This was done years ago in a teletype tone generator to generate pure sine
waves to modulate a SSB transmitter with a single tone. The filter was made
of coils and capacitors. It may be made more compact now there are many op
amps and other active devices in production.

Just something to think about.

On the other hand, the Huff-and-Puff approach can be improved by making
the reference frequency adjustable. Tune the receiver in the normal way
and when you press the button the MCU determines the appropriate frequency
to use for the PLL reference.

--

Rick



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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.

PA

On 10/6/2014 12:33 PM, Ralph Mowery wrote:
"rickman" wrote in message
...
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?


I don't know how it would work at RF, but as a square wave is made up of all
odd harmonics, if you use a low pass filter after it you should get a good
sine wave.

This was done years ago in a teletype tone generator to generate pure sine
waves to modulate a SSB transmitter with a single tone. The filter was made
of coils and capacitors. It may be made more compact now there are many op
amps and other active devices in production.

Just something to think about.

That is fine if your frequency is fixed, but where would you put the
corner frequency for a tunable VFO? Also, it is hard to get enough
attenuation of the second harmonic since it is the closest to the
fundamental and the one most likely to give you trouble in the mixer.

--

Rick

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.

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

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. 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?

--

Rick

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

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.

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.

PA

"rickman" wrote in message
...
I don't know how it would work at RF, but as a square wave is made up of
all
odd harmonics, if you use a low pass filter after it you should get a
good
sine wave.

This was done years ago in a teletype tone generator to generate pure
sine
waves to modulate a SSB transmitter with a single tone. The filter was
made
of coils and capacitors. It may be made more compact now there are many
op
amps and other active devices in production.

Just something to think about.

That is fine if your frequency is fixed, but where would you put the
corner frequency for a tunable VFO? Also, it is hard to get enough
attenuation of the second harmonic since it is the closest to the
fundamental and the one most likely to give you trouble in the mixer.


There will not be a second harmonic for a square wave, just the odd number
such as the 3,5,7 and so on.
That was one of the reasons for generating the tones and passing them
through a circuit that converted them into good square waves and then to
the low pass filter.

I don't recall if any frequency range of the VFO was mentioned. So if going
from a range of 5 to 8 MHz the corner frequency could start around 9 MHZ
and should be dropping off alot at 15 MHz where the 3 rd harmoinc of the
starting point of 5 MHz would be.



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On 10/6/2014 1:31 PM, Ralph Mowery wrote:
"rickman" wrote in message
...
I don't know how it would work at RF, but as a square wave is made up of
all
odd harmonics, if you use a low pass filter after it you should get a
good
sine wave.

This was done years ago in a teletype tone generator to generate pure
sine
waves to modulate a SSB transmitter with a single tone. The filter was
made
of coils and capacitors. It may be made more compact now there are many
op
amps and other active devices in production.

Just something to think about.

That is fine if your frequency is fixed, but where would you put the
corner frequency for a tunable VFO? Also, it is hard to get enough
attenuation of the second harmonic since it is the closest to the
fundamental and the one most likely to give you trouble in the mixer.


There will not be a second harmonic for a square wave, just the odd number
such as the 3,5,7 and so on.
That was one of the reasons for generating the tones and passing them
through a circuit that converted them into good square waves and then to
the low pass filter.

You are right, no even harmonics in a square wave. What circuit clips a
tone into a square wave just so it could be run through a low pass filter?


I don't recall if any frequency range of the VFO was mentioned. So if going
from a range of 5 to 8 MHz the corner frequency could start around 9 MHZ
and should be dropping off alot at 15 MHz where the 3 rd harmoinc of the
starting point of 5 MHz would be.

So is this a viable practice to shape a square wave with a filter to use
in a mixer? I just find it hard to imagine that the harmonics would not
create some real problems. I'd have to do the math, but I expect even a
third harmonic has potential of creating a lot of spurs. I guess I'm
used to dealing with people who want very selective receivers. But I
have to admit I have forgotten a lot of the little bit of receiver
design I did learn.

--

Rick

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