"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 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

"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

"rickman" wrote in message
...
On 10/6/2014 1:31 PM, Ralph Mowery wrote:
..

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 thought we were going to start with a square wave. Nothing shapes the
square wave. You just run it through a low pass filter just above the
fundimental or 1st harmonic if you want to call it that..

The circuit I am thinking about is from an old RTTY audio tone generator I
built and was designed by Irv Hoff years ago, around 1970 or so.

He used a unijunction to generate a tone of 2125 or 2295 Hz (rtty tones)
then fed it into a 2 transistor multivibrator to generate good square waves.
Then into a low pass filter made of two coils and I think 2 or 3 capacitors.
This filtered out all the odd harmonics ( as there are no even harmonics in
the square wave) and just leaves the fundimental frequency as a pure sine
wave.

That way you could have a VFO going from 5 to 6 MHz and set the filter for
about 8 MHz and anything above that would be filtered out. No harmonics or
anything but a pure sine wave.

As I said, not sure if this would work at RF or not, just something to
think about.



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On Tue, 7 Oct 2014, Ralph Mowery wrote:


That way you could have a VFO going from 5 to 6 MHz and set the filter for
about 8 MHz and anything above that would be filtered out. No harmonics or
anything but a pure sine wave.

But the problem is, most rigs aren't using that kind of design now.

They are single conversion (with a 9MHz or so IF) or an upconversion to
above 30MHz, so the "VFO" covers a large territory, and since most rigs
are now general coverage (since now it's almost as easy as a hamband only
rig), you need a lot more low pass filters. You're right, if this was in
the old days, with a fixed range VFO, a low pass filter would be fine,
since only one would be needed.

As I said, not sure if this would work at RF or not, just something to
think about.

One reason Irving Hoff divided that unijunction oscillator output was
because the unijunction put out a tiny pulse, which has a lot more
harmonnc content, dividing it by 2 gave the waveforme a 50% square wave,
which has harmonics, but not at every harmonic, so it's simpler to filter.

There are better ways now. With almost the same amount of division
(especially in this case since the unijunction oscillator frequency
doesn't matter, so long as it's low enough to be stable), you can generate
"stepped sinewaves" with a ripple counter and some weighted resistors,
giving you a cleaner waveform that requires less filtering. That sort of
thing was around not long after that Irving Hoff article, just a few
years.

Michael

"Michael Black" wrote in message
news:alpine.LNX.2.02.1410071258330.14980@darkstar. example.org...
But the problem is, most rigs aren't using that kind of design now.

They are single conversion (with a 9MHz or so IF) or an upconversion to
above 30MHz, so the "VFO" covers a large territory, and since most rigs
are now general coverage (since now it's almost as easy as a hamband only
rig), you need a lot more low pass filters. You're right, if this was in
the old days, with a fixed range VFO, a low pass filter would be fine,
since only one would be needed.



For new designs I am sure there are beter ways of doing things.
I was just throwing out the low pass filter sort of thing as someone was
going to start with an oscillator that put out square waves. I was just
thinking of an easy way to get from the square wave output to a sine wave
and eliminate the harmonics that may or may not cause problems.

As I said, I don't even know if it is practical to do that at RF. I don't
have a way to generate square waves at RF, but did play around with a couple
of audio filters I had with a scope and function generator. I would thing
it would work the same at RF as it does at AF if the stray components can be
managed.

Very interisting to me. I decided to play for a while in my shop. The
filter with coils in it would take out the harmonics and give a very good
sine wave output. I had a low pass filter designed to use on a frequency
counter or scope with a cut off of 5 KHz. It did a poor job of rounding
off the square wave, but of all things when I switched the function
generator to a triangle wave it did a good job of making a sine wave out of
the signal. I thought the triangle wave had even harmonics in it, but found
out it actually has odd harmonics but they decrease with the square of the
order instead of just a simple 1/N. That ment a lot less harmonic content
to filter out. It was a long time ago that I played with the equations for
the various waveforms that can be made out of a bunch of sine waves added
together. A good memory refresher.




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On 10/7/2014 10:53 AM, Ralph Mowery wrote:
"rickman" wrote in message
...
On 10/6/2014 1:31 PM, Ralph Mowery wrote:
.

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 thought we were going to start with a square wave. Nothing shapes the
square wave. You just run it through a low pass filter just above the
fundimental or 1st harmonic if you want to call it that..

The circuit I am thinking about is from an old RTTY audio tone generator I
built and was designed by Irv Hoff years ago, around 1970 or so.

He used a unijunction to generate a tone of 2125 or 2295 Hz (rtty tones)
then fed it into a 2 transistor multivibrator to generate good square waves.
Then into a low pass filter made of two coils and I think 2 or 3 capacitors.
This filtered out all the odd harmonics ( as there are no even harmonics in
the square wave) and just leaves the fundimental frequency as a pure sine
wave.

That way you could have a VFO going from 5 to 6 MHz and set the filter for
about 8 MHz and anything above that would be filtered out. No harmonics or
anything but a pure sine wave.

As I said, not sure if this would work at RF or not, just something to
think about.

You say the filter removes "all" of the harmonics... that is obviously
not correct. The filter may reduce them, but it does not and can not
completely remove them. The nearest tones (which are also the largest
amplitude tones) will only be reduced a small amount really. Or maybe
you are planning to use a brick wall filter?

My question intended to ask *WHY* would anyone design a circuit to
produce a square wave and then spend the time and trouble to filter it?
Earlier you mention that filters are easier now, but in reality analog
filters are still much more difficult than just generating a tone in the
first place.

I can use a single chip and a DAC to produce tones up to many MHz with
very high accuracy and purity.

I don't get it...

--

Rick

"rickman" wrote in message
...
You say the filter removes "all" of the harmonics... that is obviously not
correct. The filter may reduce them, but it does not and can not
completely remove them. The nearest tones (which are also the largest
amplitude tones) will only be reduced a small amount really. Or maybe you
are planning to use a brick wall filter?

My question intended to ask *WHY* would anyone design a circuit to produce
a square wave and then spend the time and trouble to filter it? Earlier
you mention that filters are easier now, but in reality analog filters are
still much more difficult than just generating a tone in the first place.

I can use a single chip and a DAC to produce tones up to many MHz with
very high accuracy and purity.

I don't get it...

When I said all harmonics, that should have been taken as to reduce them to
a very low value as not to cause any problems. If the third harmonic is 40
or 50 db down or more , it probably won't cause a problem.

My whole thoughts was that someone already had a device that was on the
corrrect frequency but it put out square waves. I just thought that if it
could be ran through a low pass filter it would knock the harmonics down
enough not to cause problems in a mixer circuit.

If starting from the start, then there are beter ways to do it just as you
are stating.



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rickman wrote in :

You say the filter removes "all" of the harmonics... that is obviously
not correct. The filter may reduce them, but it does not and can not
completely remove them. The nearest tones (which are also the largest
amplitude tones) will only be reduced a small amount really. Or maybe
you are planning to use a brick wall filter?


One thing I tried briefly, before getting into computers are doing things
with code still a bit shy of getting into digital filtering just yet, was a
switched capacitor filter, the attenuation there is excellent, and you can
easily filter out the clock signal if it's 100 times the pass frequency.

My question intended to ask *WHY* would anyone design a circuit to
produce a square wave and then spend the time and trouble to filter it?


I had a go at answerign that in my other posts just now.. Basically,
conditioning a rough wave by Schmitt trigger into somethign much easier to
process later, often for PLL input. Controlling hysteresis (using positive
feedback) can be especially useful to elimnate some problems with complex
waves that cross the centre more than twice per cycle.