"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

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

"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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"Ralph Mowery" wrote in message
...
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.

It is the sawtooth (equal slopes at rise and decay) that is made up of
even harmonics.

(I cannot speak with any authority in the triangle wave)

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

On 10/7/2014 2:07 PM, Ralph Mowery wrote:
"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.

Yes, it is good to start with a signal that has less harmonic content.
Can your function generator put out a sine wave? I bet that has pretty
low harmonic content...

--

Rick

On 10/7/2014 2:07 PM, gareth wrote:
"Ralph Mowery" wrote in message
...
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.

It is the sawtooth (equal slopes at rise and decay) that is made up of
even harmonics.

You seem to have them backwards...

http://en.wikipedia.org/wiki/Triangle_wave

http://en.wikipedia.org/wiki/Sawtooth_wave


(I cannot speak with any authority in the triangle wave)

Or much authority in the sawtooth wave as it turns out...

--

Rick

On 10/7/2014 11:29 PM, Ralph Mowery wrote:
"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.

Big "IF"... That's my point. Knocking the third harmonic down to where
it is tolerable can be hard.


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.

No one *has* a device. Someone suggested using a device to make a
circuit and I didn't notice it had a square wave output at first. So we
are starting from the start...

--

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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