"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.



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

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.




---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

"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 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

rickman wrote in :

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...


Well, he's not far wrong as it happens... I'm new to RF and radio but have
spent a lot more time with audio synthesisers. A triangle is a special case
of a sawtooth (properly, ramp) wave. Like the square, it has no even
harmonics, but the moment you change the speeds of the two parts of the ramp
in one cycle, even harmonics occur with increasing strength the more it gets
skewed. The really awkward bit with sawtooth waves is that they are a VERY
general case, and include ramps but also nonlinear slopes. This is something
I don't get into with maths, but it has strong implications for timbres
especially when emulating natural instruments like brass or strings or even
old synths that used relaxation oscillators. In radio techniques I imagine
the clipping and filtering (or other means) to precondition a rough signal
for the XOR phase comparator input of a PLL, taking advantage of its high
noise immunity even with the raw clipped signal (it still requires a 50%
width ratio), though a bit of filtering after clipping can help there.

"gareth" wrote in message
...
"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)

Oops! Got the two swapped round!

Square = odds, Triangle = evens, Sawtooth = raucous!

"gareth" wrote in news:m1agro$srh$1@dont-
email.me:

Square = odds, Triangle = evens

Nope. Both odd. Only when the pulse width ratio becomes significantly
asymmentrical do you get significant occurence of even harmonics in a ramp.
I don't know of a wave that has only even harmonics (short of artificially
contructed ones), but thermionic valves will produce a distortion that has
evens stronger than odds, I think. (I read that somewhere, I've never
measured it).

Lostgallifreyan wrote in
:

Only when the pulse width ratio becomes significantly
asymmentrical do you get significant occurence of even harmonics in a ramp.


WHich also applies to a 'flat' wave, rectangular, pulse, whatever, of which
square is the uniquely symmetrical case as the triangle is to the ramp.

Incidentally, when you skew the symmetry fully to one or other extreme, the
resulting sharp transitions have a full 'natural harmonic' series, but the
closeness and timing of the transitions in the flat wave complicate the
result and eventually all but cancel as the pulses get extremely short, while
a ramp gets a full energy, full natural spectrum set of harmonics. Even then
it's just a special case of sawtooth, natural sawtooth waves have lots of
nonlinearity.

"gareth" wrote in message
...
Oops! Got the two swapped round!

Square = odds, Triangle = evens, Sawtooth = raucous!

Still sort of wrong.

Square and trigangle are both odd harmonics.

Square wave harmonics are reduced by 1/N and triangle waves are sort of 1
over the square of the harmonic.



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com