Extracting the 5th Harmonic
 

Hi all,

Is there some black magic required to get higher order harmonics out
of an oscillator?
I'm only trying to get 17.2Mhz out of a 3.44Mhz source and am thus far
failing spectacularly. I've tried everything I can think of so far to
no avail. All I can get apart from the fundamental is a strong third
harmonic on 10.32Mhz, regardless of what I tune for. I've tried
passing the osc output through two successive inverter gates to
sharpen it up, but still nothing beyond the third appears after tuned
amplification for the fifth. I no longer have a spectrum analyser so
can't check for the presence of a decent comb of harmonics at the
input to the multiplier stage but can only assume the fifth is well
down in the mush for some reason. I could change the inverters for
schmitt triggers and gain a couple of nS but can't see that making
enough difference. What about sticking a varactor in there somewhere?
Would its non-linearity assist or are they only any good for even
order harmonics?
Any suggestions, please. I'm stumped! :(
--

The BBC: Licensed at public expense to spread lies.

In (rec.radio.amateur.homebrew), Paul Burridge wrote:
Hi all,

Is there some black magic required to get higher order harmonics out
of an oscillator?
I'm only trying to get 17.2Mhz out of a 3.44Mhz source and am thus far
failing spectacularly. I've tried everything I can think of so far to
no avail. All I can get apart from the fundamental is a strong third
harmonic on 10.32Mhz, regardless of what I tune for. I've tried
passing the osc output through two successive inverter gates to
sharpen it up, but still nothing beyond the third appears after tuned
amplification for the fifth. I no longer have a spectrum analyser so
can't check for the presence of a decent comb of harmonics at the
input to the multiplier stage but can only assume the fifth is well
down in the mush for some reason. I could change the inverters for
schmitt triggers and gain a couple of nS but can't see that making
enough difference. What about sticking a varactor in there somewhere?
Would its non-linearity assist or are they only any good for even
order harmonics?
Any suggestions, please. I'm stumped! :(

There must be something killing the fifth harmonic, which should be
present at (1/5) of the amplitude of the fundamental in a square
wave. That's a pretty strong component.

If you can amplify the output of the source and then square it up
sharply, the fifth harmonic ought to be pretty easy to extract. The
larger the amplitude of that square wave, the larger the amplitude of
the fifth harmonic, of course, so amplification is your friend here --
but you may want to shield very well indeed to keep other components
out of places where they don't belong and may cause trouble.

Have a look at
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/geowv.html
for a lot of stuff that you probably already know.

Best of luck, and please keep us posted.

--
Mike Andrews

Tired old sysadmin

In (rec.radio.amateur.homebrew), Paul Burridge wrote:
Hi all,

Is there some black magic required to get higher order harmonics out
of an oscillator?
I'm only trying to get 17.2Mhz out of a 3.44Mhz source and am thus far
failing spectacularly. I've tried everything I can think of so far to
no avail. All I can get apart from the fundamental is a strong third
harmonic on 10.32Mhz, regardless of what I tune for. I've tried
passing the osc output through two successive inverter gates to
sharpen it up, but still nothing beyond the third appears after tuned
amplification for the fifth. I no longer have a spectrum analyser so
can't check for the presence of a decent comb of harmonics at the
input to the multiplier stage but can only assume the fifth is well
down in the mush for some reason. I could change the inverters for
schmitt triggers and gain a couple of nS but can't see that making
enough difference. What about sticking a varactor in there somewhere?
Would its non-linearity assist or are they only any good for even
order harmonics?
Any suggestions, please. I'm stumped! :(

There must be something killing the fifth harmonic, which should be
present at (1/5) of the amplitude of the fundamental in a square
wave. That's a pretty strong component.

If you can amplify the output of the source and then square it up
sharply, the fifth harmonic ought to be pretty easy to extract. The
larger the amplitude of that square wave, the larger the amplitude of
the fifth harmonic, of course, so amplification is your friend here --
but you may want to shield very well indeed to keep other components
out of places where they don't belong and may cause trouble.

Have a look at
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/geowv.html
for a lot of stuff that you probably already know.

Best of luck, and please keep us posted.

--
Mike Andrews

Tired old sysadmin

According to Fourier, at some mark-space ratios of a square wave certain
harmonics may be missing from the spectrum.

Just generate a a train of very short sharp pulses from the oscillator and
you will find all the harmonics are present allbeit with reducing
amplitudes. A single transistor should do the job.

According to Fourier, at some mark-space ratios of a square wave certain
harmonics may be missing from the spectrum.

Just generate a a train of very short sharp pulses from the oscillator and
you will find all the harmonics are present allbeit with reducing
amplitudes. A single transistor should do the job.

Is there some black magic required to get higher order harmonics out
of an oscillator?

No black magic, you just need to filter for the fifth harmonic. I'm
extracting the 16th harmonic of 64 MHz out of a MMIC, and retuning the
filter, can actually extract a little more 15th than 16th. You can generate
a comb generator to do the job of generating large quantities of all
harmonics, but for this simple of a job, it'd be overkill big time. Either
make a better filter, amplify your fundamental more before filtering, (the 5
volt digital squarer ought to put out +17 dBm) or check the HP app note
AN983 and take advantage of "filter gain".

W4ZCB

Is there some black magic required to get higher order harmonics out
of an oscillator?

No black magic, you just need to filter for the fifth harmonic. I'm
extracting the 16th harmonic of 64 MHz out of a MMIC, and retuning the
filter, can actually extract a little more 15th than 16th. You can generate
a comb generator to do the job of generating large quantities of all
harmonics, but for this simple of a job, it'd be overkill big time. Either
make a better filter, amplify your fundamental more before filtering, (the 5
volt digital squarer ought to put out +17 dBm) or check the HP app note
AN983 and take advantage of "filter gain".

W4ZCB

Paul Burridge wrote:

Hi all,

Is there some black magic required to get higher order harmonics out
of an oscillator?
I'm only trying to get 17.2Mhz out of a 3.44Mhz source and am thus far
failing spectacularly. I've tried everything I can think of so far to
no avail. All I can get apart from the fundamental is a strong third
harmonic on 10.32Mhz, regardless of what I tune for.

In RF circles, the 'normal' way to do this would be a simple Class C
amplifier with a collector load tuned to the fifth harmonic. In calls C,
conduction only occurs for a small fraction of a cycle which produces a
correspondingly higher proportion of higher harmonics than a square wave.

Ian

Paul Burridge wrote:

Hi all,

Is there some black magic required to get higher order harmonics out
of an oscillator?
I'm only trying to get 17.2Mhz out of a 3.44Mhz source and am thus far
failing spectacularly. I've tried everything I can think of so far to
no avail. All I can get apart from the fundamental is a strong third
harmonic on 10.32Mhz, regardless of what I tune for.

In RF circles, the 'normal' way to do this would be a simple Class C
amplifier with a collector load tuned to the fifth harmonic. In calls C,
conduction only occurs for a small fraction of a cycle which produces a
correspondingly higher proportion of higher harmonics than a square wave.

Ian

I read in sci.electronics.design that Reg Edwards
wrote (in
et.com) about 'Extracting the 5th Harmonic', on Fri, 12 Mar 2004:
According to Fourier, at some mark-space ratios of a square wave certain
harmonics may be missing from the spectrum.


For a waveform like this (use Courier font):
_____
/ \ /
_____/ \____________/

with rise-time f, dwell time d, fall time r and period T, the harmonic
magnitudes are given by:

Cn = 2Aav{sinc(n[pi]f/T)}{sinc(n[pi][f+d]/T)}{sinc(n[pi][r-f]/T)},

where sinc(x)= {sin(x)}/x

There seems to be a number of opportunities for a harmonic to 'hide' in
a zero of that function.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk