RadioBanter - Extracting the 5th Harmonic

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- - Extracting the 5th Harmonic (https://www.radiobanter.com/homebrew/22570-extracting-5th-harmonic.html)

On Fri, 12 Mar 2004 15:02:30 -0800, John Larkin
wrote:

On Fri, 12 Mar 2004 16:31:19 -0600, John Fields
wrote:

On Fri, 12 Mar 2004 17:57:24 +0000, Paul Burridge
wrote:

Great. So without a spectrum analyser there's no way to tell? If I
examine the output of the multiplier, it's very messy. There's a
dominant 3rd harmonic alright (my frequency counter resolves it
without difficulty) but the scope trace reveals a number of 'ghost
traces' of different frequencies and amplitudes co-incident with the
dominant trace. All rather confusing. I suppose the only answer is to
build Reg's band pass filter and stick it between the inverter output
and the multiplier input? shrug

---
You may want to try something like this:

COUNTER SCOPE COUNTER
| | |
| | |
FIN--[50R]-+-[1N4148]---+----+-------+---FOUT
| |
[L] [C]
| |
GND----------------------+----+

The 50 ohm resistor is the internal impedance of a function generator,
and when I set it to output a square wave at 1.5VPP, I got 10.8kHz for
the fundamental of the tank. Then I tuned the function generator down
until I got a peak out of the tank, and here's what I found:

Fin Fout Vout fout/fin
kHz kHz VPP
-----|-----|------|---------
10.8 10.8 0.9 1.0
3.58 10.8 0.25 3.02 ~ 3
2.14 10.8 0.2 5.05 ~ 5

So with a square wave in there were no even harmonics and it was easy
to trap the 3rd and 5th harmonics with a tank.

Next, I tried it with a 3VPP sine wave in and got:

Fin Fout Vout fout/fin
kHz kHz VPP
-----|-----|------|---------
10.8 10.8 1.3 1.0
5.39 10.8 0.9 ~ 2.0
2.14 10.8 0.3 5.05 ~ 5

So it looks like the second and the fifth harmonics were there. There
were also some other responses farther down, but I just wanted to see
primarily whether the fifth had enough amplitude to work with, and
apparently it does, so I let the rest of it slide.

So, it looks like if you square up your oscillator's output to 50% duty
cycle you could get the 5th harmonic without too much of a problem. If
you can't, then clip the oscillator's output with a diode or make its
duty cycle less than or greater than 50%, and you ought to be able to
get the 5th that way.

Historical note: about 1960, a guy at HP was doing exactly this with
some new diodes, and he got way more higher harmonics than theory
predicts. To figure it out, they hooked up the just-invented HP185
sampling scope (which then used avalanche transistors to make its
sampling pulses) and discovered the diode reverse-recovery snap
phenom. Soon the scope itself was using this effect. They were
originally called Boff diodes, after the discoverer Frank Boff, but
the name didn't stick (wonder why?) and they became "snap diodes" and
later "step-recovery diodes". I think I may have the HP Journal
article around somewhere.

See page 31:

http://cp.literature.agilent.com/lit...5980-2090E.pdf

---
:-)

--
John Fields

"Paul Burridge" wrote in
message ...
Hi all,

I'm only trying to get 17.2Mhz out of a 3.44Mhz source and
am thus far
failing spectrally. I've tried everything I can think of
so far to
no avail.

Any suggestions, please. I'm stumped! :(
--

The BBC: blah de blah, yawn.

Certainly. Next time you want to put your foot in your mouth
don't remove it from the end of your leg. Then you won't
be.........

Otherwise 17MHz sits in the range of a 74HC4046 PLL. I
shouldn't suggest such things lest you start asking other
questions..... but.

DNA

The Hyperbolic Cosine is pronounced Cosh.
The Hyperbolic Sine is pronounced Shine.
The Hyperbolic Tangent is pronounced Than with a soft Th.

At least that's the way I've been doing it for the last 55 years.
==========================
Have you ever noticed no one sits next to you at meetings?
==========================

I always thought it was due to B.O.

But we live and learn!

On Fri, 12 Mar 2004 15:32:23 +0000, Ian Bell wrote:

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.

But if you want to filter the 5th, it's mighty handy not to have nuch
4th or 6th around.

Is a Smith Chart how you map an "RF circle"?

John

Paul Burridge wrote in message . ..
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.

Fifth harmonic frequency multipliers do exist, but it's usually much
easier to double and triple your way to the final frequency if possible.
(You just discovered this, I think!)

The lack of even harmonics is typical of push-pull stages ... if you
are messing around with CMOS gates, you might try using a TTL gate
(which pulls low much stronger than it pulls high) or an open collector
TTL gate, both with smmallish (100-200 ohm) pull-up resistors for
doubling.

Why not do a x3 followed by a x2 to get 17.2 MHz out of 2.866 MHz?

Tim.

On Sat, 13 Mar 2004 11:50:13 +0000, Paul Burridge
wrote:

On Sat, 13 Mar 2004 10:00:52 +1000, Tony wrote:

The 5th harmonic should be only 14dB below the fundamental, although it will
drop fairly quickly as the sides of the input square wave deviate from vertical.

Does the 3.44MHz have a 50% duty cycle?

Not quite, no. Why would that make any difference?

As the duty cycle deviates from 50%, the even harmonics start to
appear, so you need a better filter to keep them out.

John