RadioBanter - Extracting the 5th Harmonic

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Paul Burridge wrote in message . ..

Exactly how much power do you need?

Only enough to feed another inverter gate.

Egad, Paul! You've been wasting this much net bandwidth just to drive
another HC gate?? All you need is a filter/matching circuit that
steps up the voltage. This is DOG SIMPLE! See below.

Exactly how "clean" (free from
other harmonics) must it be?

Preferably filthy. It's another multiplier (this time only 3X, thank
God!)

Then you need a clean enough input that you'll get the desired output
purity. "Filthy" is likely NOT the right answer and will just get you
into further trouble. But fortunately, "clean" is simple, and "really
clean" isn't at all difficult.

Try this: square wave output -- I don't recall your exact freq; I
used 3.7MHz -- from HC gate, feeds 4.58pF capacitor (make at least
that one tuneable). Other end of cap feeds 20uH inductor, Qu=200.
Other end of that inductor connects to next gate input, and net 18.6pF
of capacitance to ground: say 15pF cap plus 3.6pF of gate input
capacitance. For DC bias, gate input to ground = 22kohms; gate input
to Vcc = 47kohms. That keeps the gate in a valid logic state when
there's no excitation. Assuming the gate's RF input resistance at
18MHz is at least 2.5kohms, you should get a voltage gain at the fifth
harmonic of about 15dB, which will be ample to drive the gate input.
The available current from the filter is low enough that the gate's
input protection diodes should clamp things nicely at the rails. Be
sure to use a gate that has input protection, or else add
low-capacitance, fast diodes externally. Gain at the third and
seventh is down 20dB or so from that. If it needs to be cleaner than
that, you can add a second resonator.

The gate biasing suggested may result in an output duty cycle
significantly different from 50%. If you will always have 3.7MHz
drive, you can bias the input more in the center of its range, or even
rearrange the circuit a bit and use a feedback resistor from output to
input to set the DC bias. The gate's input impedance is then much
lower, but you don't need much voltage to drive it. Don't use that
trick with a Schmitt trigger input, though.

69 turns of 36AWG (0.125mm) wire, spaced 2 wire diameters c-c, on an
0.375" former, should give you about 20uH at Qu=200 and first parallel
SRF about 50MHz, but you should be able to make it more compact using
something like a T-50-2 powdered iron core.

BTW, I just built the suggested circuit, on one of those old white
plug-boards which is NOT a great idea at RF. It worked fine. Easy to
tune just with a scope. Performance was as I expected: full -0.6V to
+5.6V or so swing at 18MHz at the input of the "18MHz" gate, and an
18MHz rectangular wave at its output. 74HC04 hex inverter; 3 stages
as a simple RC oscillator to generate the 3.7MHz input. Duty cycle of
that 3.7MHz was fairly close to 50%, but definitely not right on. The
performance tells me that the 'HC04 really does have a fairly high
input impedance at 18MHz, and requires under a milliwatt to
drive--perhaps well under. There's AMPLE fifth harmonic power in that
3.7MHz square wave out of an 'HC04, and you just need to filter and do
whatever impedance transformation is appropriate to get the desired
result.

Incomplete filtering of the non-fifth harmonics causes variation in
the output pulse width (in a cycle that repeats every cycle of the
3.7MHz input), and frankly if I wanted a pretty pure 15th harmonic at
the final output, I'd use a bit better filter to get a cleaner fifth.
But try the simple one first, to convince yourself that it's possible.
It's a pretty "hack" circuit, in that I just made educated guesses at
the 'HC04 input and output impedances at 18MHz, and at a reasonable
load resistance for an 'HC04. It could probably be optimized a bit,
but it DOES work as described.

Inductor Qu: The loaded Q of the circuit I described depends on the
output impedance of an 'HC04 inverter, and the input impedance of the
same with the bias resistors included. It should be running somewhere
around 20-25, I believe. That determines how well the third and
seventh are rejected. The unloaded Q of the inductor determines how
much fifth-harmonic power is lost in the filter. You can stand some
loss, but you don't want to waste too much. If the loaded Q is 25 and
the inductor has an unloaded Q of 50, then you will waste half the
power in the inductor. That probably would still work, but it would
be better if the Q was more like 100, as a minimum. Based on what
I've been hearing so far, I'm not very confident that you will be able
to tell the Qu of your junque-box inductors, especially since you need
to know it at the operating frequency. Qu of a lot of manufactured
inductors is quite low, especially at non-optimal frequencies. If you
picked a ~20uH inductor designed for use in a switching power supply
at 100kHz, it could well be awful at 18MHz. It only takes a couple
minutes to wind up an air-core one, so that's what I'd suggest. The
one I used was 49 turns on a T-94-2 powdered iron core.

Cheers,
Tom

Paul Burridge wrote in message . ..
On 18 Mar 2004 10:16:53 -0800, (Tom Bruhns) wrote:

Paul Burridge wrote in message . ..

Exactly how much power do you need?

Only enough to feed another inverter gate.

Egad, Paul! You've been wasting this much net bandwidth just to drive
another HC gate?? All you need is a filter/matching circuit that
steps up the voltage. This is DOG SIMPLE! See below.

Thanks, Tom. I'll give your suggestion a try. Just one question: is
the unloaded Q of 200 you specify critical? I probably have a 20uH
factory-made inductor laying about in my parts stash but won't know
it's Q without measuring it. Need I bother?

I don't think you can get away
with short cuts at RF, especially if you're a non-expert.

3 MHz is practically DC. Why don't you ask one of your friends at the BBC to
build it for you.

W4ZCB

On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson"
wrote:

I don't think you can get away
with short cuts at RF, especially if you're a non-expert.

3 MHz is practically DC. Why don't you ask one of your friends at the BBC to
build it for you.

I was told 40Mhz is "practically DC" too. I guess it depends on where
you're coming from.
Actually I've dumped the factory inductor as suggested by Tom and
wound-up a large, air core job on 15mm plastic water pipe. It's made
a big difference. I'm happy to report I've now got the 5th! Could be a
little cleaner but who cares? Tom made a big deal out of the
importance of high-Q so it was the obvious thing to try. Fortunately,
it's worked. God knows how I'm going to squeeze this monster coil onto
the board, though! :-|
Can I infer from this experience that SMD inductors of over a few uH
are a waste of time?
--

The BBC: Licensed at public expense to spread lies.