RadioBanter - View Single Post

March 17th 04, 12:03 PM

On 16 Mar 2004 16:54:06 -0800, (Tom Bruhns) wrote:

FWIW, just now I had not trouble at all filtering the fifth out of a
square wave, in exactly the way I suggest below. It seems very silly
to me to put nonlinear elements in when you already have plenty of the
harmonic you want.

What leads you to believe I have enough 5th harmonic in *my*
particular case? Are you satisfied with the square wave I posted? Does
it look up to the job?

I don't know what logic family you're using, but
if it's a modern one like 74AC, you should have several milliwatts of
fifth harmonic available.

Currently 74HC but I've got some ACs (shouldn't need anything special
for this relatively low frequency target, I'd have thought).

A simple series resonant LC from the logic
output to the base of a 2N2219-type transistor should get you at least
a couple mA RMS of fifth harmonic base current, assuming a
grounded-emitter stage with roughly 50 ohms input resistance.

Should do, shouldn't it.

Make
the loaded Q of the LC something around 10 to 20, and you won't screw
up the amplifier with other harmonics or the fundamental. Be a bit
careful about the coil you use, because it will be fairly high
inductance for the frequency you're interested in...that is, keep the
unloaded Q and the self-resonant frequency high enough. Use another
moderate-Q tank in the collector circuit; you should be able to get
over 100mW of fifth, with other harmonics down 40dB or more. You can
get more complicated with the filtering if it's necessary, but for
fixed-frequency operation, there's nothing wrong with simple
synchronous tuning of single resonators set to reasonably high Q. If
you insist on using the unnecessary complication of a multi-pole
bandpass filter, be sure the one from the square wave to the amplifier
starts with a series resonator, not a shunt resonator.

I'm going to have a bash at this this afternoon. Not strictly
according to your suggestions at this stage, but I just want to know
if there's a sniff of 5th about at all.

Sheesh...all you need to do is selectively amplify the harmonic you
want; it's already there. Don't add complexity trying to generate
something you already have in abundance.

Still waiting to see evidence of that "abundance" - no sign so far.
--

The BBC: Licensed at public expense to spread lies.