EMRFD Message Archive 2571
Message Date From Subject 2571 2009-01-04 01:34:07 w4zcb77 MMIC's (Reasonably long) We live in a marvelous time! I haven't noticed mention of these
wonderful little pills in a bit and thought I'd remedy the problem.
Despite Rick's mention at one time that "We have gone from way too
difficult to way too easy to generate microwave RF.", these devices
have provided us with tools we couldn't have dreamed of only a few
years ago. I have made two part oscillators out of them, an MMIC and
a crystal from front to back does it. (Make that 4 part, I forgot
you need to bypass the bias resistor, although NEC makes some with a
separate bias rsistor built in)
At any rate, I have a pair of HP 8640 signal generators in the shop,
used for almost everything I do. If you don't have at least one of
these space age oscillators, you ought to go buy one at the next
hamfest. Although they initially cost thousands for their gold
plated innards, you can find them at rediculous prices from a lot of
sources as folks move on to automated and software controlled test
gear. Their loss, and your advantage, HP never made one with better
characteristics, and you NEVER have too much test equipment!
Neither of mine had the 0.5-1GHz option installed, but with careful
searching at every hamfest, I managed to turn up an external option
doubler for the top band. Nothing but a diode doubler, you drive it
with all that the 8640 will put out (+20 dBm) and get a bit less
than 0 dBm of 0.5-1 GHz out. (HP kindly gives a bandswitch position
for that and doubles the frequency readout as well!) That served
well for a long time, and some MMIC gain chips made it useable by
turning my 432-C power detector into something that would read
reliably WAY below -30 dBm. Tuned up alot of cavities with the
combo.I DID yearn however, for the full +20 dBm out of the generator
at the top end of the spectrum and cured my limitation yesterday.
My first transmitter back in 1950, used 5 6V6 GT's. a crystal
oscillator that doubled a 40 meter rock to 20, a pair of the same
tubes as a push-push doubler to 10 Meters, and another pair driven
by a carbon mike as modulators. Great rig, I worked my first DX with
it, parked on top of an overpass to catch a KZ5 in the canal zone.
Never looked back.
Push-Push! A doubler! Just what I needed for my 8640's. Laid out a
little PC board, exactly the size of one of those State flag stamps
the post office is selling. Etched it and stuffed it. A T1-6T MCL
transformer to drive a pair of MSA-1105 MMICs in push-pull, (Similar
modern chip would be the MAV-11, but these were 20 for a buck),
separate bias resistors for the devices and paralleled the outputs
on the output side of the coupling caps. Pair of SMA's and done.
A little 12 Volts and I had +20 dBm out for a lot less than +20 dBm
in. Pretty doggone flat across 0.5-1.1 GHz. One board resonance at
about 600 MHz, but only a dB's worth. Fundamental is over 50 dB
down, another minor miracle courtesy of the MMIC.