From: neufeld@helios.physics.utoronto.ca (Christopher Neufeld)
Subject: Frequency generation at 400-1000 MHz
Organization: University of Toronto Physics/Astronomy/CITA
Date: Wed, 8 Apr 1992 17:31:55 GMT

   I've got a problem with the electronics for my thesis experiment, and
even our excellent local sources of help are at somewhat of a loss to
provide easy solutions.
   My experiment uses a continuous wave unit oscillator, a General Radio
Company 1209-C, tunable over 250-960 MHz at a power level of about 100mW.
At least, it did until recently. The oscillator is behaving badly now.
The power level is down to about +3dbm and doesn't work at all below
about 750 MHz. At startup it jumps to almost +10dbm, then rapidly decays
to its +3 equilibrium value on a time scale of tens of seconds. Sometimes
it doesn't come up at all, and I have to switch on and off the 300V
supply several times before it starts putting out the RF.
   So, the circuit diagram is very straightforward, having just a bunch
of capacitors, inductors, resistors, and a single tube. The output is
inductively coupled into the circuit with a loop of wire which just sits
hanging in space near the oscillator innards. There's very little which
can go wrong with it, I've checked the power supply when loaded, and it's
not fluctuating a bit. That more or less leaves the tube. The capacitors
are all mica, except for one 450V electrolytic which looks like a filter
on the power supply line, so I doubt it's a capacitor problem. The unit
is not designed for easy diagnostics, but the time variance of the output
suggests strongly that the tube is bad.
   I faxed to General Radio (which became Genrad, which became BCS
Electronics) and they haven't any idea what I can do. The unit was
manufactured in 1964 and they haven't made tubes for it in over 25 years.
They know of no tube equivalent, and have no modern version, since they
don't make anything above 100kHz anymore.
   Well, that was a long statement of the problem, now the questions.
First, can anybody tell me if they know of a tube equivalent to the 6481
oscillator tube (General Radio part no. 8380-6481)? Somehow our local
electronics workshop has misplaced their tube lists.
   Assuming I can't find a replacement, I've been looking for a
replacement unit. I don't need output level control, don't care
particularly about bandwidth, and even flatness is not important since
I'm only using this as a mixing signal at a single frequency for any one
data run. I have a Hewlett Packard catalogue which will sell me something
like this for $20000, but with all the bells and whistles, output level
tunable to 0.1 db, frequency to seven decimal places, computer interface
port, and similar nonsense. I just want something with a front-panel knob
I can twist while watching a scope screen for maximum amplitude on the
de-heterodyned signal. I don't care if it swings around by 5 db every 50
MHz, or if I have to stick it through a manual variable attenuator to set
the level to that required by my mixer, just so long as it resonates.
   Our local electronics shop has suggested that such a cheap and dirty
device could be cobbled together for about $20 in parts. I would get a
UHF tuner box from a television, sold at a local surplus store for $8,
and play with it until I can figure out how to tune the local oscillator
inside. The oscillator has to be continuously tunable since I'm
restricted in the frequencies I can use in my experiment by a resonant
transducer elsewhere in the system. I'm told that the television tuner
will have that capacity. Then I am to feed that oscillator into a Mini
Circuits (tm) amplifier to deliver a power level useful to my mixer
(somewhere around +5 to +10 dbm at least).
   Does anybody have a better idea? How about experience with this sort
of hack? Surely somebody's dissected a UHF tuner before.
   Any help? My experiment's idle until I can get the local oscillator
back on line.


-- 
 Christopher Neufeld....Just a graduate student  | alien: n. a being who
 neufeld@helios.physics.utoronto.ca    Ad astra  | travels great distances
 cneufeld@terranet.cts.com                       | to molest our cattle
 "Don't edit reality for the sake of simplicity" | and trample our grain