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Reverse-engineering the AIDC-3733 VCO


I don't think the VCO is in the 8-lead DIP; I suspect that the VCO is
the pile of SMDs and striplines in the middle of the board.  The mixer's
pattern is fairly obvious at the opposite end of the board, though (it
looks something like an active rat-race mixer).

OK, taking this as a working hypothesis, and looking at the PLL/mixer
board, let's see now... OK, the 8-lead DIP is probably the divider.  The
"R24" SMD is probably the VCO oscillator transistor (I noticed that the
same device is used in the IF amplifier), and the "1Pz" SMD device is
probably the oscillator's varactor diode.  The "415" SMD device is
probably a buffer amplifier (probably some sort of MMIC, judging from
the extreme lack of other components around it).

The "510" is the key part number fragment on the divider chip... the
"96XX" number is, as you say, the date code, judging from all of the
other parts.

Interesting that the varactor appears to be connected to the emitter of
the "R24" part, and not to the base (I'm guessing here on the names). 
The VCO tank circuit seems to be the squiggly stripline inductor
terminated in the capacitor and resistor.  At first blush, I'd be
thinking about changing the value of that capacitor at the end of the
VCO inductor to bring the PLL back into lock.  Wish I had a way of
accurately measuring the value of low-value chip capacitors.

Now, how/where to measure the varactor control voltage...  AH-HAH!!! A
big circular pad connected to the DC feed to the varactor!  OK, now I
have something to go on.  Remember... the DC component of the phase
detector output (aka the vco control voltage) can only be somewhere in
the range of, roughly, 1 to 4.5 volts, since the phase detector chip
runs off of 5 volts.  So, after trying two different crystals and
applying my trusty Fluke 75 to the aforementioned pad, I see that the
stock 8.898438 MHz crystal produces a VCO lock voltage of roughly 2.7
volts (funny, right down the middle, huh?).  Now, if I replace it with
my 8.828125 MHz crystal, it measures only 1.8 volts.  Wow, that's a
fairly large variation for a (relatively?) small frequency change.  No
wonder you lost lock at a reference frequency of 9+ MHz.  At that
frequency, the VCO control voltage would be well in excess of 5 volts,
asssuming a linear relationship.

Suggestions:  I suggested changing the capacitor earlier, but it might
be easier to short a hairpin loop on the VCO inductor (only a part of
one loop) than to find a different capacitor.  Both approaches are
touchy, but can be done.

It's strange, but the divider chip looks like it has two different pad
sets, to accommodate two different parts.  Do you think TranSystem was
hedging its bet against parts availability here?

OK, that's the limit of my intelligence here.  Someone else can chime in


Mahlon - K4OQ

John Harrington wrote:
> Hi, Mahlon.  The idea of using a Drake LO as a signal source is a good
> one, IF the VCO will go to 2400MHz.  I had the same idea several weeks
> ago, and ordered a 9.375 MHz crystal from ICM, intending to use a 3733
> LO.  While waiting for the crystal, I 'sacrificed' a 3733 by removing
> the LO and mixer board.  I cut off the mixer microstripline circuit
> and attached an SMA connector at the LO output point, just past a little
> pi-net attenuator circuit.  I was delighted to find a +10 dBm output
> there, with all spurious signals 30 dB down and hundreds of MHz away!
> This puppy is CLEAN.  Originally, I intended to use the LNA from the
> same sacrificed 3733 as a PA, but when I saw 10 mw WITHOUT an amplifier,
> I figured to just pass the output through a pipe-cap cavity and use it
> as-is as a beacon.  I already have some little PIC microprocessors
> programed with my call as a beacon controller.  The PIC FSK's the freq.
> a bit for the CWID.
> Then, the crystal for 2400 MHz came.  I plugged it in and got 10mw
> output,
> but couldn't find the signal at 2400MHz.  Looking at the spectrum
> analyzer,
> there it was at around 2300MHz!  After some head scratching, including
> measuring the freq of the crystal (it was right on) I determined that
> the
> VCO simply wouldn't go to 2400MHz.  The crystal would pull it upwards
> but
> it hit it's upper limit at about 2300MHz.  I hope that there are some
> VCO upper/lower limit parts which can be tweaked to get it up there, but
> don't know yet.  My immediate problem is that I can't identify the VCO
> part.  The IC next to it is a 4046A phase-comparator, a 16-pin SM
> device.
> The part which must be the VCO and divide-by-256 is a little 8-pin SM
> package (tiny!) with the markings '510',  '9612',  and 'M53' on it, but
> no manufacturers' logo.  I think the '9612' is the code date, and the
> '510'
> and 'M53' identify it.  Can anyone point me to a manufacturer or data
> sheet?
> If I can't find enough information to get the VCO to phase-lock at
> 2400MHz,
> all is not lost.  I still have a nice +10 dBm 2256 MHz phase-locked LO
> for my next upconverter.   The +10 dBm is perfect for the Mini-circuits
> level 7 double-balanced mixers.
> If you folks can help me identify the VCO and get it to 2400MHz, I
> promise
> to take the time to build a web page with the data necessary to build a
> S-band beacon with CWID, including source code for the PIC
> microprocessor.
> You can modify the source code with your call and grid or other
> information
> for automatic one-minute CW ID requirements.
> Best regards,  John  W5EME
> <snip>
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