The Apollo PM receiver is back on its original frequency!

We finally received the crystals from Laptech in Canada. Frequency is right on the money (new one at left, the old one at the "wrong" frequency at right). They were spec'ed at +/- 10 ppm and we got -1.8 ppm!

The realignment of the VCO is an "engine out" procedure, with the VCO on the bench in a test harness we had to build.

There are many, many adjustments, which are somewhat interdependent. However the VCO started to oscillate right away with the  new crystal, close to the expected frequency. We easily fine tuned it to a perfect 23.307292 MHz. That gave us confidence that we had indeed ordered the correct crystal.

We followed the alignment procedures from the manual, which had a couple errors on it, but after a bit of head scratching we were able to rectify as we went.

We slowly made our way through it. You've got to get the VCO right, then the buffer amps, then the x2 multiplier, then the x3 multiplier, mostly simple power peaking adjustments. None of the adjustments were very far off. We eventually got to about 19.5 dBm out at 139 MHz at the output of the first string of multipliers, which is 1.9 dB away from max spec. Then we got the tuning range adjusted to 600 Hz/V +/-5%. That took us a while, as the tuning rate and the center frequency controls interacted. 

We finally went on to align the x16 step recovery diode multiplier. It has three resonant tuned microwave cavities, which are pure RF magic. They are the three resonant golden posts you see to the right. You adjust the cavities with a big screw at the top. There is a fourth adjustment for the step diode impedance matching circuitry, the white cap to the top left.

At first, we could hardly see any microwave signal coming out. It was very low, in the -50 dBm range, which got me a bit nervous. But we climbed our way up by realigning the cavities, all the way up to the 0 dBm maximum spec output. The thing is super resonant and will start to self-oscillate when you get close to maximum, so you have to watch it on the spectrum analyzer and back off until the side spurs are 50 dB below the main output.

Hurray, the VCO is smack back at 2.2375 GHz, 0 dBm out, and can tune the required 100 kHz in either direction. This should put our receiver LO frequency back to its glorious Apollo frequency days, receiving our command module transponder PM downlink at 2.2875 GHz (due to the 50 MHz first IF offset).

The complete alignment took us all of 5 nerve wracking hours! Now we have to go through the transmitter, which has even more adjustments, and for which we have no manual. This will be more challenging, as we will need to thoroughly characterize it in its current state, then try to reproduce the performance with our new crystal by trial and error.

I'll do the video once we have both going and can lock the whole link with original ground equipment (hopefully!).

Marc