NASA PM Tx back on Apollo frequency, link double-locked (Patreon)

There are a lot of good looking vintage green lights in the lab tonight. We have just successfully returned the NASA PM transmitter to its original Apollo frequency. Our Apollo transponder now locks onto our NASA test PM transmitter uplink, and our PM receiver phase locks on the spacecraft transponder downlink. Solid double locked PM S-Band link, as in the good old days.

I previously posted how we retuned the receiver after we got our custom ordered crystals from Laptech. For the receiver, we followed the recently found manual, which described the alignment procedure. But no such luck for the magnificent Tx. No manual. And it has about 25 adjustments.

Without the manual, we first performed an exhaustive characterization of the transmitter with its current crystal. The goal would be to retune it to the same values with the new crystal.

We then changed the crystal, and worked our way up module per module, from the 22 MHz VCO all the way up to the 2.1 GHz output. Our hearts skipped a beat when the VCO refused to oscillate with the new crystal. But after a few tries, Mike eventually found the adjustment that got it going.  The prior Rx tuning had given us a good idea of what to expect once we had a signal. The adjustment for the VCO sweep (500Hz/V) was the most challenging, as we were not sure which adjustment did what, and they all interacted. But we got there eventually by trial and error.

We had to build an extender for the x24 multiplier, so we could access the resonating cavity multiplier adjustments on top. But after all this work, it appeared that it had been left untouched, and was still tuned for the original Apollo frequency! We were quite relieved when we saw a solid +10 dBm at the required 2.10640625 GHz Apollo frequency, more than it was putting out before.

The final touch was to get the phase modulator properly tuned. Fortunately, we had been able to characterize it thanks to a modern Keysight Spectrum Analyzer on loan from our new sponsor, Electro-Rent. Actually, we got a lot of Keysight gear from Electro-Rent, more on that in upcoming episodes. And there it is, our phase modulated carrier, finely tuned to1 radian/volt.

Finally, we setup our retuned transmitter and receiver for a grand transmission experiment, and turned them both on. We then fired up the Apollo PM #1 transponder. It settled into free running mode at first, but after a sweep from the transmitter, it dutifully captured and locked onto it. After lock, we returned our transmitter to the nominal center frequency, dragging the transponder return frequency along, and almost immediately, our receiver locked on the transponder. They are both sending and receiving at the correct Apollo frequencies again. 

This is a big milestone for the project, this had been 9 month in the making. Mike and I congratulated each other. Now, on to populating our microwave link with subcarriers, voice and data!

Marc and team