curiousmarc
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HP 8568 RF Analyzer: Restoration within a restoration
🕑 Added 2021-11-23 20:22:11 +0000 UTCIn previous posts, we explained that will need to retune our Apollo PM test receiver to the original Apollo CSM frequency, because its frequency had been changed to support another project, one of our best guesses being the Viking launch in 1974.
Fortunately we found the manual a month ago, and it describes the involved procedure in detail. For that, we'll need a RF analyzer. I was going to use my HP 8568, but when I turned it on, it had this interesting modern art "deconstructed" screen.
It's pretty but not particularly useful, and in real time the elements float around the screen making it unreadable. Seems like we have a restoration within a restoration on our hands.
Fortunately, the fault was rather simple, just a bad supply filtering cap on the +100V rail that feeds the X/Y deflection amplifier last stages.
Here it is all beautiful again, with the faulty cap in front
But before we can retune our VCO, we need a new, very special crystal.
It took the VNA to figure out what the original crystal was, and it's a 3rd overtone one with 15 pF loading, with frequency specified to 1ppm, down to the Hertz (!). The VCO is at 23MHz and hardly tunes (+/1 kHz), which is why you need such a tight spec, but after multiplication it yields the +/- 100kHz tuning range needed for acquisition under all Doppler conditions.
We found a very knowledgeable company in Canada that can make it, it's on order, 7-8 weeks lead time!
Marc
Comments
I was just watching your latest Apollo comms video. Did you know that you can bodge a Spectrum Analyzer to do FM demodulation itself? You need one with an analog output that represents the power at the centre frequency. Then, you set the SA so that the centre is near the edge of the FM signal. As the frequcy of the carrier changes the power at centre of the analyzer will go up and down, effectively doing a crude demodulation.
CuriousMarc
Hehe. I have the 8444A option 059 to go with it, of course. Captured in video when I was a baby YouTuber: https://youtu.be/_yZOW_aABlk
