Something though is getting a tuning signal from the A2A1 board as well as osc-pwr. So there is a oscillator there somewhere and it gets tuned
I’m going to have to get a SMB connector as well as 10 pin extension cables made up before I can do some measurements. The boards fir the A2A2 are stacked tightly. I do have one of the thermo cams for my iPhone but need to see the components.
osc_pwr just turns the TG on/off
the TUNE voltage (pin 5) comes from a SPI DAC on A2A1 and is 0 to 12V. It must be for tuning a VCO on BITG so I think that dark cylinder is indeed a VCO creating 1384MHz and it is fine tuned by this TUNE signal perhaps during the calibration process for TG.
Is there a frequency calibration process for TG in this instrument?
It does seem there is no phase/freq lock mechanism on this TG board and it is just tuned by A2A1 and then it is let go. That VCO is probably very stable... there is no feedback from the BITG to A2A1 board to give information about frequency accuracy, so that TUNE voltage is set just once during some calibration routine i guess ... It seems the 600MHz is there just to make up the math 1.384-0.600=3.92/5
why didnt they make that VCO just to be 784MHz then?
there is also a PWR_LVL (pin 8 ) coming from another SPI DAC on A2A1 (0 to -11V) and I think that signal controls the HMC346 variable attenuator
can you trace that TUNE signal from the connector to a pin on the BITG RF board?