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sinara-hw / Phaser

Quad channel 1GS/s RF generator card with dual IQ upconverter and dual 5MS/s ADC and FPGA in EEM form factor
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TRF LO output level varies when loading second output #138

Closed FabianSchwartau closed 3 years ago

FabianSchwartau commented 3 years ago

Hi everyone, I noticed a strange behaviour of the LO output levels of the TRF: When I connect a 50 Ohm load to the LO OUT N of the first TRF (J14B), the amplitude of the internal LO connection (connected to LO OUT P) varies significantly (something like 15 dB!). Interestingly the amplitude of P is higher, when the N output is loaded. I am not 100% sure where this is coming from. It may be, that the differential output does not like being loaded differently, I could not find any information in the datasheet about this. I have to mention, that I replaced the 10 dB attenuator with a 0 Ohm resistor, according to the discussion in https://github.com/sinara-hw/Phaser/issues/136. I will change this back shortly, and see if that makes any difference. Any idea why the board is behaving like this?

gkasprow commented 3 years ago

are you using AC or DC connection? I mean before or after the coupling cap ?

FabianSchwartau commented 3 years ago

I measured at the input of the the 10 dB attenuator (or my 0 Ohm resistor). Putting the attenuator back in place reduces the effect to ~6 dB, but that is still a problem. I noticed, that TI put a 100 Ohm resistor between N and P in the eval board, that is the only difference I can see.

FabianSchwartau commented 3 years ago

I am an idiot and I am sorry. I had a wrong power value set in my RF generator all the time, the LO outputs work as expected. Once everything is setup properly I measure roughly -2 dBm under all circumstances.