I am working on a VCO with a gain block for an FMCW radar. My schematic is shown in the 2nd pic. The output power of the 2.4 GHz VCO IC (MAX2750) is -3dBm. The gain block IC (QPA4563C) has a gain of 19.8 dB and OP1dB of 14.6 @ 2.4 GHz. This is my first active RF circuit, and I wasnt sure what running the gain block in saturation would do so I added a 3dB attenuator to the output of the VCO.
In my mind, this attenuated the VCO output to -6 dB. Then with 19.8 dB of gain from the amp, I wouldn't be running the amp in saturation (total output power of 13.8 dB -- maybe this would be considered compression?).
The part number of the attenuator is TFA20C03DBER. I used the footprint from the datasheet. When I first built the entire board last night, I got a total output power of -25 dBm! I was expecting around 12 dBm. After a few hours debugging today, I ended up removing the attenuator and just shorting the trace. This resulted in an output power of 12 dBm as expected. The spectrum without the 3dB attenuator is shown in the last slide (has an external 20dB attenuator on the analyzer).
Where did I go wrong with the 3dB attenuator? I tried another one also and flipped it around but no luck. The only thing that worked was removing it and shorting the trace.
Also for my spectrum analyzer output, why are all the peaks separated by 10 MHz? I tested phase noise and got what I expected from the VCO, just not sure if something like this is normal?