Verifying that all components are working properly enables me to run the first functional test of the MW transmitter - frequency synthesis. In the repo, there is "Basic" example that configures MAX2871, attenuation, filter and ADC for RSSI detectors (even though the latter ones do not work, presumably).
The configuration is set as follows:
- reference frequency (XTAL input): 19.2 MHz
- phase detector: R = 1, DBR = 0, RDIV2 = 0, meaning fPFD = 19.2MHz
- integer mode with N = 120, resulting fVCO = 19.2*120 = 2304 MHz
- DIVA = 0b101, meaning that fRFOUT = fVCO/32 = 72 MHz.
- RF power 5dBm.
I intentionally set the DIVA so high so that I can measure the output frequency with my RTL-SDR dongle. With 5 dBm there should be no problem detecting some signal.
The "Basic" example makes user choose register content readout and turning on/off the RFOUT.
However, things didn't go as planed. Good things first: there was some output, and I could control it (turn it on/off) with my program. Bad things: The signal was very low, way below the expected 5dBm level. And it was also very wideband and filled with spurious content. Here's an example screenshot from RTL-SDR that runs "spectrum analyzer" program.
I cannot say that the basic functionality test has been passed, so the further investigations and debugging must take place. I'm just not sure where to start looking from. Is the MAX chip configured poorly (it shouldn't be, I used configuration from datasheet example) or do I get some terrible damping from my circuit?