Hi,
I have the eval boards of the HMC394 and the HMC705 for use in prototyping the development of a PLL/VCO. Overall, I have been able to build this prototype and could get the frequencies I need locked to the REF. But the spectrum of my signal has spurs and their harmonics.
Troubleshooting for this led to finding that the spurs are due to the prescalers HMC394 and HMC705. I used the HMC394 in a translation loop to divide by 6 the output of a mixer from 150-960MHz down to 25-150MHz. And I as well used the HMC705 to divide 6GHz down to 1GHz for use in some other part of my prototype.
By measuring the spectrum of the outputs of each of the HMC394 and HMC705, I can observe spurs at around +/-320 to 370kHz in each part. And these spurs are what appear on my PLL output at same offset frequency.
I ensured to use LDOs to power the eval boards and have ruled out power line noise contributions. I am ruling out noise addition from the loop filter also, since these spurs are measured in the 'isolated' eval boards of the prescalers/dividers. There were no spurs in the input signals to these dividers, as they were generated from a signal generator as a CW tone.
The images of the spectrum are as follows:
1. Spectrum of the HMC705:
Fin = 6GHz @5dBm, N = 6, meas_Fout = 1GHz
2. Spectrum of the HMC394:
Fin = 900MHz @5dBm, N = 6, meas_Fout = 150MHz
3. Spectrum of the HMC394:
Fin = 150MHz @10dBm, N = 6, meas_Fout = 25MHz
4. Image below shows the output of my PLL, with the effects of the spurs +/-320kHz offset from the 11.76GHz carrier.
I look forward to more troubleshooting tips. And if this is a know issue with these parts, kindly clarify. If possible suggest solutions or alternatives.