I first purchased one EV-ADF5356SD1Z evaluation board, which showed off spec performance by generating 3rd harmonic with a higher amplitude than that of 1st harmonic, i.e. about 2 orders of magnitude off the specs (-14 dBc for 3rd harmonic), and revealed a spectral breakdown to odd harmonics of approximately equal amplitude: 1st, 3rd, 5th, 7th, and so on. I mainly tested at the minimal frequencies from 53 MHz to about 1 GHz and traced the outputs with a 50 Ohm and 1 MOhm switchable inputs of a 200 MHz bandwidth oscilloscope, and traced higher harmonics with a 3 GHz limited spectrum analyzer. The free outputs (RFoutB and the unused RFoutA) are terminated with 50 Ohm. I only occasionally see correct frequency at the outputs, and NEVER see a single harmonic generated, i.e. a visually pure sine! Moreover, suspecting a damage, I bought another identical board from another vendor and observed exactly the same behavior. I only used the SDP-S/USB2.0 and the v1.4.4 evaluation software, along with the 6.0V DC power supply, and proper grounding. I also do cautiously use the board to avoid static electricity or touching conductors. I checked a variety of settings and tried to terminate SMA REF inputs with 50 Ohm stubs too. Nothing looks right: neither the actual frequency (it does not coincide with the set value), nor the spectrum which is supposed to be single line. Can you please advice, what can be done in this case?
Increase the size of the choke inductors connected to RFoutA+/- to 100 nH, and the behavior will more closely match that on the data sheet.
Thank you for your reply, icollins.
The board seems to generate a wrong frequency deviant from what I set in the software. This is one problem. The other problem is that the spectrum is already contaminated, and if I filter out higher harmonics after the signal is irradiated from an output (keep in mind that MW wavelengths are just on the scale of a centimeter in the middle of the operating range of this board), it already reduces the power per fundamental mode, and does not change the oscillator regime.
The specifications are taken from the synthesizer chip ADF5356, not from the board specifications. The board, basically, does not smoothly utilize the advantages of the synthesizer chip.
Please, correct me, if I'm wrong.
What frequency range are you trying to generate?If you need to generate anything below 1 GHz, you need to increase the size of the RF choke inductor connected to the RFoutA+/- pins to Vrf. By default it is 7.4 nH, this is good for frequencies between 1 - 7 GHz, but it is not good for anything below 1 GHz. It acts as a high p[ass filter, which we do not want here.Remove 7.4 NH, and install 100 NH. The power level at the desired frequency will increase. Harmonics may require filtering, but the absolute power level will be ~+5 dBm.