I am using a custom board with three ADRV9009 devices. I have started doing performance evaluation of the RX inputs of the ADRV9009 devices. Testing includes Single Tone SFDR testing. Basically a signal generator is used (followed by an appropriate low pass / bandpass filter) to inject a signal into one of the RX inputs of the ADRV9009. The sample data is captured and the FFT is plotted. The SFDR is measured in dBc as the signal level of the input signal minus the signal level of the highest spurious component.
I am getting pretty good results at 942.5MHz and 1842MHz, in the region of 80dBc. However, there is a noticeable reduction of performance as my input frequency goes DOWN. At 79.55MHz input signal (150MHz LO frequency), I am only seeing around 60dBc.
Below is the FFT plot of such a test. The input signal is at 79.55MHz and the LO frequency is 150MHz. The level of the input signal is roughly -1dBFS. The sample rate is 208MSPS and the bandwidth is approximately 169MHz.
The numbers above the spurs indicate how they move with frequency relative to the input signal. So for example, a 1:1 spur moves by 1MHz for each 1MHz that the input signal moves, a 2:1 spur moves by 2MHz for each 1MHz that the input signal moves and so on.
I thought initially that it may be related to my custom hardware so I repeated the tests on the ADRV9009 evaluation board. However, I am seeing similar performance there.
I am aware of the limitations mentioned elsewhere about tuning the LO too low. That is why I chose an LO frequency of 150MHz.
I am performing initial calibrations (with the input terminated in 50ohm terminator) and I have enabled continuous calibration correction.
1) I know this question has been asked elsewhere but can you please give an indication of the expected ADRV9009 performance at frequencies close to 75MHz. According to RFSim, the performance of the ADRV9009 should be pretty consistent across input frequency but that does not happen with real hardware (both my custom hardware and the ADRV9009 evaluation board).
2) Can you please provide guidelines on how I can go about improving the low frequency performance of the ADRV9009. Is there any optimisation that I can do to get better results at lower frequencies?