In our system 2 Rx channels are downconverted from RF frequency to 3 GHz IF frequency signals that enter the AD9361 device.
When 56 MHz BW is defined in this AD9361 device, the Rx signal spectrum analysis looks well like in the following picture (1 MHz sine signal is injected to the RF part):
When 16 MHz BW is defined in this AD9361 device, the Rx signal spectrum analysis looks strange like in the following picture (same 1 MHz sine signal is injected to the RF part):
What can be the reason for such strange signal shape in case of using BW that is smaller than 56 MHz?
How are you changing the BW of AD9361?
Please use the filter wizard tool to generate your custom profiles.
We change BW by using the AD9361 Rx bandwidth value.
We did not use FIR in this test.
On other side, when We activate the FIR, the signal and noise values are being increased by few dB's for some reason. Do you know the source for that?
Please refer to the below post,
The half-bands are not perfectly flat over the passband which will cause inband attenuation. The FIR's main job is to provide a maximally flat passband. Using the filter wizard will design an FIR to compensate for this droop and setup the front-end analog filter correctly.
We observe the following issues:
1. When We define BW=16 MHz and also activate the relevant FIR, spectrum looks flat but the signal and noise are up by ~10 dB
2. When We define BW=3 MHz and FIR=4 MHz We see the following picture:
3. When We define BW=28 MHz and no FIR We see the following picture:
4. Another issue is the observed noise floor. Since in our spectral analysis, the recorded signal is processed with resolution BW (RBW) filter of 1 KHz (as can be observed on the pictures above), AD9361 Rx gain=40 dB,Noise floor in operating frequency (2.9 GHz) is about 3 dB, the expected observed noise floor should have been: -144 dBm+Rx gain + Noise floor that is about -100 dBm. However We observe about -75 dBm noise floor, as can be observed above.
What can be the reasons for the:
1. FIR noise and signal levels increase?
2. Noise floor value much higher than expected?
Thanks in advance.
Have you tested your receiver response by feeding signal from the signal generator?
Did you compare the results with our eval board? i.e Did you load your custom profile and tested your requirement using our eval board?
Can please share the schematic section of AD9361 along with your custom filter files?