I’m trying to use AD9361 as analog FM receiver and evaluating its characteristics.
In my evaluation, I found an abnormal behavior of DC offset in AD9361 when enabling DC offset tracking calibration and I think the calibration parameter need to be optimized from default condition.
The issue is that, when AD9361 receives FM modulated signal and the baseband spectrum is crossing over DC region (direct conversion), the received IQ signal is modulated with unwanted low frequency offset (see attached slides).
If there is relatively large DC offset in baseband signal, the demodulated FM signal is much degraded. In my measurement, the demodulated FM signal has too many and too high level harmonics around desired signal.
When moving the center frequency of the baseband signal away from DC region, for example add 12kHz (a half of signal BW) offset to LO frequency, the demodulated FM signal condition is much improved.
When disabling tracking calibrations, however, static DC offset is remained in baseband signal, so the disabling tracking calibrations are not effective for this issue.
Please advise how to correct this tracking calibration behavior.
The detail of signal condition in my evaluation is as below.
Transmitter side (Signal Generator)
Carrier Frequency (LO): 470MHz
FM signal: 1kHz single tone
FM deviation: +/- 5kHz
Receiver side (AD9361)
LO frequency: 470MHz + offset frequency if needed
RF LPF: 0.43MHz
Output Sampling rate: 0.96MSPS
Gain: Manual, 48dB
BB/RF DC offset and QEC are enabled