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AD9361 Simultaneous Tx/Rx Performance

Question asked by Tim14 on Feb 13, 2018
Latest reply on Feb 13, 2018 by Tim14

Tests were performed to evaluate the simultaneous transmit/receive performance of the AD9361 in FDD mode. The tests were conducted using the Analog Devices FMCOMMS3 and Xilinx ZCU102 development boards.

 

The tests were performed using two sets of the AD-FMCOMMS3 and ZCU102 boards. The first set of boards was configured to generate a complex tone signal using the internal AD9361 BIST generator. The complex tone was transferred to the Tx path and up-converted with the Tx LO tuned to 2.000 GHz. The RF Tx output was monitored using a spectrum analyzer. The SA displayed the desired signal and the leaked carrier signal which was approximately -50 dBc. No other harmonics or intermod products were observed above the SA noise floor.

 

The second set of boards was used to receive the RF signal. The Tx RF output port of the first set was connected to the Rx RF input port of the second set via a cable and fixed attenuation. The received signal (approximately -50 dBm) was digitized and transferred to the ZCU102 where the data was collected using a logic analyzer embedded in the programmable logic. The Rx LO was tuned to the same frequency as the Tx LO on the first set of boards, 2.000 GHz.  The AD9361 for each set of boards was configured using the reference design parameters downloaded from the AD website. The receiver AGC gain control was enabled.

 

The second set of boards was used to perform the simultaneous transmit/receive evaluation. The internal BIST generator was configured to generate a complex tone signal. And, the Tx LO was tuned to 2.030 GHz. So, The Tx signal and Rx signal were offset by 30 MHz.

 

The spectral response of the data collected when the second set of boards was configured for ‘Rx Only” (BIST disabled) appears relatively noise free (see Figure 1). The spectral plot displays the desired signal as well as the down-converted carrier signal, the image signal, and a third order harmonic. A fifth signal, which has not been identified is also observable. It is noted that the down-converted carrier signal is offset from DC due to the frequency difference between the Tx LO and Rx LO signals. The I & Q channel time-domain responses (see Figure 2 for I-Channel data) displayed minimum amplitude distortion in the tone signals.

 

The spectral response of the data collected when the second set of boards was configured for “Simultaneous Tx & Rx” (BIST enabled) is much different (see Figure 3). Only the desired signal and the down-converted carrier signal are detectable. The spectral plot displays a significant band of energy located near the image frequency as well as a higher noise floor and lots of spurious signals. The I channel time-domain response (see Figure 4) displayed significant amplitude distortion in the tone signal. The Q channel time-domain response (not shown) displayed minimum amplitude distortion in the tone signal. It is noted that data has been collected in which significant amplitude distortion was observed in both the I and Q channel data.

 

It had been assumed the AD9361 could be operated in full-duplex mode. Can you explain why the collected data exhibits significant signal degradation while operating in the “Simultaneous Tx & Rx” mode? Have you seen this issue before? If yes, what is the cause and how can it be resolved?

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