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Setting a different observation RX Frequency to TX Frequency on AD9371

Question asked by rbbris on Jun 12, 2018
Latest reply on Aug 6, 2018 by gverma

We have an application where we transmit on TX1, the signal goes through the PA and is looped back to the observation receiver ORX1 for monitoring. Normally we require the TX and ORX frequencies to be the same. This is achieved by setting

  • ad9371-phy.out_altvoltage1_TX_LO_frequency = TxFreq
  • ad9371-phy.in_voltage2_rf_port_select = ORX1_TX_LO

This works fine.

We now have an application where the observation frequency needs to be different to the transmit frequency. I thought that this could be achieved by setting:

  • ad9371-phy.out_altvoltage1_TX_LO_frequency = TxFreq
  • ad9371-phy.out_altvoltage2_RX_SN_LO_frequency = RxFreq
  • ad9371-phy.in_voltage2_rf_port_select = ORX1_SN_LO

However the signal I see on the observation port is now always shifted down in frequency by about 37.5 MHz. It does not matter what RX_SN_LO frequency I choose. Also if I change the transmit frequency, the actual transmit frequency does actually change but the observation signal remains at 37.5 MHz offset suggesting that the observation frequency is locked to the transmit frequency.

Although I first found this issue in our own application, I get the same result when using your IIO Oscilloscope application.

What is going on?