ADAR1000 Gain Setting

I am having a problem with a phased array board equipped with ADAR1000 and ADTR1107, which will not stop oscillating unless the front-end gain is reduced by at least 15dB from the maximum. The reason for the oscillation is that the layout design of the board is very poor and the isolation between the RFIO and PA output is not achieved, but could you please advise me on the gain setting of the ADAR1000.The oscillation stops when the 15dB ATT is used and the VGA gain is set to MAX (set value = 127), but oscillation occurs when the VGA gain is lowered by 15dB (set value = 13) without using the 15dB ATT. Is there any possible cause for this ,or should I not use the VGA with extremely low gain?

  • 0
    •  Analog Employees 
    on Jan 21, 2021 5:37 PM 1 month ago

    The oscillation seems to have less to do with gain and more with the attenuator being applied in the path. 

    • Does oscillation happen on all four Tx channels of the single ADAR1000?  i.e. across all four ADTR1107's?  
    • Have you tried changing the power level you apply to RF_IO?
      • You might be overdriving a subcircuit in the ADAR1000 with too large a signal
      • Putting the 15 dB attenutaor in the path would decrease the signal coming from the RF_IO pin to the TX VGA subcircuit by 15 dB
  • At maximum gain, oscillation occurs in all channels. As the gain is lowered, oscillation tends to remain in channels 2 and 4.
    The RFIO input level is about -12 dBm, so I don't think it is over-input. The oscillation occurs even when there is no input.Many of the devices oscillate around 9-10GHz, where the front-end gain is high, and when the ADAR1000 phase setting is turned 360°, the oscillation frequency varies from 800MHz to 1GHz.

  • +1
    •  Analog Employees 
    on Jan 29, 2021 6:07 PM 26 days ago in reply to okuho3190

    This sounds like there isn't enough isolation on your PCB layout.  You may have to revise your board making sure to isolate the RF paths as much as possible.