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AFE7071: Cannot suppress sideband

JMT82
Prodigy 70 points
Part Number: AFE7071
Other Parts Discussed in Thread: LMX2572LP

Hello,

I am working with your AFE7071.  As has been recommended elsewhere on this forum, I am using the LMX2572LP as the LO input to the AFE7071.   I have two boards that I am attempting to calibrate.  I am using the procedure documented in SLWU029D.

The first board achieves sideband suppression just fine with the phase and gain adjustments.  With the first board, I can easily get the sideband suppression down into the -70dBm noise floor of the spectrum analyzer that is being used, with most any input power, 0 dBm to +3 dBm or more.

On the second board, with 3.8dBm in on the LO, by adjusting the gain and phase, I can only get the sideband about 40dB below the modulated output.  If I reduce the LO input power to 0dBm, I can easily get the sideband suppression into the -70dBm noise floor.  If I go up in frequency a little bit, I can fully suppress the sideband to the -70dB noise floor as well. 

There is just one particular combination of frequency and input power on one particular board that is prevents the sideband from being supressed, and I am confused as to what might cause this.

Thank you

  • 0
    TI__Mastermind 18095 points

    JMT:

    In theory, you should always be able to tune gain/phase and suppress the image into the noise floor.  In practice, there can be issues where you run out of adjustment range.

    There are a few things that impact SBS performance.  Operating at the lower part of the frequency band translates to operating on the edge of the quadrature polyphase filter in the LO path which requires higher tuning values.  LO drive power impacts this performance (and LO suppression as well).  Driving the LO to high power tends tightens up the required correction range.  This is because the limiters in the polyphase circuit  are driven harder and will be better balanced.  LO harmonics also impact the SBS performance, and the impact shifts based on input LO power.

    My guess is that the LO harmonics are the major issue.  I recommend inserting a BPF in the LO path to suppress the HD2 and HD3 components.  You can also drive the LO at a higher level.  Don't be too concerned about the D/S limits in this case; the modulator easily handles 8-10 dBm LO power.  Do be careful of the impact to LO suppression when driving the LO harder.

    Check out these resources for more info: https://www.ti.com/lit/pdf/slaa519 and https://www.ti.com/lit/pdf/sloa313. 

    --RJH