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TRF3705: How to achieve high SFDR:Sideband suppression and harmonics

Part Number: TRF3705
Other Parts Discussed in Thread: DAC38J84

DAC38J84 + TRF3705

My design is almost a copy of EVM:

DAC38J84 + LPF550MHz + TRF3705,DAC output is quite clean:

with LO = 800MHz,TRF3705 output:

calibrate DC_offset,LO feedthrough can be suppressed to about -67 dBc,but 2xLO did not drop,and LO-Fin/LO+2Fin/LO-2Fin can not be further suppressed.Those are spurs for Fin,I expect a spur suppression of -65dBc,is that possile??and how??

  • Min:

    You have correctly achieved LO suppression by adjusting the DC offset balance on the input baseband signal.

    LO-Fin, or sideband suppression, is not impacted by DC offset. It is impacted by amplitude and phase balance between I/Q baseband input. You can adjust these parameters with the QMC of the DAC38J84 to suppress the sideband. Also note there is some dependence on the input LO harmonic content. Check out App Note:

    The 2xLO is not impacted by DC offset. There is no way to "suppress" this component other than analog filtering.

    The LO +/- 2Fin is the BB 2nd order harmonic. There is no way to suppress these components. You can back-off the input signal into the modulator to get some relief from these spurs. Typically the LO-2Fin is the strongest spur and is also the farthest away from the desired signal so some analog filtering is possible depending on the low IF frequency chosen.

  • Hi,RJH
    thanks for your reply.
    should i use a BPF before the LO input of TRF3705??

  • Yes, that is advisable to suppress the LO harmonics in order to maintain good sideband suppression. --RJH