AFE7071: Low pass filter when using LMX2572LP for LO with AFE7071

JMT82

Part Number: AFE7071
Other Parts Discussed in Thread: LMX2572LP, AFE7070, LMX2572

In some of your app notes, TI recommends using the LMX2572LP to generate the LO input to the AFE7070/AFE7071. The output from the LMX2572LP is more or less a square wave. Is a low pass filter needed to eliminate the harmonics? If a low pass filter is recommended, how much should the higher harmonics be suppressed for good operation?

over 2 years ago

0 RJ Hopper over 2 years ago

TI__Mastermind 18095 points

Yes, a harmonic LO filter is needed to maintain good performance. The App Note AFE7070 Optimized Operation in the VHF Band outlines performance with the AFE707x with the LMX2572 LO synthesizer. For that design, a Mini-Circuits MC-LPCN-180 low pass filter was used on the LO path. The impact of the LO harmonics on quadrature modulators is discussed in this app note: LO Harmonic Effect of I/Q Modulator Sideband Suppression. It is primarily the 3rd order harmonic that causes an issue with sideband suppression. With the AFE7070 + LMX2572 operating in the VHS band, I targeted a 38 dB suppression of the 3rd order LO harmonic to mitigate any degradation impact from the LO harmonics.

--RJH

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AFE7071: Low pass filter when using LMX2572LP for LO with AFE7071