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3rd harmonic of image

Question asked by on Jul 30, 2018
Latest reply on Jul 31, 2018 by Vinod

For both TX and RX, it seems that the 3rd Harmonic of the image appears to often be the strongest spur.  Is this expected?  Is there any way to improve the performance (lower the power) of this spur?


Also, I have a very odd thing I am seeing with the eval board, TES sw, and the ZC706 board.

I am looking at RX 2 and I have the LO set to 2450 MHz.   I am injecting a signal from an E8257D PSG.  As I walk that signal from above 2450 to below 2450, or vice versa I see some odd behavior.  Firstly the amplitude of the signal drops right at 2450.  I attribute this to the corrections.  The really odd behavior is when I come off of 2450, to say 2440, then the image spur (+10 MHz on the TES gui) is very strong, maybe -50 dBfs with a carrier at -4 dBfs.  If I keep walking the input down in 10 MHz steps, the image spur stays strong up until the point the carrier is at 2410.  The next step to 2400 also happens to be the jump where that 3rd harmonic off the image goes out of band.  At 2400 the image (at +50 MHz) drops down to below -90 dBfs.  Now for the really weird part: as I walk the carrier back up towards 2450, the image stays below -90 dBfs!!!  I see the strong 3rd harmonic of the image come back in band, but the image itself stays low.  That stays true as long as I keep the carrier from going all the way back to 2450.  If I bring the carrier back to 2450, and then back down to 2440, the strong image spur (-50 dBfs) returns, when just a moment ago it was below -90 dBfs!


If I offset my signal generator step, so it never lands directly on 2450 (i.e. jump from 2445 to 2455) then I never see the strong image spurs (unless they were already there, and then they go away if I tune far enough from center, and they never come back unless I land the carrier right on 0 MHz.  If I jump over 0 MHz then the image level stays low).