High harmonics on amplifier output

I have a 868 MHz input signal of around 4dBm with 2nd harmonic of -60dBm and 3rd of -80dBm thanks to a SAW filter. At the output of the ADL5320 I get around 20dBm on my base frequency as expected, but the 2nd harmonic jumps to -20dBm and 3rd to -30dBm - that's 40-50 dB gain for the harmonics. Increasing the voltage from 5V to the maximally allowed 6.5V decreases the harmonics by around 5-7dB. Signal well attenuated at the input of the spectrum analyzer, so not a distortion of the gear. Any advice on what am I possibly missing?

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    •  Analog Employees 
    on Sep 30, 2016 5:59 PM

    Is ADL5320 placed at the output of the saw filter? If that is the case, then 2nd harmonic level is increased from -60 to -20 dBm. 40 dB gain.

    ADL5320 typically 17 dB gain and 25 dBm p1dB at 868 MHz. So, 40-17=23 dB of unexpected amplifications. We have not tested the harmonic levels of ADL5320.

    You are probably operating at the nonlinear stage of the amplifier close to P1dB, so the harmonics are amplified more than expected. And when you increase the Vdd, the P1db point also increase this causes harmonics to decrease.

    Did you try to test the ADL5320 alone without the filter with the same input power level of your scenario? I suggest testing that, and if the statement above is verified, then I would replace the output amplifier with an amplifier with higher P1dB.

    Moving the SAW filter to the output of the amplifier will also help.



  • Yes, the amplifier stands after the SAW, there is only the matching LC in between. A SAW can not be used on the output since they are good only up to 10dBm. I tried a higher P1db amplifier and it helped somewhat. Tried to modify matching circuitry for both input and output but didn't help much. Any idea what else can excite high harmonic content?

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    •  Analog Employees 
    on Oct 5, 2016 1:51 PM

    I would expect both even and odd order harmonics will increase close to Psat. Changing bias conditions or matching may help with the levels, but at the end harmonics will be higher at Psatthan they are at small signal. Using higher P1dB amplifier or filter with higher power handling may help.