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Spurs of Reference Clock and its Harmonics in the Modulator output spectrum of ADRF6755 device. Need help on how to suppress them?

Question asked by anindya on Aug 21, 2015
Latest reply on Sep 7, 2015 by anindya

Hello,

 

We are using ADRF6755 device to modulate an OFDM baseband signal. The reference clock used is 32Mhz and we are trying to generate the output centred around 368Mhz. We see a lot of spurs in the modulated RF out from ADRF6755 device. These spurs seem to be generated at harmonics of 32Mhz (which is the reference clock). The spurs are significantly higher than what is mentioned in the specification. We have made several captures which i am attaching here. The ADRF6755 has a fractional PLL inside along with a Sigma-Delta Modulator. I am not sure about the source of these spurs.

 

I am attaching several images to show the issue which we are facing.

(1) ModON_chargeP5ma_1 ==> Modulator on and Charge pump current at 5mA, spur at 384Mhz

(2) ModON_chargeP5ma_2 ==> Modulator on and Charge pump current at 5mA, spur at 352Mhz

(3) ModON_chargeP325ua ==> Modulator on and Charge pump current at 325uA, spur at 384Mhz

(4) ModON_chargeP325uA_3 ==> Modulator on and Charge pump current at 325uA, spur at 352Mhz.

(5) PLLON_ModOff ==> We see these spurs even when the modulator is off and PLL/VCO is on.

 

Would appreciate some quick help/discussion on these aspects.

 

-best regards

Anindya Saha

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