Why can I detect a large RFout/2 signal from RFout port, whose power isn't negligible

Question

Dear all, 
 I used ADF4113HV and HMC738/HMC739 to form an RF sensor. 
 A very large RFout/2 signal can be detected from RFout port. 
 Although I used 50 ohm impedance to absorb RFout/2 output. 
 Some details are shown in the photos. 
 The substrate is FR-4, and the ground is complete. 
 The relative power of RFout to RFout/2 is around 17 dB. 
 The thru-holes were added densely. 
 Is it possible to use a filter to exclude RFout/2 signal? Or the chip malfunctions itself? 
 
 P.S. I hope a enable pin could be adopted to shutdown the unused port. 
 
 Thank you.

kieranb · Accepted Answer

Hi, 
 With these VCO parts, we expect an RFOut/2 tone appearing in the output spectrum. On page 1 of the datasheet of the HMC739 we quote RFOut/2 to be -20dBc. On the RFOut/2 output of the HMC738 we quote -23dBc. 
 These are typical values so it is possible to have this worst case you are seeing of -17dBc. The reason for this is that these parts are designed with a fundamental frequency of the internal resonator to be RFout/2. It is multiplied internally to obtain the RFOut frequency you see. 
 Regards, 
 Kieran)

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Why can I detect a large RFout/2 signal from RFout port, whose power isn't negligible