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

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.

Spectrum analyzer output

RF connection

  • 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. 



  • Hi,

    Thank you for answer my puzzle. It quite makes sense. I will insert a suitable RF filter to exclude the potential cross modulation in the subsequent mixer.

    Again, thank you for your kind reminder.