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HMC3716 inputs

The HMC3716 data sheet shows differential inputs with a 50 ohm differential load. Also, it shows the NREF and NVCO ports being tied to the internal bias networks.

Must the NREF and NVCO ports be bypassed, or can the REF/NREF amd VCO/NVCO inputs be driven as true differential inputs? Thank you.

Doug

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  • Hi Doug,

    The REF / NREF and VCO / NVCO ports can be driven differentially but as you noted they are matched to a 50 ohm differential load internally so depending on the impedance your driving the HMC3716 with you may want to provide some external matching. 

    Best Regards, 

    Marty 

  • Since the REF and NVCO pins are internally connected through two forward biased diodes to ground, I don't see how they can be considered differential inputs. I can see the correct waveform at NREF pin, but as expected see nothing but DC on the REF pin when driven from an AC coupled balun transformer. Seems like the part is essentially biased for single ended operation. That might explain why I had so much trouble getting the PFD to operate at all. I am going to try single ended operation of the NREF and VCO pins on the next go around.

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  • Since the REF and NVCO pins are internally connected through two forward biased diodes to ground, I don't see how they can be considered differential inputs. I can see the correct waveform at NREF pin, but as expected see nothing but DC on the REF pin when driven from an AC coupled balun transformer. Seems like the part is essentially biased for single ended operation. That might explain why I had so much trouble getting the PFD to operate at all. I am going to try single ended operation of the NREF and VCO pins on the next go around.

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