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I have a simple question:

If the frequencie of the input and the LO are diferent, how will the output voltage be? Is the one port sent out a votage meant the difference and the other sent out the phase difference? or what?


  • Hi there,

    The output of NU & ND function identically however, when the delta in frequency between the VCO and REF is great, only one output is active at a time. Refer to the last plot in the datasheet comparing LD, NU and ND where the VCO frequency is being held 11Hz higher than the REF. This is why ND appears to be constant and only NU is varying. If the VCO frequency were 11Hz below the REF frequency, the traces for ND & NU would be reversed in this plot.  Notice that the period of the saw tooth voltage waveform is 90.9mS which is the period we'd expect for an 11Hz difference in frequency. As the difference between the VCO & REF deceases and the delta approaches a limit of 0hz for the first time ( Note that 0hz is really never achieved for any real length of time as the VCO is always moving) the phase detector output pulses become very narrow and the duty cycle reduces to something much less than 50%. Now that they are close enough to declare "lock" the VCO frequency may exceed the reference frequency in one instant and in the next instant be less than the REF so both outputs will be active as they constantly work to maintain phase lock. This region is called the "dead zone" and results in increased spurious in fractional PLL's. Fortunately modern fractional PLL's can force the VCO to operate just slightly to one side of the dead zone which minimizes the fractional spurs.

    hope this helps,


  • Dear Marty

    Thanks for your kindly answer. And i finally understand the use for the PFD.

    Is there any device can detect the difference between the LO and the RF? 

    Or can you give me some porduct from your company to achieve frequency tuning.

    Thanks a lot for your help!


    Jingliang Liu

  • Hi Jingliang,

    The Phase Frequency Detector is a special "mixer" designed for synthesizer applications and using it as you would normally use a mixer isn't recommended as there can be some hysteresis. You can use a mixer if you just need to compare 2 frequencies.

    Best Regards,