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CD74HC4046A: Phase Comparator 2 Output (PC2OUT) issue

Renan Santos Adriano
Mastermind 26095 points

Part Number: CD74HC4046A

Hello Experts,

We encounter an issue while testing of the PLL IC CD74HC4046A. We are getting the expected output of Phase Comparator 1 Output and Phase Comparator 3 Output as shown below images, but for Phase Comparator 2 Output we are not getting the output as described in datasheet.

For SIGIN and COMPIN are in phase and For SIGIN is lagging than COMPIN then getting 5V (VCC) output at PC2OUT. 

For SIGIN is leading than COMPIN then getting 0V output at PC2OUT. As described in datasheet, not getting the pulses at the PC2OUT.

So I want to know the reason of the same, Kindly explain the PC2OUT behavior for 100khz to 400Khz of signal frequency.

Attachment.zip 

Regards,

Mansukh Prajapati

  • 0
    TI__Genius 16800 points

    Hi, 

    Phase comparator 2 acts as an up-down counter where a SIGIN rising edge causes an up-count and a COMPIN rising edge causes a down-count. Based on the state of the counter, the PC2 output can either be low, high-impedance (off), or high. 

    Looking at the waveform in PC2out_lagging, it seems like PC2 output is around 5V so it appears to be stuck high. However it's difficult to say when the PC2 output is off when measuring with a high-impedance probe. Depending on the probe impedance and parasitic capacitance from the PCB traces, the time for the voltage to discharge from 5V to some mid-level voltage may be much larger compared to the period of COMPIN and SIGIN, and also much longer than the pulses when the output is on (I would expect the 'high' pulses to be around 200ns based on the attached waveform). Similarly for the PC2out_leading case, the PC2 output seems to be stuck low, but in reality it may be high-impedance most of the time and the narrow 'on' pulses don't provide enough time to charge the output to mid-level. 

    If the goal here is to verify the behavior of the phase detectors, I would recommend verifying that the measurement setup can accurately measure the 3 possible states of the output. One idea is to add a pullup/pulldown resistor to bias the output to VCC/2 when the output is high-impedance. I imagine something in the 1k-10k Ohm range would be suitable for the pullup/pulldown resistors. That way if the phase comparator output is off, the output should quickly charge/discharge to the mid-level voltage. 

    Regards, 

    Connor