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CD74HC4046A: Jittered free running VCO signal

József Hermann
Expert 1010 points
Part Number: CD74HC4046A
Other Parts Discussed in Thread: SN74LV4046A

Dear Forum,

I obsered excessive jitter at VCO_out, even if loop broken (at constant VCO control)

Jitter is ca 20 Deg peak-to peak.

Test conditions:

f0=5MHz, Ct=220pF, R2 is ca 5K (using a trimmer), temporary no R1, VCO_In connectod to R2 pin.

VDD bypassed by 10uF Ta, 100nF X5R.

In my design tuning current "injected" to R1 pin (sink current source). I also experience the same jitter when no this I1 current (loop broken - of course in this case f0 somewheat lower since total internal current for Ct is the sum of (amplifeied - x6) I1 and I2).

If I use much larger Ct say 10nF, the jitter cannot be observed. I need jitter free VCO signal in case of low Cts.

Kind regards,

Joseph

  • 0
    TI__Guru* 96897 points

    Hi Joseph,

    We have other variants of the '4046, the most recent of which is the SN74LV4046A, however we don't specify or guarantee jitter on any of these older VCOs.

    Our clocking team does offer low-jitter clocking solutions, however I believe all of those run at much higher frequencies than the '4046 that you're looking at. We may not have a solution for your application.

  • 0 in reply to Emrys Maier
    Expert 1010 points

    Hi Emrys,

    thank you for the suggest! Unfortunately, I cannot purchase the 'LV4046 momentary. Meanwhile the VCO signal has been jitter-free. I think one pad of the SMD timming capacitor was not perfectly soldered or caused by the timmer resistor (a used device).

    5MHz VCO (free running, Ct=100pF), note, using a twice of cap and smaller resistor the waveform will be more regular square wave.

    However, if loop closed the VCO again jittered but I think it is not by the VCO circuitry rather then improperly set loop gain/tuning range.   

    Joseph

  • +1 in reply to József Hermann
    TI__Guru* 96897 points

    When operating as a PLL with a first order filter, you'll probably see a some ripple at the input to the VCO, which could be the jitter you're referring to.  This can be improved by increasing the RC value (decreasing the RC corner frequency), or by using a higher order filter (ie active filter) to produce a smoother response at the VCO input.

  • 0 in reply to Emrys Maier
    Expert 1010 points

    Yes, I want to operate it as first order loop (without filter). However, I previously checked with an old RCA 74HC4046 - DIP16 and it was jitter-free when loop operated. However, now i use voltage-to current converter (differential pair + current source) and i think due to this circuit problem the loop is out-of lock range when loop closed. I can came back only with this on next week.

  • 0 in reply to Emrys Maier
    Expert 1010 points

    Dear Emrys, now there is no jitter when loop closed. Threre was a problem with the external circuitry between PC3/PC1 output and R1 current input (VCO control). If no frequency divider in the loop added capacitor (loop filter) not needed, however if there is, capacitor needed (loop filter>second order loop) otherwise jitter occurs at VCOout. Anyway, the 74HC4046 free runing VCO is totaly jitter free even a small 100pF Ct used.

  • 0 in reply to József Hermann
    TI__Guru* 96897 points

    I'm glad to hear it's working for you.