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74HCT9046 VCO phase noise?

Discussion in 'Electronic Design' started by Andy F Z, May 6, 2007.

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  1. Andy F Z

    Andy F Z Guest


    I wonder what is the lowest phase noise that can be obtained from the
    74HCT9046's VCO, running at f_0=16MHz. For instance, the relative VCO
    signal power at f_0+-100Hz, +-500Hz, +-1000Hz.

    Thank you for you help.

  2. Tom Bruhns

    Tom Bruhns Guest

    Are you asking about the raw VCO performance, or the VCO in a locked

  3. Andy F Z

    Andy F Z Guest

    The raw performance. But if the data for a particular, well tuned locked
    loop is available - that would be useful as well.


  4. MooseFET

    MooseFET Guest

    IIRC: If you consider the noise to be a voltage source at the control
    input, you can say that there is a 100nV/sqrt(Hz) wide band noise and
    a 500Hz 1/F corner. The values will vary quite a lot from maker to

    Assuming the control voltage location for the noise source, suggests
    that the noise is in terms of frequency noise and not phase noise. At
    some highish frequency, actual phase noise from within the VCO will
    start to show at the output.

    Monitoring the output with a counter that samples at a 10Hz rate, I
    have seen variations under 10PPM in the frequency.

    When you close the loop, the phase margin of the system matters a lot
    to the amount of noise you see. If you use the self bias in the the
    phase detector's input amplifier, you take a fairly major noise hit.
    You are better to use an external comparitor and feed the device with
    a sharp square wave. Doing this I have seen frequency noise levels
    under 1 part in a billion per sqrt(Hz). My measurement was done at
    about 2MHz.
  5. Andy F Z

    Andy F Z Guest

    MooseFET wrote:
    What point in the input control circuit do you mean? The ratio
    dVCO_Frequency/dVCO_IN of the 74HCT9046A depends on the R1C1 product.
    I have not come across other makers of this chip than NXP/Philips.

    In my prototype, the output signal density is essentially spread in a
    few kiloHertz area around the center frequency, even with a very very
    overdamped control loop, I tend to conclude it's the noise within the VCO.
    Thanks you for the information!
  6. MooseFET

    MooseFET Guest

    It is a variation on the 4046, 74HC4046. The exact part may not be
    made by many people but lots of things very like it are.

    The signal on pin 9 is the control input to the VCO. This is where I
    referenced my noise measurement.

    I find this result a little unexpected. What was the gain crossover
    frequency of you loop? Near the carrier, the VCO's output noise
    should not be what effects the resulting noise. The VCO's noise is
    within a servo loop. If you model the VCO's output as a voltage
    instead of a phase and think about your average op-amp circuit, I
    think you will see what I mean.
  7. Andy F Z

    Andy F Z Guest

    Oh, yes, I realise that the VCO part must be the same in the 74HCT9046.
    In fact, I have a big N divider in the loop, the reference frequency is
    about 1 kHz, so the servo loop must not be able to reduce the noise in a
    few kHz vicinity of the carrier. I have now taken a look at the VCO's
    signal with the loop disconnected, its near carrier spectrum appears to
    be pretty the same as with the loop, and too bad for my purpose...

    Thank you.
  8. Winfield

    Winfield Guest

    You're using a well-bypassed SMD part? Bypassed with both ceramic
    and low-esr electrolytics? The VCOs in the '4046 PLLs are excessively
    sensitive to high-frequency supply ringing (e.g., from two ceramic
    resonating with the supply-trace inductance, which I've been able to
    observe by careful scope observations), and the '9046 VCOs are no
    doubt much the same.
  9. MooseFET

    MooseFET Guest

    I still find it strange. Have you looked at the supply voltage with
    the scope? Maybe there is ripple on it.
  10. Andy F Z

    Andy F Z Guest

    Bypassed by ceramic caps, and with an inductance in the power rail... I
    will try an electrolytics. In the meantime, I have tried a 1u film, but
    it has had no effect.
    I will try this chip in an optimized testbed, the current layout does
    not have a solid ground plane, for instance.

    Thank you.
  11. Andy F Z

    Andy F Z Guest

    No, no obvious ripple on the power rail. But it looks like this VCO is
    extremely sensitive to the supply voltage/rail (at least when pushed to
    the limits: at 15-16 MHz). I will try it in an optimized, "quiet"
    testbed, maybe with a local voltage regulator.
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