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4046 PLL question

Discussion in 'Electronic Design' started by Robert Latest, Aug 1, 2005.

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  1. A short question on the "Type 2" phase detector: On page 648 of AoE it
    says that this detector has a gain of VDD/(2*pi) which I find plausible
    because the lowpass output goes from 0V to VDD as the phase difference
    of the input signal goes from -pi to +pi.

    However, both the ON semi and the Philips datasheets say that
    Kp=VDD/(4*pi). I double-checked and made sure that all three sources
    talk about the 4046's output pin # 13.

    Why the discrepancy?

    robert
     
  2. Genome

    Genome Guest

    Yes.... I once wondered about something like that but couldn't be arsed.

    However I do believe there is a discrepency somewhere and someone else
    should sort their bum out.

    DNA
     
  3. Fred Bloggs

    Fred Bloggs Guest

    AoE is obviously referring the 4046 PC3 phase comparator, and the
    datasheets are referring to the PC2 phase comparator. The PC3 is a
    toggle FF, and the PC2 is a dual FF phase/frequency comparator.
    Your lack of comprehension. These datasheets are full of
    self-explanatory graphics, equations , and write-ups.
     
  4. Fred Bloggs

    Fred Bloggs Guest


    Just a bunch of nitpicking nonsense...
     
  5. On Mon, 01 Aug 2005 12:58:59 GMT,
    No. They refer to the same pin number (13), and Fig. 9 in the Philips
    data sheet corresponds to Fig. 9.69 in AoE. They clearly both refer to
    what Philips calls PC2. Fig. 11 (Philips) shows the output of PC3, which
    is not what AoE talks about.

    According to Philips, PC2's gain is Vcc/4pi, but AoE says it's Vcc/2pi.
    I'm trying to rectify that.

    robert
     
  6. Tam/WB2TT

    Tam/WB2TT Guest

    You don't say what family 4046 you are using. The HC/HCT parts are quite
    different, depending on manufacturer. Just look at the max frequency to get
    a clew. I think the Moto (ON), and the Philips were the most alike.

    Tam

    Tam
     
  7. Jim Thompson

    Jim Thompson Guest

    The output of an edge-matching phase detector is Vcc at +2*pi

    The output of an edge-matching phase detector is 0 (zero) at -2*pi

    So Vcc/(4*pi) is correct, and AoE is incorrect ;-)

    (I am co-inventor of the first edge-matching device, MC4046)

    ...Jim Thompson
     
  8. Tim Shoppa

    Tim Shoppa Guest

    VDD/(2*pi) vs VdDD/(4*pi)
    I think this is addressed in Appendix B of TI's app note on the 4046,
    SCHA003B is the document name.

    They say that before lock, when the correction is only in one direction
    to slew the frequency, that the Vcc/(2pi) is the gain. But after lock,
    when the corrections are small pulses and not always in the same
    direction, and the correction is made from roughly (Vcc/2), then
    Vcc/(4pi) is more appropriate.

    Tim.
     
  9. Jim Thompson

    Jim Thompson Guest

    On Mon, 01 Aug 2005 10:29:17 -0700, Jim Thompson

    [snip]
    Wrong number, make that MC4044.

    ...Jim Thompson
     
  10. Jim Thompson

    Jim Thompson Guest

    Sounds like something written by an apps engineer without a clue.

    ...Jim Thompson
     
  11. Yup. I had found out by now, too. It's not really that difficult;
    I was just confused.

    robert
     
  12. Tim Wescott

    Tim Wescott Guest

    Keep in mind that pin 13 (whatever your particular data sheet calls that
    phase detector) is high impedance most of the time in lock, and only low
    impedance while the thing is detecting phase mismatch. The thing is
    designed to connect like this:

    |
    |
    to VCO|--------------.
    | ___ |
    pin 13|------|___|---o
    | |
    | |
    | ---
    ---
    |
    |
    ===
    GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    (note that what I'm saying is still valid for more complex loop
    filters). The charge pulse that gets injected into the capacitor is
    dependent on the voltage across the capacitor as well as the phase error
    -- so if the cap is sitting at 1/5 VCC then it will have a 4:1
    difference in the response to a phase error in one direction as it will
    in the other. You need to take this into account when you design your
    loop filter, usually by including lots of extra gain margin.
     
  13. Jim Thompson

    Jim Thompson Guest

    The best embodiment is that the phase detector output is a current
    source/sink.

    Or work the 4046 "switches" thru a resistor into a virtual ground.

    ...Jim Thompson
     
  14. Tim Wescott

    Tim Wescott Guest

    The better PLL chips have some magic circuitry* to make them look like
    controlled-current sources when they're active, rather than hard shorts
    to ground or VCC. To get the same effect with a 4046 you'd have to work
    into an op-amp with a virtual ground, and you'd have to take the
    op-amp's noise into account in your circuit design.

    * that's what you do, Jim -- you put the magic into the little black
    bugs; I just use them.
     
  15. Andy

    Andy Guest

    Andy asks Jim :

    Were you working with Jon deLaune at the time ?

    Andy
     
  16. Jim Thompson

    Jim Thompson Guest

    I can't remember when I didn't use current source/sink for PD outputs.

    Yes I can, the original MC4044 used R's into a fed back Darlington.

    ...Jim Thompson
     
  17. Jim Thompson

    Jim Thompson Guest

    I remember the name, but he was in another group... maybe
    applications?

    I was in analog research/design at Motorola.

    Do you know Jon from somewhere?

    ...Jim Thompson
     
  18. Tim Wescott

    Tim Wescott Guest

    If there's anything missing from the modern line up of tiny-logic chips
    it's a 4046-style phase detector (or better yet one with controlled
    current outputs) it the World's Smallest Package. My dream chip would
    have six pins: VSS, VDD, in1, in2, phase out and current program (which
    could be tied high to emulate the 4046 phase detector). This would all
    come in a SOT-23 package or a flip-chip.

    At the moment if you want just a phase detector you have to use a 4046
    with a disabled oscillator and use up extra space not only for the chip
    but (possibly) for the aforementioned virtual-ground op amp integrator.
     
  19. Andy

    Andy Guest

    Andy writes:

    I remember seeing his name on the patent, and I think it was for the
    MC4044.
    I just went to the uspto.gov site, but they only go back to 1976, so I
    can't
    verify it just now.....
    I definitely saw his name on the Motorola app notes for the 4044 , way
    back in
    the early 70's . ( Funny, the stuff that sticks in your mind
    (grin) )

    Whoever did it, the 4044 was a fine piece of work that was a major
    breakthrough
    in the development of frequency control...
    I was designing frequency synthesizers at the time for Bendix
    Avionics, then later
    for Texas Instruments. I used the 4044 in a dozen products, then I
    started building
    my own with two D's and a reset gate ( same 4044 circuit using D's
    instead of
    individual gates --- gave me better control over the dead zone).....
    Later I expanded it into a quad of D's which gave slightly better pump
    action when the frequencies
    were widely separated ( Principles of Three State Phase Detection -
    EDN- Sharpe ).

    But I have always credited the 4044 development with the same sort
    of
    genius as the light bulb, sliced bread, and peanut butter with
    chocolate......

    Before the 4044, we were using slip counters for frequency/phase
    comparison
    ( Fairchild applications note dealing with avionics - John Nichols
    )
    and they were a pain in the ass to analyze. The 4044, especially
    when used
    with an op amp, made the analysis very straightforward..... Later
    versions in
    CMOS (4046) had more pump voltage and lower leakage. I couldn't get
    a hold
    of the dead zone problem , and leakage, until I started building them
    out of
    separate high speed flipflops.........

    Just reminiscing, and the mention of your involvement with the 4044
    yanked my
    crank enough to respond....

    Andy Sharpe
     
  20. Andy

    Andy Guest

    Andy responds to Tim:

    I would prefer separate outputs for the phase detector Pump up and
    pump down, (which I could tie
    together if desired ). And a pin for a lock detector, rudimentary
    tho it would
    have to be......

    I Andy Sharpe, retired, PE,
    etc........
     
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