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VCO basics

Discussion in 'Electronic Design' started by [email protected], Nov 12, 2006.

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  1. Guest

    I'm using an ICS AC9173-01 to recover a video dot clock. The datasheet
    offers no insights as to what's in there and what kind of power supply
    it wants. I have 5mV p-p noise as measured across the device's power
    pins. Now that's pretty good for a purely digital supply, but I think
    it's not good for a VCO that can cover 10-75MHz off a 5V supply.

    Supposing that a 0-5V swing covers 65MHz (guessing that the VCO uses
    0-5V), that means 1mV corresponds to a 13KHz variation. So the 5mV
    noise could modulate the VCO by +\- 33KHz from the center frequency.

    Anyone have any clues how a VCO is made inside an IC and how sensitive
    they are to power supply noise? I'm thinking of regulating a local 5V
    just for this guy, what's a good linear regulator for the job? LT1963
    comes to mind, any others?

    BTW, ICS/IDT is the worst company I've ever dealt with to try and get
    the application notes they suggest you read in the datasheet...
     
  2. Jim Thompson

    Jim Thompson Guest

    Can you provide links? I can't even find a data sheet.

    ...Jim Thompson
     
  3. Bob

    Bob Guest

    ICS used to be very good before IDT took them over. It was easy to contact
    people that REALLY knew the product. Good luck, now.

    It can be very important to keep the VCO's supply super clean. It's
    difficult to know what a given part's supply rejection is, so I always
    include extra filtering.

    The LT1963 is a superb low-noise LDO. It's one of the ones that Xilinx
    recommends for their FPGA serdes supplies. It may be overkill, however.

    I've had good luck, with sensitive ICS (pll-based) clock multipliers chips,
    by simply using a resistor between the supply and the chip and a parallel
    cap (chip side to gnd, of course). You have to keep the R small enough so as
    not to violate the chip's VDD min, and keep the C large enough to keep the
    noise low at low frequencies. Be sure to add smaller parallel caps so the
    noise filtering is effective even at the higher frequencies (hundreds of
    MHzs).

    We had one design that used an ICS8xxxx part. Its output jitter was terrible
    (way over spec) even though we used a series ferrite bead and small parallel
    cap. We pulled out the ferrite bead and replaced it with a 4.7ohm R, and we
    increased the cap (10uF iirc). It solved the problem.

    Bob
     
  4. Guest

  5. Guest

    Thanks Bob, I was starting to think I was losing it. The data sheet
    calls out twice to read AB01 if I'm using Hsync below 25KHz... can't
    find AB01.

    This is for a hobby project, so I just sort of tossed things that
    looked good together. I can recover my dot clock, but my initial
    measurements with my sampling scope show me some pretty poor jitter
    performance. I mean I can see on my target LCD monitor the pixels
    flashing on and off as the scan doubler's setup and hold time is
    violated, the scope just brings it home.

    I'm gonna cut and jump a heavy ferrite/RLC filter on the power supply.
    Good thing I have a quiet analog scope otherwise I'd never see that 5mV
    pp noise on the supply.
     
  6. colin

    colin Guest

    Regulators are good for rejecting low frequency supply noise but not so good
    at high frequency,
    as Bob said a resistor followed by a capacitor or 2 is often a good idea,
    ferrite beads work at high frequency but you may find dont work at the
    frequency you want,
    choose R just high enough so it only cuases a slight v drop.

    The ground and control voltage are important of course, you need to isolate
    any ground noise that will superimpose itself on the control voltage, ie
    make sure there is absolutly no digital ground currents flowing between the
    trace conecting vco gnd and PLL gnd or whatever generates the control.


    Colin =^.^=
     
  7. Do you really need a 7.5 to one VCO range?
    I am assuming the VCO runs in a phase locked loop. So the
    stability of the supply is not very important for
    frequencies well below the phase detector sampling
    frequency, since the negative feedback will correct errors
    that are changing slowly. I would first try an LC supply
    filter for the phase detector and oscillator.
     
  8. Guest

    Do you really need that much range in the VCO? If not, then use a
    different chip. The lowest noise VCOs just "pull" a crystal or LC tank.


    BTW, IDT was easily the worse place I ever worked. Len Perham rose to
    the top by the vacuum principle, i.e. someone either died or quit, and
    he got sucked up into the next level. Before you knew it, has was CEO.
    It's been some time since he was gone, but it looks like it was too
    late to fix the mess he left.
     
  9. Guest

    It's a genlock chip that has 1x and 2x outputs that covers my frequency
    of interest. There aren't many. Some have pre-set internal dividers
    that I'd have to bypass anyways.
    Since the divider is 1016, the feedback happens slowly enough that all
    kinds of stuff can happen in the meantime.
    There are other, newer video dot clock generators that cover even
    higher ranges, it's worse.
    I'm gonna go to a VCXO for the next version...
    I'm on my 4th week of sending out emails to get this AB01, still no
    reply.
     
  10. Guest

    I'll take everything I can get. I've seen VCOs have their own local
    boost converter followed by a reference chip (!) for power. Something
    like 5 to 6V, followed by a REF195 padded with tons of caps and
    ferrites. But that was for high speed serdes stuff, where jitter is
    measured in picoseconds.
    I'll take everything I can get. A ferrite with 1000ohms at 100MHz is 9
    cents, that still buys me 20dB attenuation at 20MHz. Thing with the RC
    filter is that the 7805 is kind of sloppy in its output specs, mine is
    running at 4.9V. Toss in the RI loss and I might create more problems
    than I solve. If I release my project for others to build, I can't
    control the voltage the regulator outputs. Maybe it's time to spec
    another regulator for the board.

    I'm also a bit miffed that I placed the regulator as far away as
    possible from the genlock chip... Means the 5V that reaches the chip
    has all the noise of the scan converter, microcontroller, and input
    buffer, all 5 mV of it.
    The phase detector, filter and VCO are in the same chip. The divider is
    external. The genlock chip is about 200 mils away from the clock inputs
    on the main chip. There's little else on the board. No currents except
    the return current flows underneath. (knock on wood)

    Whatever the filter is, if it's working with noisy digital signals from
    the phase comp, that's probably the real problem. Maybe I can schmitt
    the inputs too.
     
  11. colin

    colin Guest

    Well it wouldnt hurt to use a regulator ferrite bead and resistor,
    The noise you realy need to worry about is that wich is above your pll
    filter bandwidth,
    If this is about 100khz then a ferrite bead isnt going to cut it and a
    regulator might just let this through,
    however you could put a rc filter before the regulator this will solve both
    problems.


    Colin =^.^=
     
  12. Guest

    Nope, but the other genlocks have even wider ranges. I don't know what
    LCD monitors use to recover the clock on the VGA side but they have to
    use something similar to what I'm doing, so it's possible.

    When I get my 10Hz - 1MHz spec an working I'll poke around the supply
    more but I will add the 1$ of parts to try to clean up the genlock's
    supply first.
     
  13. Joel Kolstad

    Joel Kolstad Guest

    Was it during that period that IDT performed such bone-headed manuevers such
    as purchasing Quality Semiconductor and -- within months -- discontinuing a
    bunch of their products that were better than what IDT themselves offered?
    How naive...

    I would say that I've used plenty of IDT FIFOs and multi-port RAMs and have
    always been happy with them.
     
  14. Guest

    I was at IDT in the late 80's. The Quality buy out was a decade later.
    Cheun Der Lien is the only name I now recognize when I look at their
    website.

    Nothing wrong with the bread and butter IDT parts (FIFOs), though they
    fell behind on processing yield and speed due to not taking care of the
    talent. Of course, not taking care of the talent is the classic way
    Silicon Valley companies self destruct.
     
  15. Guest

    VXCO is the way to go for stability, but they are slow to lock. There
    is also a slight microphonic issue.
     
  16. Guest

    Could you offer some hints as to how to select a pullable crystal at
    32MHz? I think crystals over 27MHz are 3rd overtone, perhaps I'm out of
    date? It is my understanding that a 3rd overtone crystal is 3 times
    less pullable. Or is it 9? Are there perhaps resonators that can be
    used in this role? I think a resonator's Q would be still be higher
    than the apparent Q of a wideband VCO.
     
  17. Guest

    I did some research on this kind of project years ago, but never built
    the hardware. {cancelled project.] From memory (so use this in research
    rather than accept it to be gospel), there are two basic ways to pull a
    crystal. One is with a varactor, much like you would use if tuning the
    osc by hand. The other is a more integrated approach used in a Philips
    chip. I'm drawing a blank on how they did it, but no varactor was
    required. However, if you building a board level product, I don't see
    an issue with a varactor. There are a few that are dirt cheap since
    they are used in AM/FM radios.

    My recollection is you can only pull a crystall about 1000ppm. Remember
    the low jitter is related to the high-Q, which in turn means it takes a
    long time to move the frequency.

    There is an old book on crystal oscillators by a guy named Ferking. I
    recall using a copy from Stanford. I dug this up on Amazon:
    -----------
    "... co and go to application notes. Ferking M. E. (1978): Cristal
    Oscillator Design and Temperature Compensation, New York: Van Nostrand
    Reinhold. Hambley A. R. (1994): Electronics. ..
    --------------

    For something on crystals, a 1978 reference is actually pretty recent.
    ;-) I'm not sure if the 1994 reference is to a more recent book.

    The other source I found was from the Swiss Federal Institute, though
    the author doesn't come to mind.

    This is the company that at the time I did the research had the lowest
    "noise" crystals:
    http://www.crovencrystals.com/
     
  18. colin

    colin Guest

    You could always use a DDC, they are a bit expensive though, and use a fair
    amount of power.

    If im not mistaken pulling a crystal can be done as quickly as you want,
    the rate of pulling is not limited by the Q of the crystal just the extent.

    Once the energy is built up in the circuit it is bound to oscillate at the
    resonant frequency.
    But its not something ive looked into.

    A ceramic resonator however makes a pretty good VCO.

    Colin =^.^=
     
  19. lemonjuice

    lemonjuice Guest

    If you are not using a fixed overtone crystal oscillator 32 MHz should
    be a frequency lying between the resonant and antiresonant frequency
    of the crystal. You would have to add a capacitor in series(assuming
    the crystal is looking into a low impedance load in the VCXO circuit)
    to make it pullable. That will only raise the frequency. An inductor
    in series will change that. Or you can add a phase lag around the
    oscillator circuit to lower the frequency, best if you are interested
    in setting your oscillator to a fixed frequency.

    An LC selector tank substituting one of the capacitors in the typical
    capacitor stack (of your VCXO) fed into your amplifying unit should
    fix that. At low frequencies, this circuit appears inductive, while
    above resonance,its capacitance dominates. By setting the resonant
    frequency to bemidway between the highest unwanted overtone and the
    next desiredovertone, C and L will be of opposite sign at all
    lower-frequencyunwanted overtones, but of the same sign (capacitive
    reactance) at the desired overtone above the selector resonant
    frequency.
    Yes ... bout 20MHz right up to 60MHz then 5th overtone take over,
    though above 50MHz you would need to add a shunt inductance to make
    the crystal resonate. As at such frequencies the parallel motional
    capacitance shorts out the series motional inductance and capacitance
    in the crystal.
    From what I heart the pullability is limited by about 0.15% the
    Crystal's fundamental frequency, though units specifically designed
    to resonate at an overtone frequency resist being pulled at that
    frequency but have good frequency variability at their fundamental
    value.

    That shouldn't be your worry ... by varying the external capacitance
    as I suggested above should fix things if you are using a crystal not
    resonating at a given overtone frequency.

    lemonjuice
     

  20. A few companies make fundamental crystals to 125 MHz. We used them
    at Microdyne. They were in welded metal TO-5 cans, and custom made.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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