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Oscillator buffer

Discussion in 'Electronic Design' started by o pere o, Nov 14, 2012.

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  1. o pere o

    o pere o Guest

    The thread related to large signal PSpice models and an emitter follower
    comes from the following problem:

    I have an oscillator that should drive a digital part of the system.
    In short, what is the best way to achieve this?

    My first attempt has been a common base Colpitts oscillator that gives a
    signal riding on the +Vcc rail. This has been AC coupled to a 74AC gate
    biased to the point that gives square output signals.

    This works more ore less ok, but: the startup transient, which is
    important here, is different depending if the oscillator amplitude is
    sufficient to toggle the gate. This translates into an envelope that
    raises more ore less smoothly until the gate begins toggling, where the
    envelope raises more abruptly -and I guess that the instantaneous
    frequency changes.

    I have thought of two causes for this. The first one is feedback via the
    DC supply: the spikes generated by the gate switching get coupled back
    to the oscillator. The second one could be the change in input impedance
    seen by the oscillator -does this make sense? The cure for #1 could be
    better supply bypassing. The cure for #2 a buffer stage.

    So, what could be a good way to generate a digital signal from an
    oscillator without loading it? Ideally I would like to preserve the
    instantaneous frequency of the unloaded startup transient. And: power
    consumption should be low, say preferably (much) less than 1 mA.
    Operating frequency should be initially 27 MHz, but ideally scalable up
    to ~1 GHz.

    Pere
     
  2. Why can't you use a power on reset circuit that keeps the clock signal low
    until the oscillator has stabilized?

    petrus bitbyter
     
  3. o pere o

    o pere o Guest

    Thanks for the suggestion for improving the coupling. I will have a look
    at it. However, in the 4046 the VCO is giving a stable amplitude, so the
    effect of the load is constant. Without having looked at it, I am afraid
    that in the gradual transition from small signal to large signal, the
    input impedance of the inverter will change significantly...

    Pere
     
  4. o pere o

    o pere o Guest

    Are you building some kind of synchronous oscillator? In our application
    an external signal influences the startup transient (think superreg.
    principle) and the information contained therein should be more or less
    preserved.

    Pere
     
  5. o pere o

    o pere o Guest

    Unfortunately not in this application. The startup transient is the
    relevant fact here.

    Pere
     
  6. o pere o

    o pere o Guest

    The response from an unloaded oscillator is already ok, even taking
    these facts into account. Loading it more or less directly with a CMOS
    gate is not ok.
    Yes, I have to investigate both ways. In the meantime I wanted to hear
    if there were other facts that I had overlooked.

    Pere
     
  7. o pere o

    o pere o Guest

    I have used ceramic coaxial resonators to build oscillators at ~433 MHz,
    and they are quite stable.
    I have also seen that you make stuff on FPGAs. IIRC there are
    interesting techniques to achieve high timing precision making use only
    of digital resources (keywords: time to digital FPGA)...
    I have used several oscillator topologies. At 800 MHz the tapped-C // L
    resonator plus emitter follower works ok, i.e. it is a non-inverting
    topology. But the problem is more or less the same: how to tap the
    signal out.
    You mean something like the DS90C032? You mean sensing with the LVDS
    side and feeding back from the CMOS side to provide the gain? I guess
    that this would not work, as I need a wide linear part where the
    oscillator grows up more or less slowly...

    Thanks for your inputs.

    Pere
     
  8. Am 15.11.2012 09:25, schrieb o pere o:
    Download the operating & service manual for the HP 5370 A/B time
    interval counter. It has startable oscillators that really work.
    Circuits included.

    regards, Gerhard
     
  9. Nice thread, (wiki calls the tickler the Armstrong.)
    The only RF oscillator I know is used to drive a Rb discharge lamp.
    It's a Hartly basically copied from an efratom lamp circuit.

    To Opere, I don't quite get your problem.
    To sense the circuit you're going to have to take a bit of energy
    out.
    This must change the Q and (thus) the resonant frequency.
    If it's a changing Q when you switch in your circuit.. (?)
    then you have to balance it out.

    George H.
     
  10. Guest

    Reason # 3 : Logic gate input is clamping the signal to one or more rails.
     
  11. o pere o

    o pere o Guest

    The end effect has to be just driving a digital input when the amplitude
    is sufficiently high. It does not need to replicate the transient. If it
    does, I guess that it will be better behaved in terms of constant
    loading, but it is not necessary.

    Pere
     
  12. o pere o

    o pere o Guest

    On 11/16/2012 01:25 AM, George Herold wrote:


    Current oscillator version is

    Vcc Vcc Vcc
    | | |
    C L R1
    | | |
    | C-*--*-----C---*----Dig. Gate
    *-B C1 |
    | E-*-* R2
    R | | |
    | Re C2 gnd
    ctrl | |
    gnd

    If the Gate impedance were constant for small and large signal, this
    would be ok (controlled loading). What I am seeing is that when the Gate
    sees enough signal to begin toggling the transient changes. Hope this
    sketch helps explaining:

    envelope: *************
    *
    *
    *****
    ***** |
    0****** | --------------------------0
    <smooth rising> | <saturated>
    |
    start toggling-> (very) fast rising

    As mentioned in my original post, I see 2 possible causes for this: 1.
    Coupling through the Vcc rail (although more ore less well bypassed)
    and 2. Different behavior of the gate for small and large signal.

    The cure for 2 is what I am looking for. Cure for 1 would be better
    bypassing.

    The other configuration I have used (at ~900MHz) is

    Ctrl-
    Rb C---Vcc
    --C-*---B
    | | E
    L C1 |
    | | |
    | *----*
    | C2 Re
    | | |
    gnd gnd gnd

    And a similar one, with a transmission line instead of L has been used
    at 2.4 GHz.

    A minicircuits gain block with a hairpin resonator has also worked ok at
    2.4 GHz, although this was not low-power (tens of mA):

    ·---Amp---·
    | |
    ·-· ·---· <-Phasing lines
    || ||
    || || <--Resonator
    || ||
    ·-·

    (Maximum simplicity)
     
  13. Bill Sloman

    Bill Sloman Guest

    One weird and expensive approach would be to use something like an
    AD834 as your gain stage; set it up with enough initial gain to get
    the oscillator to start up respectably fast, then drop the gain back
    to a level that sustains the oscillation when the digital logic
    detects an edge. If you can live with a little bit of clipping, the
    "sustain" gain wouldn't have to be too well defined.

    http://www.analog.com/static/imported-files/data_sheets/AD834.pdf

    The power consumption is rather higher than you want.

    Multipliers are designed to have the same gain over a respectable
    range of input amplitudes, so the start-up ought to be well-defined.
     
  14. o pere o

    o pere o Guest


    Well, that could be the reason if the oscillator amplitude became large
    enough. However, in practice -though not in theory- the oscillator
    amplitude however tends to saturate at levels that, even riding on 0.5
    Vcc should not reach the rails (iirc less than 1 Vpp). But I will have
    to check that to be sure...

    Pere
     
  15. o pere o

    o pere o Guest

    A multiplier would offer a better control of gain, which could translate
    into a bigger linear range of operation. Of course, at the price of cost
    and power consumption but it could serve as an idealized prototype.
    However, when I played with analog multipliers in the past (at
    frequencies much much lower than the 500MHz of your suggestion), they
    did not perform as well as announced in the datasheets -it could have
    been my fault...

    Pere
     
  16. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    Oh, I thought LO stood for Local Oscillator...
     
  17. Bill Sloman

    Bill Sloman Guest

    The Analog Devices multipliers all have added extra Barry Gilbert. He
    invented the concept and has run with it for a long time now. There
    are a variety of copies available, all much cheaper, but not as good.
     
  18. o pere o

    o pere o Guest

    I had used the 1648 in the past! A simple and nice VCO indeed. Now it
    seems the only successor is the MC100EL1648 from OnSemi, but I may have
    overlooked something.

    I remember having tried to use that chip to make a quenchable oscillator
    but with modest success. Perhaps you could suggest a clever way to tweak
    the gain to allow for this? Power consumption would still not be within
    the desired range, but I might learn something from it.

    Pere
     
  19. o pere o

    o pere o Guest

    <snipped circuit>
    Why don't you use small C's instead of L2 and L3? That should give
    similar impedance transformation at less expense...

    For mmic and coaxial resonator I have used a tapped C at the input and
    output:

    --C--*----*-----C--*----
    | | |
    C res C
    | | |
    -----*----*--------*-----

    Pere
     
  20. o pere o

    o pere o Guest

    It is the oscillator that has to be quenched. I recall fiddling around
    with ramping up the Vcc of the oscillator part (with the last original
    1648's that I had left) and perhaps I even tried something else -it is
    quite a long time ago. Do you see a possibility via the AGC pin, for
    instance?

    Pere
     
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