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LC Oscillator Questions

Discussion in 'Electronic Design' started by Anthony Fremont, Mar 19, 2007.

  1. Pictures available in ABSE

    The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan)
    is taken at the base of the transistor. There is a switchercad file, but
    the simulation will show allot of distortion that really isn't present in
    the prototype circuit, because of lots of circuit capactance I suspect. R1
    was something I was playing with to try and tame the voltage across L1/C3
    being applied to the base.


    Hello all,

    I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I
    arrived at the values of C1 and C2 empirically after starting with a crystal
    oscillator circuit. The values in the original circuit created a horrid
    waveform that looked allot like the simulation. After much tinkering around
    and simulating, I come to the conclusion that getting a perfect waveform is
    nearly impossible, especially with big swing. It seems that the transistor
    likes to take a bite out of the right half of the peak of the wave.

    What is the secret to beautiful waveforms? Do I need another LC resonator
    on the output to fix it up? I mean, I'm getting a pretty nice wave now, but
    there is still some distortion that you can just see at the top of the peaks
    on the yellow trace.

    How do you control the peak voltages of an LC resonattor without mangling
    the waveform? The waveform at the junction of L1/C3 is of course quite
    beautiful, how do I get it from there to the output? ;-)

    I realize that I will need a buffer stage(s) before I can make any real use
    of the signal, but I want the input to the buffer to be as perfect as
    possible.

    Thanks :)
     
  2. K7ITM

    K7ITM Guest


    The waveform in a high Q tank that's lightly coupled to the amplifier
    should be very nearly sinusoidal. If in addition, the amplifier
    remains linear and represents a constant impedance over the whole
    cycle of the waveform, then the waveforms should everywhere be
    sinusoidal. If the amplifier+tank has barely enough loop gain to
    sustain oscillation, then clipping will be minimal, but it's also
    possible to detect the level and control the gain of the amplifier.
    You could, for example, use a light bulb like HP did in their original
    audio oscillator. Beware, though, that best oscillator performance in
    other regards may not be achieved the same way you achieve lowest
    harmonic distortion. Be careful that you optimize the right things
    for your application.

    In the work I do, I need to measure distortion, and the generators I
    use don't have low enough distortion in their outputs to be directly
    useful. The distortion levels in the "raw" outputs are generally
    about -40 to -50dBc. I use filters to make things better, and can get
    to -140dBc distortion levels fairly easily. If it's low harmonic
    distortion you want, I'd suggest that it may be better to just put a
    filter on the output of the oscillator that has only moderately low
    harmonic output, and not worry so much about that aspect of oscillator
    performance. Filters work well when the oscillator frequency range is
    about 1.5:1 or less. Much more than that and you'd need to switch in
    different filters depending on the oscillator frequency.

    Cheers,
    Tom
     
  3. Hello Anthony,

    1.
    Please set the following option to sitch off data reduction/compression
    in the result file..

    ..options plotwinsize=0

    2.
    You have to set a small maximum timestep in the .TRAN line too.
    Maybe a value of 0.01*Period of oscillation if you hunt for very low
    distortion.


    Can you send me your file (.asc-file and model-file?) to check it?

    Best regards,
    Helmut
     
  4. Tim Wescott

    Tim Wescott Guest

    The secret to a beautiful waveform is -- you usually don't need it
    straight from the oscillator.

    There are a lot of things that you want out of an LC oscillator. Low
    phase noise, frequency stability, consistently strong oscillation, pure
    tone, etc. Of these, the only two that you can't clean up later in the
    following amplifier chain is low phase noise and frequency stability.
    Concentrate on those, & don't sweat the nice waveform.

    Frequency stability and phase noise performance are often achieved by
    intentionally designing the amplifier so the active element operates in
    class C, without ever going into voltage saturation. This keeps it's
    drain (or collector) impedance high, yet delivers a large voltage swing
    to the gate (or base) to keep phase noise low. It also gives you a more
    or less consistent standing voltage in the tank, which helps the design
    of the following buffer stages.

    If you absolutely positively must tap the World's Most Beautiful Sine
    Wave off of the oscillator section, consider a parallel-tuned tank
    that's loosely coupled to the active element. Then loosely couple your
    output tap to that -- it's your best chance.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  5. After reading the other replies, it seems aparent that the shape of the
    signal from the first stage is not that critical, it is stability and phase
    noise that are most important. So, I should put things back where there is
    clipping to be sure that the oscillator oscillates and then clean up the
    signal in subsequent stages.
    Thanks. :)
     
  6. In alt.binaries.schematics.electronic I have posted the schematic, the
    asc-file and an oscilloscope screen shot from an actual circuit. Here is
    the asc-file contents:

    Version 4
    SHEET 1 880 708
    WIRE -704 -96 -784 -96
    WIRE -400 -96 -704 -96
    WIRE -224 -96 -400 -96
    WIRE -704 -16 -704 -96
    WIRE -400 -16 -400 -96
    WIRE -224 32 -224 -96
    WIRE -544 80 -592 80
    WIRE -400 80 -400 64
    WIRE -400 80 -464 80
    WIRE -288 80 -400 80
    WIRE -592 128 -592 80
    WIRE -400 144 -400 80
    WIRE -784 160 -784 -96
    WIRE -400 240 -400 208
    WIRE -224 240 -224 128
    WIRE -224 240 -400 240
    WIRE -80 240 -224 240
    WIRE 48 240 -16 240
    WIRE -784 272 -784 240
    WIRE -704 272 -704 48
    WIRE -592 272 -592 208
    WIRE -400 272 -400 240
    WIRE -224 288 -224 240
    WIRE -592 384 -592 336
    WIRE -400 384 -400 336
    WIRE -400 384 -592 384
    WIRE -224 384 -224 368
    WIRE -224 384 -400 384
    WIRE -400 448 -400 384
    FLAG -784 272 0
    FLAG -400 448 0
    FLAG -704 272 0
    FLAG 48 320 0
    SYMBOL voltage -784 144 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 5.8
    SYMBOL res -416 -32 R0
    SYMATTR InstName R3
    SYMATTR Value 100k
    SYMBOL npn -288 32 R0
    SYMATTR InstName Q3
    SYMATTR Value 2N3904
    SYMBOL cap -416 144 R0
    SYMATTR InstName C1
    SYMATTR Value .01µ
    SYMBOL res -240 272 R0
    SYMATTR InstName R7
    SYMATTR Value 1k
    SYMBOL cap -416 272 R0
    SYMATTR InstName C2
    SYMATTR Value 500p
    SYMBOL ind -608 112 R0
    SYMATTR InstName L1
    SYMATTR Value 20µ
    SYMATTR SpiceLine Rser=.1
    SYMBOL cap -608 272 R0
    SYMATTR InstName C3
    SYMATTR Value 200p
    SYMBOL cap -16 224 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C4
    SYMATTR Value 270p
    SYMBOL res -448 64 R90
    WINDOW 0 0 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName R1
    SYMATTR Value .001
    SYMBOL cap -720 -16 R0
    SYMATTR InstName C5
    SYMATTR Value 10µ
    SYMBOL res 32 224 R0
    SYMATTR InstName R2
    SYMATTR Value 10000k
    TEXT -792 360 Left 0 !.tran 50uS

    Thank you for your time.
     

  7. Hello Anthony,

    The large capacitance of C1 (10nF) has caused an interrupted oscillation.
    Please change its value to 1000p and the oscillator will work as expected.
    I have also added MEASURE-commands to measure the frequency.
    View -> SPICE Error Log

    Another method is using the FFT in the waveform viewer.

    Best regards,
    Helmut

    Save as "osc1.asc".

    Version 4
    SHEET 1 880 708
    WIRE -688 -96 -784 -96
    WIRE -576 -96 -688 -96
    WIRE -304 -96 -576 -96
    WIRE -784 -64 -784 -96
    WIRE -688 -64 -688 -96
    WIRE -576 -16 -576 -96
    WIRE -304 32 -304 -96
    WIRE -784 48 -784 16
    WIRE -688 48 -688 0
    WIRE -576 80 -576 64
    WIRE -480 80 -576 80
    WIRE -432 80 -480 80
    WIRE -368 80 -432 80
    WIRE -576 128 -576 80
    WIRE -432 144 -432 80
    WIRE -576 240 -576 208
    WIRE -432 240 -432 208
    WIRE -304 240 -304 128
    WIRE -304 240 -432 240
    WIRE -240 240 -304 240
    WIRE -160 240 -240 240
    WIRE -64 240 -96 240
    WIRE -32 240 -64 240
    WIRE -576 272 -576 240
    WIRE -432 272 -432 240
    WIRE -32 272 -32 240
    WIRE -304 288 -304 240
    WIRE -32 368 -32 352
    WIRE -576 384 -576 336
    WIRE -432 384 -432 336
    WIRE -432 384 -576 384
    WIRE -304 384 -304 368
    WIRE -304 384 -432 384
    WIRE -432 416 -432 384
    FLAG -784 48 0
    FLAG -432 416 0
    FLAG -688 48 0
    FLAG -32 368 0
    FLAG -64 240 out
    FLAG -240 240 e
    FLAG -480 80 b
    FLAG -576 240 lc
    SYMBOL voltage -784 -80 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value 5.8
    SYMBOL res -592 -32 R0
    SYMATTR InstName R3
    SYMATTR Value 100k
    SYMBOL npn -368 32 R0
    SYMATTR InstName Q3
    SYMATTR Value 2N3904
    SYMBOL cap -448 144 R0
    SYMATTR InstName C1
    SYMATTR Value 1000p
    SYMBOL res -320 272 R0
    SYMATTR InstName R7
    SYMATTR Value 1k
    SYMBOL cap -448 272 R0
    SYMATTR InstName C2
    SYMATTR Value 500p
    SYMBOL ind -592 112 R0
    WINDOW 39 36 108 Left 0
    SYMATTR InstName L1
    SYMATTR Value 20µ
    SYMATTR SpiceLine Rser=.1
    SYMBOL cap -592 272 R0
    SYMATTR InstName C3
    SYMATTR Value 200p
    SYMBOL cap -96 224 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C4
    SYMATTR Value 270p
    SYMBOL cap -704 -64 R0
    SYMATTR InstName C5
    SYMATTR Value 10µ
    SYMBOL res -48 256 R0
    SYMATTR InstName R2
    SYMATTR Value 100k
    TEXT -824 -152 Left 0 !.tran 0 200uS 0 4n
    TEXT -824 -184 Left 0 !.options plotwinsize=0
    TEXT -816 472 Left 0 !.measure tran t1 FIND time WHEN V(out)=0 TD=90u RISE=1
    TEXT -816 504 Left 0 !.measure tran t2 FIND time WHEN V(out)=0 TD=90u
    RISE=101
    TEXT -816 536 Left 0 !.measure tran f0 PARAM 100/(t2-t1)
    TEXT -816 576 Left 0 ;View -> SPICE Error Log \nfor the measured frequency
    TEXT -520 -184 Left 0 ;C1 changed to 1000p!
     
  8. Okay, that certainly explains why all the sample circuits I find don't
    expend any great effort at creaing a nice sine wave, and none at explaining
    why. What you say certainly makes sense, especially if there are no really
    negative consequences of having the oscillator make a "less than perfectly
    shaped" wave.
    Ok, thanks for the information. :) I did allot of googling but found
    nothing that explained it like this. I was thinking of building a little
    single conversion superhet WWV receiver for 10MHz, if I continue with that
    I'll just concentrate on cleaning it up in another stage.

    Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc
    (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on
    that? Right now I have way too much inductance for 3.5MHz by that theory,
    and judging from other circuits I've seen. <10uH seems to be the going thing
    for around 4MHz?
     
  9. Tam/WB2TT

    Tam/WB2TT Guest

    I have never seen clipping. These things are supposed to limit in cutoff,
    not saturation. As the signal build up, the conduction angle gets smaller
    and smaller until the device runs out of gain. That is another way of saying
    that the DC value of the gate voltage gets more negative the bigger the
    amplitude. This works out automatically with a JFET. You need about 10K -
    100K DC resistance from gate to ground. Using a bipolar transistor is not a
    good idea.

    Tam
     
  10. Tim Wescott

    Tim Wescott Guest

    That sounds more or less right. With a Clapp oscillator the main tank
    is isolated by the series cap, so more of the energy is kept in the coil
    and C3, and less of it shows up in C1, C2, and the transistor.

    If you're driving a balanced mixer you want to have an LO signal that
    doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but for a
    casual receiver that's the least of your worries. Since you're
    operating at a fixed frequency it may be a good idea to just feed the
    oscillator output into a single-tuned resonant circuit to clean it up,
    then send it on to the mixer.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  11. I was wondering about the load that a bipolar would present. I will see if
    I can find a JFET in my junk pile, thank you. :)
     
  12. Chris Jones

    Chris Jones Guest

    In some LC oscillators, the amplitude of the oscillation is controlled by a
    feedback loop. For example, a rectifier can be used to create a DC voltage
    proportional to the oscillation amplitude on the LC tank, and then an
    op-amp can be used to compare the rectifier output signal to a reference
    voltage. The output from the op-amp can be filtered and then used to
    control the current in the oscillator core. It is difficult to do all of
    this in a way that keeps the phase noise low, but given the right
    simulation tools (e.g. SpectreRF which is rather expensive), good results
    can be obtained. In particular, a well-defined oscillation amplitude can
    help to keep the KVCO well controlled, which is useful in PLLs.

    Chris
     
  13. colin

    colin Guest

    This can more difficult to acheive well than it first seems,
    adjusting the bias conditions to adjust level is often not stable,
    as it draws more current as the oscilations build up,
    a fast feedback loop wich monitors the current can be used to adjust the
    bias to
    keep the average current at a set point,
    wich can be just a single npn transistor,
    an op amp can then be used to set the set point,
    although just ensuring a set current is usually sufficient to maintain a
    very clean stable waveform,
    and yet not have any startup problems,
    for all but very wide range oscillators.

    Colin =^.^=
     
  14. Thank you very much. :) I have now switched to using an MPF102 JFET
    instead of the bipolar and much less capacitance for C1 (now 470pF). I only
    get a 2V peak to peak signal out now, but it's quite nice looking.
     
  15. Ok, I've now put in an MPF102 and changed R3 to a pull-down. I lowered C1
    to 470pF and I get a nifty 2V p-p sine wave on the output. It really tamed
    the tank circuit voltage down as well. Which brings up a question, with the
    tank now completely DC blocked from Vcc and Vss, where does it get it's
    energy. I assume that it must come thru the gate. How does that happen?
    :-? My circuit is much like Figure 1 here, without the diode though:
    http://www.electronics-tutorials.com/oscillators/clapp-oscillators.htm
     
  16. john jardine

    john jardine Guest

    The prettiest waveforms come from balanced oscillators. Distortion then
    turns up as 3rd 5th 7th etc harmonics which are far less ugly than the 2nd
    3rd 4th 5th etc generated by the single ended types. Balanced ALC is also
    easier and more effective.
    My own experience says that 'prettier' is better. Those oscillators offering
    gross distorted outputs also seem to suffer badly in other areas and gross
    distortion always causes problems further down the line.
    Procuring good quality is a classic black art, one aspect is to allow the
    LC just an occasional vague glimpse of the maintaining amplifier. Another is
    to cause limiting by use of an amp having a gentle gain change (eg Fet v
    bipolar) and the other is ALC. (Or all three together).
    Failing that, there is always the cop-out of an output filter :)
    john
     
  17. Tim Wescott

    Tim Wescott Guest

    It comes from the source, through the coupling capacitors -- Cfb-a and
    Cfb-b in your link.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  18. I have now changed it to an MPF102 that I've had laying around for many
    years. It works great, thanks.
    :)
     
  19. Tam/WB2TT

    Tam/WB2TT Guest

    Glad it worked out. By the way the feedback path is through the capacitive
    network between source and gate. That would be more obvious in the
    configuration that uses a tapped inductor, but works the same way. Leaving
    out the diode was the right thing to do; it just adds to the noise. I don't
    know what kind of stability and linearity you need, but if that is
    important, do not use the common type of ceramic capacitors that are meant
    for bypassing. They are lossy, and their value varies with applied voltage.
    Use mica, NPO ceramic, or Mylar and similar for larger values.

    Tam
     
  20. rebel

    rebel Guest

    Without going to the purity levels that Tom requires, I've always found that
    bipolars can be used to produce a fairly reasonable "visibly sinusoidal" (see
    note) waveform. Follow the oscillator with an amplifier stage which drives a
    limiter/clipper, and use that to control a gain element in the oscillator. It's
    like the incandescent non-linearity arrangement except the oscillator stage
    waveform remains fairly clean.

    (Note: Harmonic distortion not readily discernible on a CRO)
     
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