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Simple phase-shifter?

Discussion in 'Electronic Basics' started by eromlignod, Feb 24, 2008.

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  1. eromlignod

    eromlignod Guest

    Hi guys:

    I know there must be a super-simple way to do this, but I keep getting
    caught up in PLL stuff.

    I have a simple square wave in the audio-frequency range. I would
    like to simply shift its phase for experimental purposes. The output
    would be a square wave at the same frequency, but with the phase
    shifted X-degrees. I would like for the shift angle to be variable if
    possible. It seems to me there should be an easy way to do this with
    some sort of passive R-L-C setup, but I can't seem to find
    specifically what I want in any books.

    Thanks for any help you can provide.

    Don
     
  2. Jamie

    Jamie Guest

    Assuming that the freq is fixed, a dual 555 timer.
    first unit, a time on delay for the shift. Second
    unit, a one shot equal to the fixed square wave.

    for variable frequency, something like a shift register
    would work. You can simply vary the clock rate to adjust the
    offset how ever, there would be an error in frequency reproduction
    due to the bit res and clock rate..

    Other option, use a uC (microchip) like PIC's, AVR's etc and
    use a hardware timer event that inputs the signal from your existing
    square wave and shifts it through a pack of bytes using the bits of each
    byte as the value as circular buffer that gets output to an IO as the
    phase offset.
    the rate can be adjusted via one of the ADC inputs which would simply
    adjust the interrupt and offset.

    Many solutions.



    --
    "I'd rather have a bottle in front of me than a frontal lobotomy"

    "Daily Thought:
    that's just it, too much smoke for that!

    http://webpages.charter.net/jamie_5"
     
  3. John Larkin

    John Larkin Guest

    A simple RC lowpass filter, followed by a comparator, will work. Vary
    the R to vary phase shift. You can reasonably get up to, say, 45 to
    maybe 90 degrees of lag (more if the comparator has hysteresis), and
    you can cascade stages to get more. Phase shift will vary with
    frequency.

    PLLs can do a lot more, lead and lag, but are more work.

    You could also pipe it into a clocked shift register, and vary the
    clock frequency to directly add delay. But that would introduce a
    1-clock jitter.

    John
     
  4. Strictly speaking, phase shift applies only to sine waves,
    and a square wave is made up of many different sine wave
    components. What you are needing is not a phase shift, but
    a variable time delay, that is independent of frequency
    (equal time shift for many frequencies). But the time delay
    for a 10 degree phase shift for the fundamental frequency in
    the square wave is a 30 degree phase shift for the third
    harmonic, and a 50 degree phase shift for he fifth harmonic,
    etc. So an analog circuit that does what you want must
    produce a phase shift that is proportional to frequency,
    with a variable proportionality constant. A tall order.

    Variable time delays are usually performed with a sampled
    representation of he waveform passed through a shift
    register with variable speed clocking. The signal
    representation may be analog (voltage samples stored in a
    bucket brigade analog delay line) or digital (binary numbers
    made by an analog to digital converter that are shifted
    through a multi bit shift register, the output converted
    back to voltage samples by a digital to analog converter).

    DSP (Digital Signal Processor) chips have all the stuff
    needed for the second case, except, possibly for memory to
    hold the shift register.

    The analog delay lines are getting really hard to find
    because DSP chips have gotten so good and so cheap.

    You might find some on eBay:
    http://cgi.ebay.com/Reticon-SAD512-...et-Brigade-Chip_W0QQitemZ160156411067QQihZ006
    or from surplus places electronics sellers.
     
  5. John Fields

    John Fields Guest

    ---
    View in Courier:



    HC123A
    +------+
    Vcc-----+-----------+--|1B |
    | | | _ | Vcc
    [Rt]<-+ +-O|1R | |
    | | | | | |K
    +---+--+----|--|1RC | [1N4148]
    | | | | _ _| |
    SQIN>---|------|-+--|-O|1A 1Q|--[10nF]--+------+
    | | | | +------+ | |
    | | | | [10K] |
    | | | | HC123B | A
    | | | | +------+ GND NOR Y-+--OUT>
    [Ct] | +--|--|2B | +-B |
    | | | | _ | Vcc | A-+
    | | +-O|2R | | +-Y NOR
    | | | | |K B
    | +-------|2RC | [1N4148] |
    | | _ _| | |
    GND>----+-------------O|2A 2Q|--[10nf]--+---------+
    +------+ |
    [10K]
    |
    GND


    ________________ __________
    SQIN___| |________________|

    Tdr-->| |<-- -->| |<--Tdf
    ________________ _______
    OUT_______| |________________|


    Tdr = Tdf = 0.45 RtCT
     
  6. Bob Masta

    Bob Masta Guest

    The RC approach that others have described is probably as close as
    you're going to get to "super simple". But just to toss another idea
    into the pot, you can integrate the square wave into a triangle, then
    use comparators to set the trigger points. The simplest way to do
    that is probably to run your original square wave at twice the
    frequency you really want, with a single comparator that triggers
    at the desired level (which is now linearly proportional to phase).
    Its output will be a pulse of variable width, namely only as wide as
    the portion of the triangle wave that is above the threshold. But
    the output of the flip-flop will give you a clean square wave, at the
    desired frequency.

    Note that this approach is best with a fixed frequency. If you vary
    the frequency, the triangle out of the integrator will change its
    amplitude.

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
     
  7. eromlignod

    eromlignod Guest


    I'm intrigued by the low-pass filter idea. I'm using a comparator to
    square the wave already.

    Now, do I have to be close to the cut-off to get the phase lag, or do
    I get the lag even if I'm well below it? In other words, can I make
    the filter with a cut-off far above my working frequency and still get
    an appreciable phase lag?

    Thanks for all the ideas so far.

    Don
     
  8. I'm intrigued by the low-pass filter idea. I'm using a comparator to
    square the wave already.

    Now, do I have to be close to the cut-off to get the phase lag, or do
    I get the lag even if I'm well below it? In other words, can I make
    the filter with a cut-off far above my working frequency and still get
    an appreciable phase lag?

    Thanks for all the ideas so far.

    Don
     
  9. whit3rd

    whit3rd Guest

    Alas, the easy ways are either very limited or require sinewaves.
    If you had a quadrature oscillator, outputs SIN(wt) and COS(wt)
    you could feed those to a pair of balanced mixers that accept
    low-frequency local oscillator input, sum the outputs, and get

    cos(phi)sin(wt) + sin(phi)cos(wt) = sin(wt + phi)

    where some trickery with custom potentiometers makes the
    constants cos(phi) and sin(phi)...

    For square waves, you could generate with a PLL a signal
    at 20*f, then use a CD4017 to generate ten staggered output pulses.
    A toggle flip/flop on any of those pulses will have a characteristic
    phase (you
    might want to reset all flip/flops with (Q #0 .and. pulse# 0)
    to ensure repeatable phase). Between the Q and /Q outputs of
    ten flip-flops, you'd have twenty phases of the same square wave to
    choose from.
     
  10. Bob Masta

    Bob Masta Guest

    If you tell us more about how the whole system is supposed to work,
    perhaps we can come up with an optimized approach. For example,
    are you generating the initial square wave yourself? Does the phase
    shifter have to work across varying input frequencies automatically,
    or can you readjust when the frequency changes?

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, FREE Signal Generator
    Science with your sound card!
     
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