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Phase shifting a sine wave smoothly

Discussion in 'Electronic Design' started by Greysky, Jan 7, 2004.

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  1. Greysky

    Greysky Guest

    I have a bit of a problem... I have to be able to shift the phase of an 5
    MHz sine wave oscillator by plus / minus 90 degrees. It needs to be done
    via analog (continuous) means, and without distorting the waveform in the
    process. I once saw a circit that did something like this, by using a
    voltage controlled resistor circuit, but that was soo long ago I don't
    remember any particulars. Are there any nice methods of doing this and also
    keeping the phase distortion of the output signal as small as possible?
    Thanks
     
  2. Joel Kolstad

    Joel Kolstad Guest

    Luckily for you, the integral and derivative of a sine wave just so happens
    to be the same sine wave with a phase difference of -90 or +90 degrees.
    You can build very good integrators out of op-amps. Making the various
    errors 'as small as possible' becomes a function of how good your op-amp and
    support components (i.e., the integrating capacitor) are.

    Differentiators tend to be more difficult to build due to the their rising
    gain with frequency -- this is especially problematic if you're shooting for
    very low noise designs. You could instead just invert your signal and then
    integrate, although this is only the same as differentiating at one
    frequency (since the trend of the magnitude response is 'wrong' -- if you
    don't care about the magnitude response or can fix it later though, this is
    still viable).

    ---Joel Kolstad
     
  3. sin(wt +/- a) = sin(wt) * cos(a) +/- cos(wt) * sin(a)

    If digital in the control signal was acceptable, you could probably
    use a couple of digital pots and a LUT in a micro with ADC input.


    Best regards,
    Spehro Pefhany
     
  4. Do you need the fixed phase shift, or does this have to be variable?
    The former is easy, the latter is more trouble.

    Just look up single sideband generated by the phasing method. You need
    a carrier with two signals 90 degrees apart. At a fixed frequency it's really
    easy, but with more parts one can come up with a network that works
    over a wider range.

    The trick is usually to have two networks, each shifting by 45 degrees,
    and when they accumulate you get the 90 degrees between points.

    Michael
     
  5. Active8

    Active8 Guest

    Do you need the fixed phase shift, or does this have to be variable?
    The former is easy, the latter is more trouble.

    Just look up single sideband generated by the phasing method. You need
    a carrier with two signals 90 degrees apart. At a fixed frequency it's really
    easy, but with more parts one can come up with a network that works
    over a wider range.

    The trick is usually to have two networks, each shifting by 45 degrees,
    and when they accumulate you get the 90 degrees between points.

    Michael
    [/QUOTE]
    I bet the Mini-Circuits Phase modulators would do the job, if not,
    there's enough info on circuit topology to get started, at least in
    the handbook.
     
  6. John Smith

    John Smith Guest

    Source on the left, output on the right. I saw this one somewhere a long
    time ago. Finding an op-amp to handle the 5 MHz is left to those who know
    op-amps.


    ------------+--------------.
    | |
    .-. |
    | | '
    | | ---
    '-' ---
    | | |\|
    +----------------------+-|-\
    | | | >-
    | +--------+|+/
    | | |/|
    ' .-.
    --- | |
    --- | |
    | '-'
    -------------+--------------+
    |
    ===
    GND
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
     
  7. But this is just another occurance of someone trying to extend audio
    ideas to radio.

    It's almost like "is the glass half full, or half empty". You've
    shown something that works, but then it may not be the best choice
    at RF because of the active element limitations.

    Yet anyone with radio background would automatically suggest
    an LC network, and the op-amp would be nowhere to be seen.

    Michael
     
  8. John Larkin

    John Larkin Guest

    Nowadays, 1 GHz opamps are cheap, and there are a few faster parts
    around. Lots of classic "radio" functions can now be done with an
    opamp or two.

    John
     
  9. John Smith

    John Smith Guest


    Well... not really trying. It just happens to be an approach I remembered.


    I didn't say it was the best choice. As with so many topics here, there are
    a myriad of approaches which the reader must evaluate and eventually choose.
    It's just an idea.


    I'm not trying to compete with you, Michael. Perhaps your idea is better and
    the OP may choose to use it. My feelings won't be hurt. I just assume people
    would like to have a number of different approaches from which to choose and
    I didn't see any harm in presenting it.

    John
     
  10. I wasn't intending to attack your solution. It was more a general observation
    (and I believe someone else mentioned op-amps in this thread too).

    In the hierarchy generally, I've seen plenty of times when people say
    "I need to build a 2MHz oscillator, and I used an 8038 [or sometimes a 555]
    and it's not working right. What am I doing wrong?". Or "I need to
    get 3KHz bandwidth at 10MHz, but when I tried to design it with an
    active filter it wouldn't work." Their vision is from the standpoint of
    audio, and they just want to push the device a bit. But people with radio
    background would come up with more suitable solutions that would work fine.
    Sometimes things scale, sometimes they don't.

    Look at audio circuitry, and you want to avoid inductors as much as possible,
    because at low frequencies they are bulky, difficult to wind or procure,
    and in some applications end up being more a problem than a solution.
    But get to RF, and the values become much more manageable. And it's harder
    to synthesize actual needs at radio with RC components than at audio.

    So there often is a point where one wants to flip from "audio" thinking
    to radio thinking. 1MHz for an RC oscillator is often pushing it,
    yet it's real easy to make an LC oscillator at that frequency. But you've
    got to have a really good reason to build an LC oscillator at 1KHz.

    Michael
     
  11. That reminds me of a guy who thought he could build Cable TV
    descrambler using a 741 op-amp in an active notch filter to remove the
    inband jamming signal from Ch 3 TV (60 - 66 MHz) He saw a sample circuit
    for a twin T audio filter and he was convinced that, "Its just a matter
    of picking the right resistors."

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  12. Mac

    Mac Guest

    I don't thing 5 MHz really qualifies as RF these days. At least in the
    sense that no special techniques are required, other than using a ground
    plane on your circuit board, and some small ceramic chip capacitors near
    the op-amp power supply inputs.

    Mac
     
  13. Mac

    Mac Guest

    I know you said digital is out, but have you considered a DDS? If I had to
    do what you are doing, I'm sure I would use a DDS from Analog Devices, but
    that is because I don't really know how to design a variable phase
    shifter. Although, come to think of it, I think Analog Devices has those,
    too.

    You should also try minicircuits.

    Just a thought.

    Mac
     
  14. Keith Buck

    Keith Buck Guest

    Hi, A Goniometer works a treat and you may be able to find one in an old Tek
    Vectorscope.
    Regards Keith Buck
     
  15. Phil Hobbs

    Phil Hobbs Guest

    I usually do this by using an active all-pass filter. You don't need an
    op amp, any differential amp with high-impedance inputs works fine, as
    long as the + input has twice the gain of the - input. The op amp
    circuit below does this automatically, but you can arrange it with a
    voltage divider otherwise. (Twenty years ago, I built two of these in
    cascade, as part of a successive-approximation 60 MHz phase digitizer.
    I used NE592s running at low gain for the amplifier, and put a
    divide-by-2 voltage on the inverting input.)

    R3

    +--RRRRR--+
    | |
    Vin R2 | |
    | |\ |
    0---*----RRRRR--*--|-\ |
    | | \ |
    | | >--*----0 Ouput: Amplitude = Vin
    | | /
    +--RRRRRR---*--|+/ phase=2*arctan(R1/X)
    | |/
    R1 | where X is the total
    | reactance of the LC circuit
    +---------*
    | |
    | __L__
    | \ / Hyperabrupt varactor,
    | \ / e.g. MVAM series
    | V
    | ---+---
    | ---+---
    L2 | L1 |
    LLL LLL L1 chosen to resonate with varactor at about 1V
    LLL LLL L2 chosen to resonate with series circuit just
    LLL LLL above whatever upper voltage limit you like
    LLL LLL
    | |
    | *----------RRRRRR-----O Phase shift control
    | | 2-15V -> 150 degree range
    | ---+--- +- 4 degree linearity
    | ---+--- Big cap
    | | Xc << R1
    +---------*
    |
    GNDGNDGND
    GNDGNDGND
     
  16. Phil Hobbs

    Phil Hobbs Guest

    I usually do this by using an active all-pass filter, which has the huge
    advantage that the amplitude doesn't change as you adjust the phase
    shift. The circuit below gets you a very wide range, reasonable
    linearity, and low distortion if you don't let the bias voltage get too
    small or the signal swing too large. You don't need an op amp, any
    differential amp with high-impedance inputs works fine, as long as the +
    input has twice the gain of the - input. The op amp circuit below does
    this automatically, but you can arrange it with a voltage divider
    otherwise. (Twenty years ago, I built two of these in cascade, as part
    of a successive-approximation 60 MHz phase digitizer.
    I used NE592s running at low gain for the amplifier, and put a
    divide-by-2 voltage on the inverting input.) The math is at the bottom,
    but it's easy to work out.

    R3 (R3= R2)

    +--RRRRR--+
    | |
    Vin R2 | |
    | |\ |
    0---*----RRRRR--*--|-\ |
    | | \ |
    | | >--*----0 Ouput: Amplitude = Vin
    | | /
    +--RRRRRR---*--|+/ phase = -2*arctan(X/R1)
    | |/
    R1 | where X is the total
    | reactance of the LC circuit
    +---------*
    | |
    | __L__
    | \ / Hyperabrupt varactor,
    | \ / e.g. MVAM series
    | V
    | ---+---
    | ---+---
    L2 | L1 |
    LLL LLL L1 chosen to resonate with varactor at about 1V
    LLL LLL L2 chosen to resonate with series circuit just
    LLL LLL above whatever upper voltage limit you like
    LLL LLL
    | |
    | *----------RRRRRR-----O Phase shift control
    | | 2-15V -> 150 degree range
    | ---+--- +- 4 degree linearity
    | ---+--- Big cap
    | | Xc << R1
    +---------*
    |
    GNDGNDGND
    GNDGNDGND


    Vout/Vin = (-1+2(jX/(R1+jX)) = (R1-jX)/(R1+jX) = exp(-j*2*arctan(X/R))
    (gain is always 1)
     
  17. Reg Edwards

    Reg Edwards Guest

    Draw a bridge. Series C and R in the left-hand arm. Series R and C in the
    right-hand arm. All 4 components with the same fairly low impedance.

    Connect sinewave generator to top of the bridge.

    Ground bottom of bridge.

    Connect a high-value sliding potentiometer between the centres of the right
    and left-hand arms. Tap off a phase-shifted voltage from the pot slider to
    a following high input impedance circuit such as a source-follower. The
    output voltage will vary between + and - 45 degrees relative to the
    generator.

    Do it twice in succession with a 2-gang pot and the phase will vary between
    + and -90 degrees. If I remember correctly the output amplitude is constant
    versus phase shift. A little elementary circuit analysis will soon show.
    ---
    Reg.

    --
    ............................................................
    Regards from Reg, G4FGQ
    For Free Radio Design Software go to
    http://www.btinternet.com/~g4fgq.regp
    ............................................................
     
  18. Tom Bruhns

    Tom Bruhns Guest

    As one who has worked in the RF domain for quite a while now, I might
    suggest LC, but that likely would not be my first suggestion. The
    common way to do this task is to generate contra-phase signals, equal
    amplitude, and put an RC between them, picking the output at the
    junction of the R and C. Then vary the R or the C, and you vary the
    phase of the output without varying its amplitude (assuming a high
    input impedance buffer amplifier). Works fine and easily up into the
    few-MHz region. Can be made to work much higher. No op amps
    required, but they are allowable if you wish. Cascade a couple stages
    of it to get more variation. You can use varicap diodes for the
    variable C if you keep the RF voltage low enough; an anti-series pair
    helps. You can see this circuit used as an PM modulator, for example,
    in ICOM equipment and I suppose lots of other places. I may have
    first seen it in a WWII radar training book, but surely it's been
    known longer than that.

    Cheers,
    Tom
     
  19. Fred Bloggs

    Fred Bloggs Guest

    Right- you are talking about something like this in radio circuits:
    Please view in a fixed-width font such as Courier.




    V
    +-------------+
    | |
    o-----+ | /
    | | \ R
    )||( /
    V )||( \
    in )||(----+ |
    )||( | +----->Vout
    )||( --- |
    | | COM |
    o-----+ | ===C
    | |
    +-------------+
    -V


    1
    --- - R
    jwC
    Vout= V x -------- = V x /-2 ATN(wRC)
    1 ------------
    --- + R
    jwC


    The phase is NOT a very linear function of R or C.


    It is good for jumping between discrete phase angles.
     
  20. Fred Bloggs

    Fred Bloggs Guest

    Okay- you're getting very close to a good vector modulator...BTW, what's
    a LUT ? Oh- Look-Up-Table, there's a name for people who acronymize
    everything.
     
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