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variable phase shift of signals

Discussion in 'Electronic Design' started by axr0284, Mar 16, 2007.

  1. axr0284

    axr0284 Guest

    Hi,
    I am looking for possible solutions to be able to phase shift a
    signal in the 1 to 4 MHz range by a certain amount between 0 and 180
    deg. Originally we were using the AD9501 delay chip but they are going
    obsolete. The resolution should be less or equal to 50ps over the
    above frequency range.
    Some solutions that we have already come up with are
    1) using FPGA with DCMs to induce the phase shift. <-- This might not
    be realizable at that resolution.
    2) Use FPGA to control a DDS and produce the phase shift. <-- Best
    solution we have right now but we have never tested it.
    3) Substitute the AD9501 with MC10EP195 <-- The ECL voltage levels
    might need to be converted to lvcmos/lvttl

    I am looking for other possible external solutions. An FPGA would be
    controlling the phase shifting. I figured people who work with this
    kind of stuff on a daily basis might know. Thanks a lot for any
    answer,
    Amish
     
  2. linnix

    linnix Guest

    How about the ELMEC programmable delay?
    6 bits from 0 to 750 ps in 50ps steps.
    How many steps do you need?
     
  3. axr0284

    axr0284 Guest

    John, this is for using a class D power amplifier. I am not too sure
    of the details of the amplifier part. All I know is that I need to
    produce two square waveforms that have a certain frequency and phase
    shifted by a certain amount wrt each other. Most probably an FPGA will
    be used to create the square wave and control the phase shift. The
    frequency will also be tracked in such as way that a frequency
    variation by +-5% should NOT cause the phase shift to change by more
    than 50 ps.
    So if the signal was running at 4 MHZ, with 45 degree phase shift.
    This would mean that signal B would have a delay of 31250 ps wrt
    signal A. Now if the frequency changes by +5% for whatever reason.
    this gives 4.2MHz. With the same 45 degree phase shift, it would mean
    that signal B would have a delay of 29763 ps wrt signal A. In this
    case, the phase would be adjusted so that signal B would have a delay
    of approximately 31250 ps wrt signal A. I hope this is clear enough.

    I will be designing a board that has all the components on it so I
    would need a solution like a chip or a circuit that can be mounted on
    a board. The ELMEC programmable delay looked interesting but they do
    not have a rohs compliant version which is required by my company.

    I really appreciate you guys taking the time to answer me. Thanks,
    Amish
     
  4. John  Larkin

    John Larkin Guest


    What's the nature of the signal? Is it just a single digital edge, or
    a square wave? What parts need to be delayed? Must it automatically
    track changes in frequency, or is a programmable time delay enough?

    Would this work?

    http://www.highlandtechnology.com/DSS/T560DS.html

    I can get you the s.e.d. discount!

    John
     
  5. Tom Bruhns

    Tom Bruhns Guest

    Seems to me that if you are tasked with generating the signals, and
    you don't have to accept some arbitrary external signal, the DDS is
    just about an ideal solution. I believe you should be able to find
    DDS parts from Analog Devices that will do the job for you with
    relatively few external parts. Consider that a DDS works by
    incrementing a phase accumulator; a phase offset is just a constant
    added to the phase accumulator value. The phase between the
    accumulator and the accumulator plus offset is then independent of the
    amount you increment the phase accumulator each clock. It will be
    important to have the two channels have identical delays; 50psec is
    only about 5mm of trace length on an FR4 PC board, I believe. If you
    have a lot of FPGA resources left over, you could implement the DDS
    sine generation there, and just drive a dual DAC from the FPGA. If
    you need a square wave instead of sine, you can low-pass filter the
    DAC outputs and drive comparators. -- Of course, it's also possible
    to calculate the time delay required for a particular phase difference
    for any known frequency, and that can be reduced to a number of clock
    periods plus an analog-controlled delay. But the DDS likely makes it
    easier to generate frequencies with arbitrarily fine resolution.

    Cheers,
    Tom
     
  6. Guest

    The Analog Devices range of Direct Digital Synthesis chips include
    some parts that produce two separate outputs at a programmable
    frequency and with a programmable phase shift. '

    http://www.analog.com/en/subCat/0,2879,770%5F843%5F0%5F%5F0%5F,00.html

    The DDS chips are listed in the RF/IF catagory.

    The AD9854 and the AD9958 offer dual outputs and the AD9959 offers
    four.

    The master clocks for the DDS chips now run up to 1GHz (but that is an
    expensive part). 500MHz gives 2nsec sample spacing which is a lot more
    than your 50psec, but DDS chips adjust the amplitude of the signal
    coming out of their 10-bit or better output DACs to get much finer
    control of frequency and phase.

    50psec phase shifts in a 4MHz waveform coming from a DDS (via a low
    pass filter) should be entirely practicable.

    The parts can be reprogrammed pretty rapidly - this could well be an
    interesting solution.

    I've not worked with these DDS chips (or any others)., but Win Hill
    had an interesting device on his web-site at one time that did use DDS
    chips in this sort of way.
     
  7. Jamie

    Jamie Guest

    Coax cut to length ?
     
  8. Paul Mathews

    Paul Mathews Guest

    Check out Dallas Semi DS1021 series.
    Paul Mathews
     
  9. Guest

    http://datasheets.maxim-ic.com/en/ds/DS1021-DS1021-50.pdf

    The minimum delay step of 250psec is a good bit bigger than the 50psec
    the OP asks for, and the maximum delay the OP asks for - 180 degrees
    at 4MHz, or 125nsec - is more than 256 times times this minimum delay.

    It isn't likely to be an attractive solution. The Dallas parts also
    have rather poor jitter specifications

    The last time I had to do something like this I planned to use the
    predecessor of the MC10EP195 to generate fine increments of delay,
    interpolating between 2nsec increments generated by a 500MHz crystal-
    controlled clock.

    The MC10EP195 offers a larger minimum delay range - 7.85nsec - than
    the part I planned to use, so one could use a slower clock - down to
    128MHz for a single MC10EP195 - which would make life easier and
    cheaper.

    The problem is that the delays generated by the MC10EP195 increase
    with junction temperature at about 1300ppm/C. In my application I
    planned on using frequention autocalibration (which would have taken
    about 1msec) to measure the actual delays being generated by the fine
    delay generator, and expectd that the digital words generating the
    delay would be recalulated after every calibration.

    The calibration procedure involved generating an 80MHz pulse-width
    modulated waveform whose mark-to-space ratio depended on the
    programmable fine delay, running it through a low-pass filter and
    digitising the analog output voltage.
    This would have allowed me to compare delays being generated by the
    analog delay line against the period of the 500MHz crystal controlled
    clock with quite good precision

    I doubt if exactly this approach would work for the OP. In the
    appIication decscribed, you might want to built three swappable delay
    modules, and cycle around all three, re-calibrating one against your
    crystal-controlled coarse clock while the other two controlled the
    inverter drives.

    In theory, one could string together 16 MC10EP195 parts and stick them
    all in something like a crystal oven, but this would be rather bulky.
    Granting the rather wide manufacturing tolerances on the MC10EP195,
    you'd have to calibrate each delay module at manufacture, and the data
    sheet doesn't say anything about the long term stability of the
    delays, which could make built-in autocalibration rather desireable -
    though this rather depends on exactly what the OP is actually doing.
     
  10. Fred Bloggs

    Fred Bloggs Guest

    That is not called a "phase shift," it is called a time delay. Your
    understanding of the requirements are leading you down the road to a
    complicated system with little chance of success. It would be better if
    you drop your microscopic view of things and describe the offset clocks
    and the class D amplifier in plain language.
     
  11. axr0284

    axr0284 Guest

    Thanks for the response everyone. This is going to be an interesting
    project since i am pretty new in the RF field. Fred, I definitely do
    not have an expert understanding in the matter of phase and delay
    difference. Maybe you can explain it to me in a little more detail.
    Luckily for me I believe there is another company project using the
    DDS AD9958. The board is supposed to be manufactured in the near
    future. If the results from that are conclusive, we will probably go
    with that but it is still good to have other options under hand. Keep
    the ideas coming. Thanks a lot for the help,
    Amish
     
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