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Idea for a Vector Network Analyzer

Discussion in 'Electronic Design' started by Adam S, Dec 2, 2005.

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  1. Adam S

    Adam S Guest

    I had the following idea for a home built vector network analyzer project.
    The block diagram is shown below. Essentially channels A and B are mixed
    down to separate 1kHz IF and sampled with a stereo 24bit DACE for the
    PC. Phase and magnitude of the resulting 1kHz signals are then
    calculated with DSP methods.
    f1-f0 = 1kHz as f0 sweeps from 0 to 20MHz.
    I know there will be problems as f0 gets close to 1kHz and again at near
    0Hz, due to SA602 not being an ideal mixer.

    Anyone think concept won't work ?


    To PC
    *
    |
    -------
    ------ |FT245BM|
    ch A ----- | | | |
    o----|SA602|---| 2kHz |------. |USB I/O|
    |mixer| | LPF | | -------
    ----- ------ | |
    | LO | |
    | ------ -------
    o----. |24bit | | AVR |
    | | | ADC |---| |
    | | |stereo| | uC |
    | | ------ -------
    | | ------ | |
    ch B ----- | | | | |
    o----|SA602|-----| 2kHz |----' |
    |mixer| | | LPF | |
    ----- | ------ |
    | |
    | |
    | ------ ------ |
    f1| | | |AD9834| |
    '--| 20MHz|---| DDS |-----o
    | LPF | | | |
    ------ ------ |
    |
    /| ------ ------ |
    SIG / | f0| | |AD9834| |
    OUT__/ |--------| 20MHz|----| DDS |----'
    f0 \ | | LPF | | |
    \ | ------ ------
    \|
    BUFFER

    [view in monospaced characters]
     
  2. Guest

    Yes, it won't work. You are mixing down to DC, and you have lost the
    ability to sense phase. For example, if you read an amplitude of 0 on
    'B', you don't know whether this is due to a large attenuation between
    Sig Out and B, or whether it's due to a 90 degree phase shift between
    Sig Out and B.
    Try a quadrature downconverter:

    .. To PC
    .. *
    .. |
    .. -------
    .. ------ |FT245BM|
    .. ch A ----- | | | |
    .. o-+--|SA602|---| 2kHz |------. |USB I/O|
    .. | |mixer| | LPF | | -------
    .. | ----- ------ | |
    .. | | LO | |
    .. | | ------ -------
    .. | '----. |24bit | | AVR |
    .. | | | ADC |---| |
    .. | | |stereo| | uC |
    .. | | ------ -------
    .. | | ------ | |
    .. | ----- | | | | |
    .. '--|SA602|-----| 2kHz |----' |
    .. |mixer| | | LPF | |
    .. ----- | ------ |
    .. | | |
    .. | | |
    .. | | ------ ------ |
    .. | f1| | | |Quad. | |
    .. | '--| 20MHz|---| DDS |-----o
    .. | SIN | LPF | | | |
    .. | ------ ------ |
    .. | | |
    .. | ------ |
    .. | | | |
    .. '-------| 20MHz|-----'
    .. COS | LPF |
    .. ------
    ..

    You will need one of these for chan A, one for chan B. You can get by
    with one if you only want to do 'through' measurements.

    I recommend using a single DDS for both stimulus generation and
    downconversion LO. This avoids problems with phase shifts between the
    two DDSs in your first solution.

    Regards,
    Allan
     
  3. Guest

    Maybe I should read your post more closely before I comment next
    time...

    Your use of a frequency offset does indeed allow you to sense phase
    differences between A and B.

    Regards,
    Allan
     
  4. Adam S

    Adam S Guest


    wheew, I spend quite a while with pencil and paper playing around with
    trigonometry identities trying to see if phase and magnitudes of the two
    IFs do indeed represent that of inputs A and B.

    Thanks

    Adam
     
  5. Chris Jones

    Chris Jones Guest


    Are you already aware of the N2PK network analyser? (Try google). I think
    your idea of using an IF frequency rather than mixing down to DC might be
    preferable but there probably isn't much difference. Using a non-DC IF,
    you might be able to make the receiver digitally filter out responses due
    to the third (and other) harmonics of the source frequency, which could
    make the signal source purity less difficult. The third and fifth
    harmonics will unavoidably be present in the receive LO (since the mixer
    only works well with hard driven LO port, creating odd harmonics), but
    since the source and LO frequencies are different by 1* fIF in your system,
    the harmonics of the source will be received and downconverted to
    frequencies 3fIF and 5fIf etc. which are not the same as the desired IF
    signal so you can filter them out.

    I would also like to make a VNA, but to be interesting to me I would really
    want it to cover the 2.4GHz WLAN frequencies so I could test filters,
    antennas etc. The receiver part looks feasible to me using AD8343 mixers,
    or maybe diode samplers, but the signal source and the s-parameter test set
    seem like the challenging parts to me. Rather than a 10kHz to 3GHz signal
    generator, I wonder is it easier to generate a very wideband pseudo-random
    bit sequence using ECL type comparators or latches, and to use that as the
    stimulus, recording the response of the network in the time domain using a
    second shifted PRBS generator driving a correlator, and computing the
    frequency response. I fear that this would take too long for a very wide
    sweep however.

    Chris
     
  6. Adam S

    Adam S Guest


    That last part paragraph flew over my head. I've heard of PRBS
    generators used as a stimulus for audio speaker frequency analysis.
    Maybe there is someway of correlating two of these generators and use
    digital sampling (ones and zeros) at super high speeds to somehow
    compute the frequency response. Could just be my imagination.

    I have seen the renowned N2PK network analyzer published on the web. I
    would of though dynamic range could be seriously improved by using AC,
    as DC would have drift problems. Once the 1kHz is inside the PC then
    there are no noise/drift sources to speak of. There is also the other
    advance as you mentioned, about harmonics of the source signal being
    blocked by the 1kHz since they will not be 1kHz relative to harmonics of
    the LO. But the catch is the DDS generates a bunch of non-harmonic
    related spurs that shift all over the place as you sweep the frequency.
    At some frequencies multiple harmonics will get mixed down to 1kHz and
    cause error on the output. The DC mixing method of the N2PK will have
    every spur joining together from each DDS and mixing down. Some will
    subtract some will add. Needs more thinking..

    Adam
     
  7. Chris Jones

    Chris Jones Guest

    I think that the N2PK design would not have bad DC offset problems because I
    think he flips the phase of the LO and then subtracts the two readings,
    thus getting rid of any offsets in the mixer and LO leakage into the
    receiver input. In some ways this is equivalent to having a very low
    frequency offset between the LO and source, the frequency offset being
    equal to one half cycle as often as the PC feels like it. It's probably
    not quite as good as a proper offset between the two frequencies though.
    With regard to the DDS spurs, I think that the ultimate answer is to have a
    different kind of synth which I want anyway because I want coverage up to
    2.5GHz at least. If that is too much work, then a big improvement could be
    had by moving from the AD9851 to the AD9951, which I think has more DAC
    resolution and much better spurs (and is more than twice as fast too).

    Chris
     
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