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Network Analyzer Query

Discussion in 'Electronic Design' started by Paul Burridge, Dec 1, 2005.

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  1. Hi guys,

    It turns out there a a few essential parts missing from the VNA I
    bought. Nothing too serious, but mainly some RF interconnects (patch
    leads) that go between the VNA and the transmission/reflection test
    set. These are three leads of 24" and one of 34" all terminated by
    N-type (male) plugs. The length of these cables is critical to
    preserving the phase relationship of the signal under examination, it
    seems.
    Now I can't find any on E-bay or elsewhere, so my first inclination is
    just to make them up myself. But is that going to be problematical WRT
    subsequent measurement errors? How critical is the electrical length
    quoted and what's the best method for attaching the plugs so as to
    preserve Zo so far as possible?
    Thanks,
    p.
     
  2. qrk

    qrk Guest

    Do you have a manual? I would suggest reading it or getting it. HP
    (Agilent) are amassing manuals (service and operation) on-line for
    their older instruments. If you give the model number, perhaps someone
    could help you get a PDF copy of the manual.

    Normally, the instrument and test fixture(s) are normalized each time
    you turn the instrument on. Normalization will null phase and
    amplitude issues. Some HP instruments allow registers to be set for
    cable length, impedance references, ... The operation manual should
    have this information.
     

  3. You should be able to find a specification table for the VNA parts from the
    supplier or from the Internet. I also assume you really are going for the
    higher frequencies as the cable length will be critical.

    Most VNA's have a calibration or normalisation process. This means it should
    be possible to compensate for basically any length of cables ( within
    reason of course!)

    The electrical length should not be that critical after you recalibrated
    your VNA. Make up your cables yourself but be as thorough as possible and
    look for ready made Belkin rigid or semi-rigid cables.

    Cheers


    Dan
     
  4. Okay, thanks Dan (and qrk).
    Yes, I would like to cover up to the full 1300Mhz if possible so need
    to be as accurate as I can be.
    AFAIA, the interconnects (which is all I'm talking about here - they
    interface the VNA with the transmission/reflection bridge) can be
    compensated for electrical length variation from the control panel of
    the VNA by a couple of centimeters either way; that's all.
    Ideally, yes, rigid would be best. In practice, I'm stuck with
    whatever I can get. I've seen these N-type (coax-feed) plugs on e-bay:

    http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=5816184863&rd=1&sspagename=STRK:MEWA:IT&rd=1

    which looked great until I saw the part about "UR43 cable entry." UR43
    is a pretty skinny, flossy cable. I know the cables only need to be 2
    feet long, however, so I guess that's not worth worrying about? - or
    is it? Is the cable capacitance figure relevant? Will it give rise to
    undue phase shift? I'm a bit out of my depth with these finer points.
    :-(
     
  5. Paul,


    1300 MHz is by now way high frequency...

    You should be able to get away with very crude cables.

    Added capacitances etc. You null them while calibrating.
    Should not be anything you need to know about!

    What kind of VNA is it?


    Cheers

    Dan
     
  6. Thanks, Dan. Yeah I know it's practically DC by today's standards, but
    to me with 150Mhz being the fastest thing I've ever worked on, it's
    very unfamiliar territory, believe it or not!
    It's an old HP 8754A dating back to '79. There's a photo of it here:

    http://www.newconservativeparty.org/sub/guestspace/network/vna.jpg

    I like it because 4-1300Mhz is way enough for me and the thing is
    totally repairable - no custom chips or SMDs to screw things up and
    for two hundred quid it was an absolute snip. I basically bought it
    simply to get a better intuitive understanding of how the properties
    of lumped and distributed reactances change over frequency and see how
    components *really* look at higher frequencies. Whilst this doesn't
    require any absolute degree of accuracy, I'd still like to do all I
    can to minimise any measurement errors. I might want to measure some
    active-device S-parameters at some point.
    Thanks for your input on the subject.
     
  7. JosephKK

    JosephKK Guest

    IIRC i actually used one back in the mid 80's. you had to tell the
    instrument the physical length and it would calibrate and analyze out the
    rest of the electrical length and residual impedances.
     
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