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Vector network analysis question

Discussion in 'Electronic Design' started by [email protected], May 21, 2006.

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  1. Guest

    hi guys,

    After an hour or more of pushing and twiddling various buttons and
    knobs on my VNA I have at least established two standards: I connect a
    short and the (polar) spot flys off to the extreme left-hand-side and
    sits stably on the central horizontal axis. I then remove the short and
    replace it with an open and the spot jumps across 180 degrees to the
    extreme right-hand side on the same plane. All well and good so far.
    Next, I replace the open with a lab-grade 50 ohm termination and I wind
    up with an arc on the outermost circle of the polar chart at about
    two-thirds of the way round the circumference (clockwise) from "12
    o'clock" as opposed to a spot in the dead center of the display as I
    would expect. What have I done wrong? I have checked this with 3
    different 50 ohm loads and they all show an arc at the 'south west'
    corner of the chart. What's goin' on here, guys??
  2. K7ITM

    K7ITM Guest

    You haven't fully described your setup, so it's hard to tell. What are
    you using for a bridge? If I recall correctly, your VNA is an HP8754A,
    which won't do S-parameter measurements by itself; it requires an
    external bridge (an S-parameter test set, or a reflection-transmission
    test set, for example).

    The fact that you see an arc on the outermost circle, assuming that you
    have it set as a unit circle on the linear reflection coefficient plane
    (the Smith Chart outer circle), implies that you have a (nearly) unity
    reflection coefficient. Is something almost disconnected so it looks
    like a tiny series capacitance?
  3. Paul,
    did you do the calibration process ?
    Usually the calibration is to be done for
    the mode you want to measure, S11, S12, S21, S22

  4. Guest

    Okay, thanks. I must admit it is symptomatic of the load's center pin
    not quite making contact for some reason, even though the load is fully
    screwed home. But the same thing happens with 3 different loads!
    I'm using the T/R test set, by the way, although I don't see how that
    could explain the problem. I don't have the full S-parameter bridge but
    am in the market for one.
  5. Dave Platt

    Dave Platt Guest

    Okay, thanks. I must admit it is symptomatic of the load's center pin
    If you're using N connectors, check the center pins of the females.
    Make sure that they aren't splayed open.

    The local repeater association I'm a part of has had repeated problems
    with splayed N connectors. They can be caused by a couple of
    problems. A botched install of an N male can cause its center pin to
    project too far forwards, out of the shell. We've also seen some N
    adapters (Chinese-made I believe) whose center pin was larger than
    usual in diameter, or was not tapered in the usual way.

    In either case, if you screw an N male connector with a bad center pin
    into an N female, the male's pin can force the split segments of the
    center female pin outwards. Once this happens, a _good_ N male
    connector's pin may not make reliable contact with the female's
    damaged center pin. Intermittent nastiness can occur.

    It's sometimes possible to bend the female's split pin sections back
    together with needle-nose pliers, creating a repair which may or may
    not be permanent. A better solution is to replace the damaged female
    connectors entirely, and inspect all N male plugs and discard any
    whose center pins are too large in diameter, not tapered properly, or
    project too far out of the shell.

    I suppose that similar problems might occur on BNC or SMA connectors
    as well, for related reasons.
  6. Guest

    Okay, thanks for that guys.
    One clue that might assist in tracking down this problem is that the
    aforementioned arc that appears at about '37 minutes past the hour' on
    the outermost polar circle when the 50 ohm termination is fully screwed
    in first begins to appear at just before 12 o'clock and gradually
    circumambulates around this outer circle anticlockwise to its final
    position as it's fully screwed home. I'd have (intuitively) thought
    that once the centre pin makes proper contact with its counterpart, the
    trace would suddenly flick to close to the centre. But with no
    experience of VNAs, that's just a hunch. Any suggestions?
  7. Guest

    Sounds like someone has mixed 75 ohm N connectors with 50 ohm N

    50 ohm N males will damage a 75 ohm N female by spreading its contacts,
    often to the point of damage.

    75 ohm N males will fit loose in a 50 ohm N.

    Many people obtain N connectors without actually knowing which one they
    are getting.
  8. Guest

    Oh dear. It appears you may be right.
    I've just checked with the suppliers and they tell me the plugs I
    bought are 75 ohm ones. This really stinks as in their catalogue,
    although they don't specifiy the impedance of these plugs, they do
    clearly state that they're suitable for RG214 coax. AFAIK, RG214 is
    only available in 50 ohm - if anyone knows otherwise please tell me. It
    took me a good deal of time and trouble to make up these patch leads so
    all in all, I ain't exactly pleased.
    Here's the URL for the catalogue page so you guys can tell who's at
    fault here:
  9. Guest

    Since posting earlier I've been on to the actual manufacturers and they
    reckon these plugs are actually 50 ohms. So the supplier's insist
    they're 75 and the makers say they're 50! I need to know for sure, as
    one or the other is in error. How can one tell, by visual inspection
    and or physical measurement, which type is which? There must be
    something that's visibly different about the two types.
  10. Yes, look at the thickness of the central pin. A 50 Ohm connector has
    a central pin of some 1.6mm thick. The 75 Ohm version is a mere needle,
    something like 0.8mm.

    It's one of my long-time gripes about coaxial connectors that N and
    BNC series of the two impedance levels are sufficiently compatible
    to mate together, but not enough to do so without damage. In a lab
    with someone ignorant of this fact, it's a source of endless trouble.

    Jeroen Belleman
  11. Guest

    Thanks, Jeroen.
    Well on that basis, all the plugs I have here are indeed 50 ohms, then.
    In fact I pulled a centre pin out of a new packet from the same batch
    and it fits the sockets in the VNA perfectly. They all accept the pin
    with a nice snug fit and what's more the sockets show no signs of ever
    sustaining any damage/spreading/flairing/bending and that goes for
    those on the T/R bridge, too. On reconnecting everything, however, the
    problem still persists, although the 50 ohm load is now showing as very
    close to a dead short; virtually the same spot on the Smith chart
    overlay as when the socket is shorted, in fact. Once again, the other
    loads do likewise and they all check out fine as 50 ohms with a DVM.
    Very strange! I wonder if there's some setting I may have got wrong
    somewhere. :-/
  12. Paul-

    What would be the result of calibrating the system with an inductive
    "short"? If it can be done, results of testing with a good 50 Ohm load
    might be skewed.

  13. Highland Ham

    Highland Ham Guest

    Having had a look in my (junk)box with N-connectors I found that most
    silver plated (professional) units have 50 Ohms stamped into the body
    ,whereas the nickel plated units either have no identification or a
    specific manufacturer's code .
    The Farnell catalogue you referred says these N-connectors are nickel
    plated . Try to find any stamped code on your units . If any I'll be
    happy to compare with those on my connectors.

    Frank GM0CSZ / KN6WH
  14. Guest

    Good thinking, Fred. That's theoretically possible, I guess. But I'm
    carrying out these checks purposely at only around 5Mhz to minimise
    such high frequency effects. You may well be on the right lines, though!
  15. rick H

    rick H Guest

    Have you tried buzzing your home-made leads through with a DVM to
    confirm that they're ok (at dc) - both centre and shield of the coaxial

    Try discarding the test-set for a moment and just connecting Ref to the
    A and B inputs on the VNA (configured to measure loss/gain). If you know
    that your coaxial leads are ok, then you'll be checking that all the
    I/O ports on the VNA are working correctly.

    Have you confirmed that the test-set is correctly connected to the VNA?

    Try terminating unused I/O ports with a 50 Ohm load or with a pad (a
    10dB pad makes a reasonable 50 Ohm load) and then repeating the cal
    and measurement.

    Try calibrating the VNA/test-set without the leads, just using a
    Male-Male adaptor instead. Then see what happens when you measure gamma
    with 50 Ohms dangling off the adaptor.

  16. Roy Lewallen

    Roy Lewallen Guest

    All the 75 ohm BNC connectors I have, have the same diameter center pin
    in the connection region as 50 ohm connectors. The difference is in the
    thickness of the dielectric around the pin in the connection region,
    being much thinner in the 75 ohm connectors. The pin of the 75 ohm
    connector is smaller only where it's in solid dielectric at the bottom
    of the connector.

    Roy Lewallen, W7EL
  17. Mike Monett

    Mike Monett Guest

    Hi Roy,

    There's lots of room for confusion. The Kings tool FAQ implies the
    center contact diameter got smaller when the pin was redesigned and
    the old crimp die set would not work:

    Why won't my Kings 2025-X-9 die set work with the 2065-X-9

    With the redesign of the True 75 Ohm BNC connectors, the center
    contact diameter got smaller to maintain a constant 75 Ohm
    impedance through the connector. Consequently, the center contact
    crimp dimension got smaller also.

    The problem with the inability of the die set to crimp this new
    connector is probably because it is the older, larger die
    dimension. The Kings website has a Cross-Reference Search that
    lists the applicable die set for most Kings connector part

    However, another manufacturer claims their new 75 ohm hardware is
    interchangeable with 50 ohm connectors:

    TRUE 75 Ohm BNC & Connectors

    Where Controlled Impedance is a Necessity

    Emerson Network Power Connectivity Solutions is introducing its
    premium line of TRUE 75 Ohm BNC connectors and adapters. These
    flexible cable 3 Piece BNC Connectors are essential for analog and
    digital high frequency signals.

    They have been designed around MIL-STD-202 for up to a 3GHz
    frequency range, and are constructed of high performance materials
    including a machined nickel plated over brass housing, a teflon
    dielectric and gold plated brass contacts. These connectors are
    designed to eliminate distortion and impedance mismatching caused
    by using 50 ohm connectors on 75 ohm cables, and they safely
    inter-mate with standard 50 Ohm BNC connectors.

    If your junkbox is anything like mine, it may have stuff purchased
    20 years ago mixed with more recent purchases. So it might be
    worthwhile to check carefully before mating different hardware.


    Mike Monett
  18. Mike Monett

    Mike Monett Guest

    So the thought finally strikes me. How do you check the female socket
    dimension? Do you stick a male pin in and see how much friction there is?
    That could bend the contacts and damage the pin.

    How about checking the male pin diameter when it is already installed on a
    connector? There's not enough room for calipers to reach the pin.

    I often find the BNC connectors on my scope go intermittent due to a loose
    center pin. Often the only choice is to use a pair of pliers to squeeze the
    contacts together again. The damage appears to have been caused by some
    off-brand 50 ohm terminators that require an unusual mount of force to seat
    on the connector. But the repair doesn't last. Once the female pin has been
    forced open, it quickly goes intermittent again. This is a big problem
    since the scope connector is usually a special item that is difficult to


    Mike Monett
  19. Roy Lewallen

    Roy Lewallen Guest

    For sure. It looks like it's not safe to mate a 75 ohm connector with
    anything, even another 75 ohm connector, without checking pin diameter.

    Roy Lewallen, W7EL
  20. Maybe your "N" female connector has "splayed" leaves on the center
    contact. This sometimes happens when someone tries to screw a PL-259 UHF
    connector in place by accident.

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"©

    "Treason doth never prosper: what's the reason?
    For if it prosper, none dare call it treason."

    "Follow The Money" ;-P
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