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HP8753A measurement help required

Discussion in 'Electronic Equipment' started by David, Jul 13, 2007.

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  1. David

    David Guest

    Hi,

    I have just purchased a HP8753A VNA and need some help setting the thing up.

    I want to start off just measuring impedance of lumped components but I
    cannot get any sensible readings. I have tried all sorts of Calibrations
    from the operation manual but still no luck.

    I have the HP85044A Test Set and the 50 Ohm Calibration set.

    I am trying to connect a coaxial lead to the Test port on the Test Set
    and so a 1 Port measurement. I have tried both RESPONSE cal and 1-Port
    Cal and placed a short,oopen and 50 Ohm load at end of the coax and then
    after calibration I place a leaded cap but do not get anything like I
    see in RF books I Have.

    Any help much appreciated.

    Regards

    David
     
  2. g

    g Guest

    David

    For a rudimentary measurement, first select S11 under measure.

    Leave test port 1 open.
    Next select Cal and to perform a response calibration by
    selecting "open"
    Next under Format, select Smith Chart.

    You can now place a DUT at the test port and see S11 directly or else,
    by selecting marker, read the R +- jX at the marker position.

    g
     
  3. David

    David Guest

    G,

    Thanks for that.

    The Test port is an "N" connector. How can I attach a capacitor to it ?

    I tried setting up as you suggest and used a length of coax from Test
    port to component.

    When I select Smith chart I get what looks like child's scribble all
    over the chart.

    Regards

    David
     
  4. g

    g Guest

    David
    I'd suggest using a solder pot type N female. Connect the solder-N to
    your test cable and calibrate with the DUT absent.
    That probably indicates that it is measuring correctly. Your DUT is
    being rotated (transformed) by the length of cable. Calibrating with
    the cable and N fitting (sans DUT) first will eliminate that, leaving
    your measurement plane located *at* the DUT instead of some distance
    down your test cable from it.

    g
     
  5. David

    David Guest

    G,

    I connected a coaxial cable (about 300mm length) to the test port.

    Sweep was set between 300kHz and 1.3Ghz

    The measurement was set up for Smith Chart to measure S11

    Auto Scale was applied

    A 1-port calibration was performed. (Q: What do I use for a load at the
    end of the flying coaxial lead. The precision Load I have is "N" type) I
    used 2 x 100 Ohms resistors as "Load" for Cal.

    I attached a 27pF leaded capacitor (with leads cut short as possible) to
    end of coaxial cable.

    When I looked at the response I saw a Circle that indicated the
    component became inductive around 500MHz (I presume this is SRF).

    The odd thing is that the capacitance reads only 3.1pF at 1Mhz and does
    not read 27pF until 369 MHz ?
     
  6. g

    g Guest

    David
    Not much point in using a load for measuring a low loss/reactive
    component like a capacitor. Just use the Response:Open cal and nothing
    else. That should leave you seeing a high impedance after the cal and
    before the device is connected.
    Perhaps. Or maybe you tried to do a full S11 cal rather than response
    only? You really don't need the load to get pretty close for measuring
    a reactance. Using a load will help your accuracy for well matched
    devices. Using open gives you a measurement for small capacitances and
    using a short for small inductance. You can do an open:short:load cal
    if you have all standards in the same connector type for which you have
    a defined cal kit in the instrument. Otherwise just use response cal.
    What do you see before you attach the DUT? Should look pretty much like
    a dot on the right side of the Smith Chart and read out as a high Z with
    perhaps a small capacitance portion. This represents the modeled
    fringing capacitance of an "open".


    g
     
  7. David

    David Guest

    G,

    Thanks heaps, I am starting to actually measure something now. It was
    frustrating as I was looking forward to receiving the VNA but found once
    it arrived that I didn't actually know how to use it :)

    When I do the Open only Response cal I get a dot on the Resistance line
    on right hand side. R is jumping around but reads several "K" Ohms.

    When I connect now a 22pF leaded cap it shows 22p at low freq and starts
    increasing above 30MHz. The response curve of the component crosses the
    Resistance line at around 350MHz and the cap looks inductive above this.

    By 150MHz this leaded cap is 150pF. No wonder I had problems building
    prototypes using leaded components at VHF in the past. I did not realise
    the capacitance changes so much with frequency.

    Do I use this same methodology if I want to measure SMD caps and
    inductors ? Just attach the cable to PCB without component and cal for
    open than solder in the cap or for inductor, form a short to gnd plane ,
    cal and then replace short with inductor ?

    Finally, What would I do to measure an RF IC in place on a PCB ? I have
    a SA605 Receiver IC that should have input impedance that looks like
    around 1K Ohm in par with 2.5 pF. As this is capacitive, would I do
    something like solder coax to pcb pad and have coupling cap to IC
    removed for OPEN cal and then insert the coupling cap to device to
    measure impedance.
     
  8. g

    g Guest

    And you're only using about 1% of it at this point, there's lots more,
    particularly if you have time domain! :)>)
    That's right. If you look carefully at 3 GHz you may notice that it is
    starting to rotate clockwise around the edge. That's the modelled
    fringing capacitance of the "open" standard you have selected as part of
    the Cal kit you are using.
    The capacitance almost certainly doesn't change this much. You're seeing
    the actual component characteristics which are closer to series
    connected RLC. The L is "tuning out" the C on the way to series
    resonance. When you cross the horizontal axis, you are at resonance and
    all that is left is the ESR of the DUT.

    Yes, basically. For a really refined measurment you would use cal
    standards designed for the environment you are measuring in (PCB).
    However for many purposes, a simple resopnse cal in situ but sans DUT is
    enough. You use the cal device(s) most appropriate for the DUT since the
    measurement effectively is comparing the known cal with the unknown DUT.
    If you have a situation such as you mention where the DUT looks both
    capacitive (low freq) and inductive (higher) you may want to use more
    than a single device for the response cal.
    Same thing, but use a thru cal device. You'll have to figure out how to
    connect the test ports together in the measurement environment. Maybe
    just use a couple of semirigid cables soldered to the PCB and connected
    by a piece of microstrip.


    The goal is to make the test fixture as much like the
    application/measurment environment as possible and find cal standards
    that give you as much information about the desired paramater as possible.

    I think Agilent might have some VNA basics app notes on their web site.
    Have you looked?

    g
     
  9. David

    David Guest

    G,

    Thank you for all the helpful information you have given me.

    I also looked at a document from Agilent that explains how to make Test
    Fixtures to use as standards using microstrip.

    Another site shows actual "N" connectors and SMA that are shorted, open
    and a Load (2 x 100R) and then another where the DUT is soldered across
    the back of the connector.
     
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