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Smith Chart question

Discussion in 'Electronic Design' started by Paul Burridge, Apr 29, 2004.

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  1. Hi all,

    Firstly, apologies for raising a matter of relevance to electronics on
    this group.
    I have posted a .gif image to a.b.s.e under the same subject heading
    in which a Smith chart is displayed with four impedance
    transformations shown marked in black, blue, red and yellow. The black
    arrow shows the effect of adding series capacitance; the red arrow
    shows the addition of shunt inductance. What do the blue and yellow
    arrows indicate, though? I'm particularly interested in the effect of
    the yellow arrow, which starts off in the direction of increasing
    series inductance, peaks and then starts heading south again towards
    increasing C. Is that really what it's showing and would such a
    transformation that passes through a zenith be used in real life? This
    has got me completely baffled for some reason. The answer should be
    obvious but for some reason I have a complete mental block on seeing
    straight. Any informed advice welcome.

  2. Active8

    Active8 Guest

    Shit like that will get you killfiled.
    It's orange, Paul.
    Series L
    It's *not* heading toward series C or even decreasing series L. It's
    moving in a positive direction along a constant reactance circle.
    Positive reactance being inductive. It's heading for infinity.
    you have a zenith? good TV.
    Maybe, the actual impedance it's heading for is unknown since we
    don't know how the chart's been normalized. In real life, you'd want
    to head from this end point to the center of the chart normalized to
    the characteristic impedance of a transmission line or the input
    immpedance of a filter maybe. OTOH, you could be feeding a high Zin
    FET, so IDKWTF.
    A pshrink and an optometrist? A pshrink and an optometrist were out
    golfing one day...
  3. I know. Jim Thompson in particular would be furious at me.
    It looks orange through this scanner. The 'black' line is actually
    dark green!
    Eh?? One's in the inductive region, the other's in the capacitive. And
    "series L" by itself doesn't really tell me anything.
    Er, no. All the lines I've marked are on constant *resistance* circles
    which is I assume what you meant to say. I'm still none the wiser
    though. :-|

    <equally baffled>
  4. Tim Wescott

    Tim Wescott Guest

    Really, if you don't feel qualified to comment about Bush you could at
    least snipe about Blair.
    You don't care if it's in the "inductive" region, you only care where
    it's going. Series L will add inductance with constant resistance,
    shunt L will add inductance with constant conductance, similar for
    capacitive series & shunt. The blue line is entirely in the capacitive
    region, but it will take you all the way over to inductive if you keep
    it up.
    Only two of the lines are on constant resistance circles; the other two
    are on constant conductance circles. The circles centered on the right
    edge are usually taken as resistive; the circles centered on the left
    are usually taken as conductive (you can do it the other way if you feel
    like it).
  5. Active8

    Active8 Guest

    effect of adding series inductance.
    The top one is. The bottom is headed for 0 net reactance, but if it
    doesn't change it's way, it'll keep on heading from net capacitive
    thru 0 to net inductive.
    right. They're on const R circles. They're both traversing the
    reactance circles in the same positive direction, in this case
    clockwise. I can't get any clearer without drawing pictures or using
    many more words than necessary. Opfen der book.
  6. Active8

    Active8 Guest

    Good observation.
    Which brings me to what I forgot to mention. That looks like a real
    good friggin' chart like the ones from AD. All on has to do is look
    at the printing to see if they're headed in the negative or positive
    resistance, conductance, reactance or susceptance direction. Then
    it's a matter of knowing that negative net reactance is capacitive
    as is net positive susceptance.
  7. Jim Thompson

    Jim Thompson Guest


    I really could care less... I don't see Paul's posts unless someone
    replies to them, then I kill the thread ;-)

    ...Jim Thompson
  8. Tom Bruhns

    Tom Bruhns Guest

    Getcherself a Smith chart program, and experiment with it. In fact,
    you probably already have one in the form of RFSim99. Shunt L's and
    C's result in arcs that follow a constant admittance curve. For
    example, set up a port1 source, and a load consisting of parallel 150
    ohm resistor and capacitor of, say, 0.01pF. Simulate. Set the plot
    to smith x/y. Set the frequency range to 100MHz to 101MHz. Note that
    the point is on the first larger admittance circle from the center of
    the plot. Now "tune" the capacitor up in steps and see where the
    point goes. Stop when you get to 14.8pF. Now add in a series
    inductor between the source and the parallel RC. Start with it very
    tiny, and adjust it up. Note that you are moving along a constant
    resistance line. Note that when you get to about 112nH, you're at the
    center of the chart: 50 ohms in this case. You just designed a
    matching network to match from 150 ohms resistive to 50 ohms resistive
    at 100MHz. Note that if you swap to a shunt inductor and series
    capacitor, you can do the same thing, on the upper half of the chart.
    Works for any sort of ladder network you want to put together: pi,
    L-pi, T, several sections...

    RFSim99's Smith chart, used this way, isn't as nice as something like
    WinSmith, but it's servicable.

    Try also putting in a section of transmission line: note that the
    point moves in a circle centered on the line's characteristic
    impedance. In a line with loss, it will be a spiral inward as you
    lengthen the line.

    Shunt L and shunt C don't change the admittance but do change the
    suseptance: the two are orthogonal. Series L and series C don't
    change the resistance, but do change the reactance. Again the two are
    orthogonal. Inductors add suseptance or reactance; capacitors
    subtract (in that their reactances and suseptances are negative).


    (I'm quite sure I've seen a good Smith chart tutorial on the web. Do
    a google search...)
  9. That would profit me not a jot. However, I've a much more practical
    solution to our lying PoS Prime Minister: I'm leaving the country -
    and mostly because of him and his 'government, too.
    Enough's enough.
    I find it better personally to simply convert all the susceptances to
    reactances by taking the inverse of each before I "un-normalise" so
    don't worry about that aspect.
    Sorry to say that notwithstanding both yours and Mike's efforts so far
    I'm still pretty much in the dark and my original question hasn't been
  10. I have geoffneted der buch but all the examples in it are really
    easy-peasy ones that use combinations of tracks along those constant
    resistance circles where those tracks are all near vertical - like the
    black and red ones on the chart I posted. I understand those alright.
    What I'm having difficulty with is when a track/trace/arrow - call it
    what you will - follows a near *horizontal* path. It appears to be
    moving sideways and neither towards the inductive region or the
    capacitive region.
    Are you saying - I *think* this is what you are saying - that the
    orange line, for example, solely represents an increase in series L
    right along its length, notwithstanding that it starts to drop back
    down towards the capacitive region after about the halfway point?
    Similarly, are you saying that even though the blue line is nearly
    horizontal that it actually, in fact, represents an increase in series
    L, too?
    If that's the case then surely it's misleading for publishers to show
    the top hemisphere of the chart as being inductive territory and the
    bottom hemisphere as being the capacitive region?

  11. Tim Wescott

    Tim Wescott Guest

    Get a copy of the ARRL's UHF Experimenter's handbook, and a good book on
    microwave amplifiers (or a book on transmission lines). They'll give
    you more info on Smith chart's than you can handle.

    Isn't there info to google on for this?
  12. Reg Edwards

    Reg Edwards Guest

    The Smith Chart went out of date with the invention of the personal

    As an approximation it suffers from serious misleading errors at low

    It was adopted in the late 1930's from simplification of similar charts
    which had been in use since the Victorian age to cater for the splurge in
    the use of HF coaxial lines particularly during WW2.

    Old timers now use it only for sentimental, recreational purposes.
  13. Rick

    Rick Guest

    no - they follow a curve of constant CONDUCTANCE. No such thing as
    a curve of constant admittance.
    conductance circle
    Only if the line's characteristic impedance is the same as the
    normalising impedance of the chart. Otherwise, the equation for
    the centre of the circle is a bit of a bitch.
  14. Active8

    Active8 Guest

    Piles. And the UHF book isn't nearly as good at explaining the Smith
    Chart as Chris Bowick's little book, which Paul has. I'm suspecting
    either a lack of fundamentals, a lack of an intuitive feel for
    electronics in general, or something else yet to be revealed.
  15. Active8

    Active8 Guest

    Yeah. Look at the *whole* const R circle that the line is on and
    realize that clockwise is moving in a positive net reactance
    direction. positive net reactance is inductive, so you're moving in
    an increasing inductance direction.
    I know what picture yer referring to, but I didn't have a problem
    with that because the fundamentals of reactance have been in my
    beaner since I was a teen. I could also see that the lines were arcs
    drawn on a circle and no one said I had to start or stop at any
    particular point on the circle. He did draw them with an arrow.
    Remember rays? the keep going in one direction.
  16. This is another problem one faces in trying to get to grips with this
    aspect of the science. Admittances, susceptances, reactance,
    conductance, resistance, impedances and so on. Quite a lot of similar
    sounding terms to differentiate between. The difficulty being
    compounded when someone makes a mistake like the one above. And
    earlier Mike used the word "reactance circle" when he meant
    *resistance* circle. Thank god I'd done enough homework to be able to
    spot those errors for myself and the only outstanding query was the
    one I originally posted about - which Mike has answered to my
    satisfaction now.
    Thanks, all.
  17. I did - at the outset a few weeks ago- come across a rather nifty
    little interactive site for Smith chart manipulation. Analogue
    Electronics, IIRC. Very useful for getting a feel for it. My original
    questiion though was not fathomable from it as the scale of the
    virtual chart was very small. But in all other respects it was very
    helpful. As for Bowick's book, again, no specific info on what happens
    when an arc is moving through what *appears* to be neutral points. I
    reailse now that it's not; just the necessarily distored nature of the
    chart making it appear so at first sight.
    So yeah, if the yellow arrow were to continue all the way down to the
    horizon at infinity, it still wouldn't be going any more capacitive or
    less inductive; it's just the distortion of the chart making it appear
    as if it is.
    HTF I've got the above right or I *am* in trouble!
  18. As someone who's probably older than the Chart, Reg, I'm surprised to
    hear you say that. I believe a lot of folks would disagree with you...
  19. Active8

    Active8 Guest

    Yeah. Sounds like you got it. When I wrote my (unfinished) smith
    chart app's GUI, I had to write the equations for the circles,
    calculate the intersections, plot them, and then I said "f it" and
    captured different color schemes to use for the app. The whole time
    I thought the process sucked because of what the chart really is. A
    distorted cartesian plane.

    Think of resistance as being the y axis with all horizontal grid
    lines representing const R, and the others, const X. remember the
    basic triangle that desribes a phasor? the base is the R, the height
    is the net X, and the hypotenuse is the Z = R + jX = sqrt(R^2 + X^2)

    if you take the vertical lines at infinity ( 00 ) , pinch the ends
    together and bend them back around to the same point ( 00 ), you get
    the R circles. With the horizontal lines, you pinch one end together
    and spread the other ends out. Actually, you're pinching both ends,
    but bending the top and bottom back around in their respective
    directions (leaving the x axis alone) to make 2 families of circles
    but the 0 R cicle that bounds the chart makes it look otherwise.
  20. Reg Edwards

    Reg Edwards Guest

    "> >Old timers now use it only for sentimental, recreational purposes.

    Well, half a dozen perhaps. And even they never use it in anger.

    Face the facts!

    As for me, after 60 years of experience with transmission lines, from 0.1 Hz
    to 3 GHz, I have never used the chart other than out of mild curiosity to
    discover what other people might do with it. ;o)
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