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Impedance matching / LPF question

Discussion in 'Electronic Design' started by neddie, May 5, 2009.

  1. neddie

    neddie Guest

    Hi to all.
    RF is not really my strong point , so I could use some advice here.
    I have a simple Collpits oscillator running at 433.92Mhz. It has a
    strong output signal at 433Mhz , but unfortunately also at 868Mhz.
    In fact the signal at 868Mhz is also very strong. I would like to
    attenuate this harmonic , and the others
    that I can't see(My Spectrum analyser only goes to 1Gig), which are
    definitely there.
    I want to put in a 5th order low pass filter. I have Chris Bowick's RF
    design book , which shows some
    easy ways to design a filter for this. The problem is I don't know how
    to calculate the output impedance
    of the oscillator , which is obviously needed in the calcs. I'm
    assuming the load , which is a 1/4 length piece
    of wire to be about 75 ohms , give or take.
    The data sheet I have for the BRF93a only shows S11 at 400 and
    500MHz , so I have to interpret the
    impedance at 433Mhz , but it gives ma a starting point. This is also
    at Vce of 8V and 25ma , which is not exactly where I'm sitting , but I
    think is close enough.
    I get a s11 of 0.211 , angle -137deg.
    Plotting this on a Smith chart shows an impedance of 35.3-J10.5 ohms.
    I assume this to be roughly the output impedance of the transistor.
    I'm not even sure if I'm on the right track at all :0(
    Assuming this is correct , Do I use this as the source impedance in my
    calculations , or do I then have
    to take into account the other components around the transistor.(I
    assume so)
    It would then be (attached is a schematic in LTSpice)
    (35.3-j10.5 + 47) // (L1 // (C1+c2)) . Hope this makes sense. It's the
    output impedance of the transistor
    in series with the emitter resistor. All this in parallel with the
    parallel combination of the inductor and
    the 2 caps in series , phew.....
    What I need to know is am I on the correct path at all , or am I
    missing the boat all together :0(
    Any pointers would be great.
    Cheers
    Rob


    Version 4
    SHEET 1 880 680
    WIRE -16 16 -160 16
    WIRE 96 16 -16 16
    WIRE 176 16 96 16
    WIRE 176 32 176 16
    WIRE -16 80 -16 16
    WIRE 96 96 96 80
    WIRE -160 144 -160 16
    WIRE 176 144 176 112
    WIRE 288 144 176 144
    WIRE 432 144 288 144
    WIRE 448 144 432 144
    WIRE 560 144 528 144
    WIRE 576 144 560 144
    WIRE 672 144 656 144
    WIRE 704 144 672 144
    WIRE 784 144 768 144
    WIRE 176 160 176 144
    WIRE 288 176 288 144
    WIRE -16 208 -16 160
    WIRE 112 208 -16 208
    WIRE 784 224 784 144
    WIRE 432 240 432 144
    WIRE 560 240 560 144
    WIRE 672 240 672 144
    WIRE 176 272 176 256
    WIRE 288 272 288 240
    WIRE 288 272 176 272
    WIRE -16 288 -16 208
    WIRE 176 288 176 272
    WIRE 288 288 288 272
    WIRE -160 400 -160 224
    WIRE -16 400 -16 336
    WIRE -16 400 -160 400
    WIRE 176 400 176 368
    WIRE 176 400 -16 400
    WIRE 288 400 288 352
    WIRE 288 400 176 400
    WIRE 432 400 432 304
    WIRE 432 400 288 400
    WIRE 560 400 560 304
    WIRE 560 400 432 400
    WIRE 672 400 672 304
    WIRE 672 400 560 400
    WIRE 784 400 784 304
    WIRE 784 400 672 400
    WIRE 176 416 176 400
    FLAG 176 416 0
    FLAG 96 96 0
    SYMBOL res 160 272 R0
    SYMATTR InstName R1
    SYMATTR Value 47
    SYMBOL ind 160 16 R0
    SYMATTR InstName L1
    SYMATTR Value 68n
    SYMBOL cap 272 176 R0
    SYMATTR InstName C1
    SYMATTR Value 1.5p
    SYMBOL cap 272 288 R0
    SYMATTR InstName C2
    SYMATTR Value 3.3p
    SYMBOL voltage -160 128 R0
    WINDOW 123 0 0 Left 0
    WINDOW 39 0 0 Left 0
    SYMATTR InstName V1
    SYMATTR Value PULSE(0 12 0 10n)
    SYMBOL res -32 64 R0
    SYMATTR InstName R2
    SYMATTR Value 10k
    SYMBOL cap 80 16 R0
    SYMATTR InstName C5
    SYMATTR Value 100n
    SYMBOL cap 544 240 R0
    SYMATTR InstName C6
    SYMBOL cap 656 240 R0
    SYMATTR InstName C7
    SYMBOL ind 544 128 R90
    WINDOW 0 5 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName L2
    SYMBOL ind 672 128 R90
    WINDOW 0 5 56 VBottom 0
    WINDOW 3 32 56 VTop 0
    SYMATTR InstName L3
    SYMBOL res 768 208 R0
    SYMATTR InstName R3
    SYMATTR Value 75
    SYMBOL cap 768 128 R90
    WINDOW 0 0 32 VBottom 0
    WINDOW 3 32 32 VTop 0
    SYMATTR InstName C8
    SYMATTR Value 1n
    SYMBOL cap 416 240 R0
    SYMATTR InstName C3
    SYMBOL npn 112 160 R0
    SYMATTR InstName Q1
    SYMATTR Value BFR93a
    TEXT 0 -16 Left 0 !.tran 0 100u 0
    TEXT -88 360 Left 0 ;433.92Mhz SAW
    RECTANGLE Normal 0 336 -32 288
     
  2. Guest

    Hello Rob,

    I would recommend you to make a band pass filter (or a low pass filter
    with extreme ripple) as this requires less components for the same
    attenuation. The antenna itself (for example) a resonating loop) can
    be part of the bandpass filter. Don't make the BPF too narrow in
    bandwidth to avoid component tolerance issues.

    Regarding the antenna. When the physical size of your 433 MHz device
    is very small with respect to a quarter wave length, the impedance of
    your antenna may deviate significantly from 75 Ohms. The behavior of
    a quarter wave depends on the ground. The ground is in your case your
    transmitter + supply wiring (if present). When you fed the quarter
    wave antenna over a large flat piece of metal, you will see something
    between 30 and 50 Ohms.

    Best regards,

    Wim
    PA3DJS
    www.tetech.nl
     
  3. Andrew Holme

    Andrew Holme Guest

    [snip]

    It's more a case of what input impedance the filter presents to the
    oscillator. If the oscillator works with a load RL, then you could design a
    filter with that input impedance.
     
  4. neddie

    neddie Guest

    Does that mean I should load the oscillator with a resistive load to
    find out where I can get max power out and maintain oscillation ,
    then use this as my source resistance for the filter calcs?
    Cheers
    Rob
     
  5. Andrew Holme

    Andrew Holme Guest

    [snip]
    Sucking maximum power from an oscillator is not a good idea. It lowers the
    working Q of the resonator and increases phase noise. In the extreme, it
    may also make startup unreliable. It is common practice to follow the
    oscillator with a buffer stage. This improves frequency stability by
    isolating it from load impedance variations. In your case, it sounds like
    you want to feed the antenna directly from the oscillator. Without a
    buffer, you may find hand-capacity effects around the antenna troublesome at
    433 MHz. There's a trade-off: the more you load the oscillator, the more
    output you will get - up to a point, at the expense of stability and purity.
    But yes you could test with a load resistor and then design your filter for
    that input impedance.

    Another way to reduce loading might be to replace the 3.3p cap with two in
    series (e.g. 6.8p + 6.8p) and take your output from the middle.
     
  6. neddie

    neddie Guest

    Thanks for the help , I'll try what you suggested. I'm not going to
    try and get to much power from the transmitter ,
    so I won't try and "overload" it to much. Range does not have to be
    huge , so maximum power is not that
    much of an issue.I'm more interested in getting the principal
    correct.I also assume over loading the transmitter
    would make the harmonic issue worse!!

    As to my original post. If I wanted to match an impedance to the
    output of the transmitter , would my
    methodology have been correct?

    Cheers
    Rob
     
  7. Baron

    Baron Guest

    Andrew Holme Inscribed thus:
    He will probably find that a tuned buffer amp also helps with a
    reduction in the second harmonic !
     
  8. neddie

    neddie Guest

    Thanks for the tips.
    In fact I think I do have a copy of Wes Haywards book around here
    somewhere. Time for a tidy up :0)

    I see what you are saying about trying to get to good a match , and in
    the process killing the
    oscillator.I'm still not sure if my methodology that I described in my
    first post is correct.
    Any comments?
     
  9. neddie

    neddie Guest

    Thanks to all for the help :0)
     
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