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3 port S-parameters

Discussion in 'Electronic Design' started by Philip Newman, Oct 23, 2003.

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  1. If one was designing a mincrostrip passband filter for a 2-port network, one
    would look at s21 and s11 (to see how much signal was being reflected).

    However, if one had two filters cascaded with each other, with a trunk line
    terminating each filter segment (ie two multiplexer segments) so that the
    first segment had a passband at 5.2GHz and the other at 6.3GHz, the circuit
    would effectively be a 3-port network.

    Which S-parameters would we need to observe in order to see the filter

  2. Don Pearce

    Don Pearce Guest

    Assuming Port 1 was the common filter terminal, you would want S11,
    S21, S31 for a quick stab. If the filter provides out of band
    termination, then S22 and S33 would be useful. Finally, to check the
    isolation between the two filtered ports, S32.


  3. i think port 1 is the input, port 2 is the end of the trunk, and port 3 the
    terminal of the second trunk line

    does this matter?

  4. Don Pearce

    Don Pearce Guest

    Well, I'm not sure I've grasped the topology of the circuit fully, but
    I am assuming a sort of T, with the input on the stem, and the two
    different frequencies on the branches. In those terms, port 1 should
    be the input. You can make the measurement on a two port analyser by
    swapping connections, but be sure to terminate the unconnected filter
    ports properly.


  5. o-filter_segment_one__o___filter_segment_two-o
    trunk line

    input on filter segment one, the other ports are terminated by 50ohm

    hope this helps (hope it posts properly too!)

  6. Don Pearce

    Don Pearce Guest

    If the input is on filter segment one, and segment two is on a
    different frequency, what is port two (right of diagram) for? Looks
    like nothing can ever come out there.


  7. port 2 isnt really a port, its a terminating resistor at 50ohms. the trunk
    line is also terminated in this manner.

    i perhaps should have said this before, but we are trying to implement a
    5-channel contiguous multiplexer filter, that can be seen in Christan
    Rauscher's IEEE paper, Efficient Design Methodology for Microwave
    Multiplexers using infinite-array prototype circuits.
    this is readily available of the IEEE website.

    We are trying to implement it in Agilent's Advanced Design System, and
    seperatley, both filters work, but when they are cascaded together, it
    becomes a little more tricky to determine which of the s-parameters we need.

  8. Don Pearce

    Don Pearce Guest

    OK, the measurement you need primarily is the one which represents the
    way you will be using it. But you may still find value, particularly
    troubleshooting, to treat that central 50 ohm point as a port and
    measuring outwards through both filters.


  9. so which s-parameters is that? pardon me if that's a stupid question

  10. Don Pearce

    Don Pearce Guest

    If you go back to my first reply, and assume that the centre (dummy
    load) port is what I called the input. Otherwise, if you simply need
    the straight performance then your original idea is dead right.


  11. ok, thanks for all your help!! will try it out tomorrow

  12. i made a mistake with my circuit topology

    should have looked like this:


    but the s-parameters were correct, so thanks!! s11, s21, s31 and s41 were
    the ones we wanted


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