3 port S-parameters

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

1. Philip NewmanGuest

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
characteristics?

Phil

2. Don PearceGuest

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.

d

_____________________________

http://www.pearce.uk.com

3. Philip NewmanGuest

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?

phil

4. Don PearceGuest

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.

d

_____________________________

http://www.pearce.uk.com

5. Philip NewmanGuest

o-filter_segment_one__o___filter_segment_two-o
|
|
trunk line
|
o

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

hope this helps (hope it posts properly too!)

phil

6. Don PearceGuest

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.

d

_____________________________

http://www.pearce.uk.com

7. Philip NewmanGuest

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.

phil

8. Don PearceGuest

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.

d

_____________________________

http://www.pearce.uk.com

9. Philip NewmanGuest

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

phil

10. Don PearceGuest

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

d

_____________________________

http://www.pearce.uk.com

11. Philip NewmanGuest

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

phil

12. Philip NewmanGuest

i made a mistake with my circuit topology

should have looked like this:

filter_3___
|
trunk
|
filter_2___
|
trunk
|
filter_1___

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

cheers

phil