How to test a directional couplers? - RF Cafe Forums
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Post subject: How to test a directional couplers? Posted: Tue Jun 16, 2009 12:31 pm
Joined: Tue Jun 16, 2009 12:25 pm
I have got few directional couplers pretty
old one manufactured by company named anzac electronics its not in existence was closed in 1960 itself...I dont
have the data sheets ...
So I need to find the specifications for those directional couplers and need to test
them whether they are working properly or not...Can any body please suggest me some experiments to test a
directional coupler using Vector network analyzer..
Please do reply
Thanks a lot...
Post subject: Re: How to test a directional couplers?Posted: Wed Jun
17, 2009 11:38 am
Joined: Thu Sep 25, 2003 1:19 am
Basically, performing return loss measurments on all ports and insertion loss measurements between
all ports will tell you if the couplers are performing well and what the frequency range and coupling value is.
You could also determine directivity which is Coupling - Isolation.
Is one of the ports terminated?
I'm sure RFCafe has appnotes, if not, checkout Macom and Minicrcuits for some tutorial.
Post subject: Re: How to test a directional couplers?Posted: Tue Jun 23, 2009 11:55
Joined: Tue Jun 16, 2009 12:25 pm
the isolation port is terminated
how do i perform the test? I have a tow port VNA...
should i just give input and check
whether i am get output for the desired frequency range? I did check rf cafe and mini circuits they dont provide
how to test DC for unknown configurations...
Post subject: Re: How
to test a directional couplers? Posted: Tue Jun 23, 2009 12:43 pm
Joined: Sun Aug
03, 2003 2:02 pm
Location: Erie, PA
Here is a page I have on
the basics of directional couplers, but it does not have explicit instructions for how to test a directional
So, here is a
brief description of how to determine the fundamental parameters of a directional coupler. Since your couplers are
configured with the reverse port (Port 4 in the drawing) internally terminated, that makes the process a bit
simpler. Physically, the layout of the directional coupler is similar to the drawing, with the mainline path along
the long dimension of the body, and the coupled port(s) perpendicular to that.
For clarity when referring
to port numbers, network analyzer ports will be referred to as Port1NA and Port2NA, and directional coupler ports
will be referred to as Port1DC, Port2DC, Port3DC.
1) Calibrate your network analyzer (NA) across the
frequency range that you will be testing. If you do not know the frequency range, then cal across the entire
bandwidth of the NA.
2) The first test will be the mainline loss. Connect Port1NA to Port1DC (input), and
Port2NA to Port2DC (output). Place a calibrated 50-ohm termination on Port3DC (coupled). The insertion loss within
the operational bandwidth of the coupler will typically be less than 1 dB, so if you did not know the BW to begin
with, the 1dB points will be a good ballpark number. You can note the S-parameters as needed in one or both
directions (S11, S21, S12, S22).
Note: If you cannot obtain an insertion loss value of less than 1 dB anywhere
in the band, then either your network analyzer does not cover the DC's bandwidth, the assumed DC port
configuration is not correct (try other port combinations), or the coupler is broken.
3) The next test will
be the coupling factor. Connect Port1NA to Port1DC (input), and Port2NA to Port3DC (coupled). Place a calibrated
50-ohm termination on Port2DC (output). The measured S21 value in the bandwidth determined in the first
measurement is the coupling value wrt to the input. Again, you can note the S-parameters as needed in one or both
directions (S11, S31, S13, S33). If you measure -20 dB in the middle of the band, then you have a 20 dB
4) If you need to know the isolation between the directional coupler output port and
the coupled port, then connect Port1NA to Port2DC (output), and Port2NA to Port3DC (coupled). Place a calibrated
50-ohm termination on Port1DC (input). The measured value is the coupled port isolation. Again, you can note the
S-parameters as needed in one or both directions (S22, S32, S23, S33).
Directivity is the numerical
difference between the value measured here and the value measured in the previous step. Directivity should be
greater than 20 dB, typically 30 dB or better.
- Kirt Blattenberger
Progenitor & Webmaster