How to Test a Directional Couplers? - RF Cafe Forums
Post subject: How to test a directional couplers?
Posted: Tue Jun 16, 2009 12:31 pm
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 am
Jun 16, 2009 12:25 pm
the isolation port is terminated
how do i perform the test? I have a tow port
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 directional coupler.
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
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster