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Author Topic: Testing 3dB couplers  (Read 1967 times)
G7MRV
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« on: November 22, 2012, 09:48:48 AM »

Ive aquired a number of 4-port RF couplers (i believe 3db couplers) ex-broadcast, i think they are UHF, but all are completely unmarked.

Can anyone suggest a test setup that will allow me to test these and determine their characteristics? I have access to a signal generator, sweep generator and spectrum analyser usable to 1.5GHz.
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WB6BYU
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« Reply #1 on: November 22, 2012, 11:41:13 AM »

The main setup would be one to measure the loss through any two ports.
Most likely this would involve driving one port with the signal generator and
measuring the output with the spectrum analyzer.  Connect the two cables
with a barrel to set the reference level on the analyzer, then check the
various pairs of ports (while terminating the remaining ports).  Direction may
important in some cases.  For many directional couplers, there will be two
ports with very little loss that represent the through path, and two others
that will be down 10dB or 20dB that are the coupled ports.  For a 3dB splitter
there should be 3dB of loss from the input port to either of the others, but
higher attenuation between the two output ports.

And, of course, the measurement would be done using a sweep over
frequency.  The operating bandwidth, then, is the range over which the
device maintains performance within the limits of what you need for your
application.
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AE6RV
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« Reply #2 on: December 07, 2012, 03:11:36 PM »

I have access to a signal generator, sweep generator and spectrum analyser usable to 1.5GHz.

Assuming this means the analyzer isn't yours, then talk to the owner.  Chances are he already has experience in this sort of measurement, and has a tracking generator to go with his analyzer.
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WB2WIK
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« Reply #3 on: December 07, 2012, 03:39:23 PM »

Is each one a "coupler" or actually a "splitter?"

If it's a 4-way "splitter" (divider/combiner) I don't see how each port could be -3dB. Wink
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VK2TIL
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Posts: 344




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« Reply #4 on: December 08, 2012, 12:38:59 AM »

Look at physical features first.  What connectors are used?

If you can open the case, look inside.

If it is a stripline coupler, consider the length of the lines as a guide to frequency.

The Through line (In & Out) connectors are usually on the case ends whilst the Cpl connector(s) are on the side.

Connect the signal generator to one of the case-end connectors; connect the SA to the other case-end connector.  Terminate the other connector(s) in 50 ohms.

Determine the through loss.

Then terminate the through line in 50 ohms; terminate one (if there are two) of the side connectors in 50 ohms and connect the SA to the other side connector.  Determine the Cpl factor.

If there are two connectors, interchange the SA and the termination and repeat the test.

Follow this procedure for various frequencies and record the results.

You will then have a good idea of the device characteristics.

A photo may assist us; at least the exterior but a clear interior photo would be more useful.

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