RF SPDT Switch

Thread Starter

Titia

Joined Feb 14, 2013
7
Hi everyone,
I receive C-Band signals from 1.9m dish via a C-Band PLL LNBF. This one has a L.O. frequency of 5.150GHz.
I intend to switch the signal out of the LNB alternatively to two different receivers. The switch used for this is
the HMC435 module SPDT RF non-reflective switch. Making some isolation ports measurements with a steady signal (the satellite beacon frequency 1.199GHz after down-conversion) the module did not respond to what I was expected. From the datasheet I would expect an isolation of approximately -62 dB. The attached pix indicate the various measured values:
Direct connection (i.e. without SPDT switch) -56.7dBFS (FS = full scall)
Output A (i.e. trough SPDT port A operational) -55.9 dBFS
Output B (i.e. as Output A but measured on port B) -60.5dBFS
The isolation between port A and port B is only 4.6dB which is far away from the specs.

Doing measurements with port B operational provides almost same figures. So I suspected a faulty module, but since I have two of them, I repeated the test with the second one and unfortunately it provides figures in the same range.
I am really puzzled. What's wrong? Could someone give me a start of explanation.
Many thanks
 

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drjohsmith

Joined Dec 13, 2021
499
Sounds like you have coupling between the ports,
Check how the switch is specified
do the outputs have to be "50" ohm terminated, even if not used.

Share a picture of your test set up and the board please
 

sagor

Joined Mar 10, 2019
754
Is the ground (GND) of the device the same GND as the signal? Looking at the datasheet, the isolation relies heavily on the termination of the unused port.
 

sagor

Joined Mar 10, 2019
754
Datasheet also says when A high, B low, you are switched to RF2 but your picture seems to show connection to RF1/out1 (assuming red wire from A goes to +5V)
 

Thread Starter

Titia

Joined Feb 14, 2013
7
You are right, Pin A is connected to +5V, Pin B and GND are connected to GND. In this configuration the signal should only be available on out2. But when I connect the receiver on out1 (as shown on the picture) the signal is also present.
 

sagor

Joined Mar 10, 2019
754
But out 2 had no load, so all the signal will reflect back into the switch. Put a 50 ohm load on out2 and then measure out1 signal.
 

Thread Starter

Titia

Joined Feb 14, 2013
7
But datasheet specifies that the module is a non-reflective system and functional diagram shows a 50 ohms connected to the not operational output according to truth table.
 

sagor

Joined Mar 10, 2019
754
But datasheet specifies that the module is a non-reflective system and functional diagram shows a 50 ohms connected to the not operational output according to truth table.
That may be non reflective for input. No matter what, without a proper termination/load on the output port (out2), all the RF is being reflected back into the switch where it looks for some path. You must have all ports with proper loads for the specifications to mean anything
 
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