S-Parameter Test Set
for the SDR-Kits VNWA

August 2016:

This was a project that I embarked upon simply for the fun of it. In fact, I don’t really need an S-Parameter Test Set; although I’m familiar with such things, having used one during my time with Racal. I had found a beautiful Transco Transfer Relay in my box of microwave odds and ends and was wondering what it would be useful for. As it happened, at about the same time there was some ‘chatter’ on the VNWA Yahoo Group which mentioned using a transfer relay to turn a VNWA into an S-Parameter Test Set. I downloaded several documents from the group and made the decision that I was going to enhance my VNWA by building it into an S-Parameter Test Set, not because I need one but because I could do it. The finished article is pictured on the right. I have left the front panel deliberately blank for the simple reason that I actually don’t know how to ‘identify’ the two connectors. I will explain as I go on.

With reference to the images on the left, I will try to shed some enlightenment on the transfer relay and how it is used to turn the VNWA into an S-Parameter Test Set.

The top image is the schematic of the transfer relay itself. As you can see it has 4 connections. Since it is a coaxial relay, these connections are N-Type sockets and are numbered 1 thru’ 4. Unlike a conventional ‘failsafe’ relay which has one solenoid coil, this being a transfer relay, has two: One for each of the two positions. The connections for the two solenoids come out to pins A & B with C being the common. Pins D, E & F can be used as a position indicator. To assert Position 1, 28V DC is applied between A & C. Current flows, the relay activates and the current is automatically disconnected. The relay is said to be latched. In position 1, connectors 1 & 3 and 2 & 4 are connected. In position 2, connectors 1 & 2 and 3 & 4 are connected.

Now referring to the bottom image: TX & RX are the Output and Input ports on the VNWA whilst DUT 1 & 2 are the connectors on the Test Set for connecting the ‘Device Under Test’.

Note that when Position 1 is asserted, RF from port TX is fed through the relay to DUT 1. RF appearing at DUT 2 is then passed through the relay to port RX. This is essentially the same as connecting the Device Under Test directly to the VNWA. However with an S-Parameter Test Set it should NOT be necessary to turn the Device Under Test around in order to measure the S-Parameters in the opposite direction, and this is where the transfer relay comes in.

When Position 2 is asserted, RF from port TX is now fed to DUT 2 and any RF at DUT 1 is fed to port RX.

Simple and elegant!