S-Parameter Test Set
for the SDR-Kits VNWA, page 2

August 2016:

The original article describing how to use the VNWA in conjunction with two RF relays implied (correctly) that the relays could be controlled directly from the VNWA. The current software embodies support for using the VNWA in conjuntion with an external S-Parameter Test Set. However there are essentially three variants of the VNWA. The current hardware model is version 3, whilst I own a home-built model 2.6. The version 3 hardware has an RJ-11 connector on the rear which conveniently carries a control line which can, with appropriate circuitry, be used to activate a relay. There is no such connector on my VNWA. It took a bit of hunting but I eventually discovered that there is an output on the ATMega chip on the USB Interface board in my VNWA that can be used to control a relay. If you have an older, pre-USB VNWA, then there is a pin on the serial connector that will do the job.

Anyway, attaching a wire to the USB board looked easy enough, there is even a pad on the board for the wire. However, getting the wire out of the enclosure was not so easy. In the original article, and others have done the same, a 3.5mm jack socket had been fitted. I wasn’t keen on that idea since such connectors can be pulled out and I wasn’t happy with the idea that this might happen and maybe short the connection to 0V.

Another solution was to use a stand-alone USB controlled switch to activate the relay. The schematic for this is shown below.

This is a tidy solution but it would require obtaining and programming an ATTiny45 microcontroller. More on that later.

I even found Peter DC2PD’s article on using a latching relay as an S-Parameter Test Set. The schematic on the left is Peter’s relay driver circuit for use with the USB controller above.

Peter was even able to power his 28V relay from the USB port. I wasn’t so fortunate … more on that later too!