HF switch part 1 - a few words about the project

Introduction

At the ham radio club, several antennas are used connected to the transceiver through a mechanical switch. This mechanical switch is rather convenient for people using the gear from the room, but totally unconvenient for people using it in remote.

I volunteered myself to develop a computer controlled switch. But I did not want to make a simple electromechanical relay switch: I wanted to take this opportunity to try to experiment solid state switches.

Two techniques are common to switch RF: the PIN diodes and the MOSFETs.

The ham radio club would need to switch from 1 to 30 MHz at max 1300 W. PIN diodes work very well, even at high power, but have an high static current consumption, and need a complex bias scheme with a current bias in the ON state and an high reverse voltage in the OFF state.

MOSFETs are routinely used in integrated circuit switches to switch low powers (≲ 10 dBm), but in this case the gate drive is easy because all voltages are low the the gate can “simply” be tied to Vdd in the ON state. On the contrary, power MOSFETs have often a much lower Vgs breakdown voltage than their Vds breakdown voltage, which makes impossible to just tie the gates to the maximum voltages, let alone the difficulties to handle of the peak voltages at such high powers.

To solve this issue, I wanted to try a technique borrowed from the switching power supplies world: the use of isolated gate drivers. From a pure V/ns point of view, transistors able to hard switch at maybe 3 MHz seem to be worth trying for a sinus switch at maybe 30 MHz.

Co-author comes to help

I invited Gönül Demir to contribute to this project since it is a very good exercice to sharpen her RF skills. A rather though one actually.

She made a great job in this project: selected MOSFETs, new MOSFETs after the previously selected one had too much Coss, selected new isolated power supplies and isolated gate drivers, and so on.

Project status

The project is not yet finished and I lack time to make a full summary. Usually I use cat pictures, but this time, AI gave me very funny pictures worth posting. Here we go: