Electronic T/R Switching and the Ameritron QSK-5
by Greg Latta AA8V

Building an Electronic Transmit/Receive Switch Using PIN Diode Switches

Safety Note: Working on or testing the QSK-5 and associated circuitry may involve operating the amplifier containing the QSK-5 with the cover off. This is extremely dangerous since very high voltages are present when the amplifier is turned on, sometimes in close proximity to the QSK-5. If at all possible, do all work with the amplifier off and unplugged. The operator assumes all risk and liability in such matters! Never operate the amplifier with the cover off unless you are experienced with working around very high voltages!

Basic Circuit:
To build a complete amplifier transmit/receive (T/R) switch requires three PIN diode switches. The switches are connected as shown in the figure below:

A Complete Electronic T/R Switch Using PIN Diode Switches
Click on any of the switches in the diagram for more information on that particular switch.

The transceiver is connected to one side of the receive switch and the amplifier input switch. The antenna is connected to the other side of receive switch and one side of the amplifier output switch. The other side of the amplifier input switch goes to the input circuit of the amplifier, and the other side of the amplifier output switch goes to the amplifier output circuit.

During receive, the receive switch is turned on and the amplifier input and output switches are turned off. The signal from the antenna then passes through the receive switch to the transceiver. The amplifier input and output switches ("transmit" switches) keep the amplifier isolated from the transceiver and antenna during receive periods.

In transmit, the receive switch is turned off and the "transmit switches" are turned on. The output of the transceiver is then routed to the amplifier input, through the amplifier, and out to the antenna. During receive the receive switch isolates the amplifier output from the amplifier input, preventing feedback, oscillation, and distortion.

Each of the three PIN switches has different requirements, as discussed below.

Receive Switch
During receive, the receive switch doesn't have to handle any power or appreciable current, just the feeble receive current from the antenna. However, during transmit it must withstand the full output voltage of the amplifier (274 volts for a 1500 watt amplifier) without allowing any appreciable leakage. If too much of the output of the amplifier should leak back to the input through the receive switch, the amplifier could oscillate or generate severe distortion. This could also damage the transceiver. In the QSK-5, eight MA4PH301 diodes are used in series to obtain the required isolation.

It is absolutely imperative that no RF from the transmitter be allowed to flow through the receive switch at any time. Even more important is that the receive switch must not be turned off or on when any appreciable current could flow through it. This is known as "hot switching" and it must be avoided at all costs.

Amplifier Input Switch
The amplifier input switch is turned on during transmit and allows the output of the transmitter to pass to the input of the amplifier. This switch must be able to handle approximately 100 watts (1.4 Amperes) maximum. Since the load is the amplifier input, which is a fairly stable load, a large safety factor is not really needed here. In the QSK-5 a single MA4P4001B diode is used.

During receive this switch simply needs to isolate the amplifier input circuit from the antenna and transceiver. A single PIN diode has more than enough isolation for this purpose.

Amplifier Output Switch
The amplifier output switch is turned on during transmit to connect the amplifier output to the antenna. It must be able to handle the full output current of the amplifier, about 5.5 amperes for a 1500 watt amplifier, plus a large safety margin in case of antenna mismatch, feed line arcing, and other problems that seem to occur with alarming frequency in amateur stations. In the QSK-5, four MA4P4001B PIN diodes are connected in parallel to handle the load. These are floated above large holes in the circuit board to increase their power handling capabilities.

During receive this switch, like the amplifier input switch, simply needs to isolate the amplifier output circuit from the antenna and transceiver. The four PIN diodes used in parallel have more than enough isolation for this purpose.

Why Can't I Just Use More Diodes?
It is natural to ask why extra PIN diodes or larger PIN diodes can't be used for a larger safety margin. In fact, they can, but, unlike conventional power diodes, PIN diodes like those used in the QSK-5 are very expensive. The replacement cost of a single MA4PH301 diode used in the receive switch is $14, and the replacement cost of a single MA4P4001B used in the amplifier input and output switches is a whopping $37. At such prices, you don't throw in a few extra diodes for good luck. You carefully choose exactly how many you need to do the job, with some extra margin. The folks at Ameritron have done just that. They have chosen the correct number of diodes to do the job yet keep the cost reasonable.

In the case of the receive switch, the cost of the diodes prevented the use of larger, heavier diodes to cover mistakes made by the operator, such as hot switching, poor transceiver timing, and forgetting to hook up the T/R line from the transceiver to the QSK board. A much more economical solution to cover such mistakes was to use a pair of fuse lamps that could light to indicate malfunctions and blow to protect the eight receive diodes. It was a good compromise on the part of the designers at Ameritron.

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