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

Handshake Circuit

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!

General Circuit Description:
Some transceivers, such as the TenTec Jupiter, are capable of operating in QSK Loop Mode. In this mode of operation, the QSK circuit sends a signal back to the transceiver over a "Loop Return" line whenever it is in transmit mode. The transceiver monitors the QSK loop return and is programmed so that it will not transmit unless it receives this "handshake" signal from the QSK circuit. Since the transceiver cannot transmit unless the QSK circuit is in transmit mode, hot switching cannot occur.

In the QSK-5, the handshake circuit samples voltages from the receive and transmit diode switches and feeds them into an op-amp that is wired as a comparator.

The connection to the receive diode switch is at a point where less than 12V volts are present in receive and at 600V volts are present on transmit. For more information see PIN Switch Driver - Turning A PIN Diode Switch Off And On

The connection to the transmit diode switch is made at a point where 600V are present on receive and less than 12V are present on transmit. For more information see PIN Switch Driver - Turning A PIN Diode Switch Off And On

The output of the op-amp drives a transistor that controls the QSK loop return line back to the transceiver, which must be connected to the point marked "AUX" or "K" in the schematic diagram below:

QSK-5 Handshake Circuit

Receive Mode:
The handshake circuit is connected to the receive diode switch at a point where less than 12V is present when the QSK-5 is in receive mode. This connection to the receive diode switch is fed to a voltage divider consisting of R8 and R12 that feeds 1/50th of the sampled voltage to the "+" input of the comparator. Since 12V/50=0.24V, we can obviously say that less than 12V is present at the comparator "+" input during receive.

The handshake circuit is connected to the transmit diode switches at a point where 600V is present when the QSK-5 is in receive mode. The connection to the transmit diode switches is fed through 1Mohm resistor R11 to diode D15 and the "-" input of the comparator. Since 600V is large than 12V, diode D15 is forward biased. The voltage at the "-" input of the comparator is thus 12V plus the forward voltage drop of the diode. In other words, the "-" input of the comparator is higher than 12V.

Since the voltage at the "-" input of the comparator IC1-C higher than the voltage at the "+" input, the comparator is off. The output of the comparator is then low, and transistor Q6 is also turned off. See the diagram below:

QSK-5 Handshake Circuit in Receive Mode

Transmit Mode:
The handshake circuit is connected to the receive diode switch at a point where 600V is present when the QSK-5 is in transmit mode. This connection to the receive diode switch is fed to a voltage divider consisting of R8 and R12 that feeds 1/50th of the sampled voltage to the "+" input of the comparator. Since 600V/50=12V, 12V is present at the comparator "+" input during transmit.

The handshake circuit is connected to the transmit diode switches at a point where less than 12V is present when the QSK-5 is in transmit mode. The connection to the transmit diode switches is fed through 1Mohm resistor R11 to diode D15 and the "-" input of the comparator. Since less than 12V is on the anode of diode D15, D15 is reverse biased. The voltage at the "-" input of the comparator is thus less than 12V.

Since the voltage at the "-" input of the comparator IC1-C lower than the voltage at the "+" input, the comparator is turned on. The the output of the comparator is then high, and transistor Q6 is also turned on. This signals the transceiver, if properly connected by the "Loop Return Line" to the point marked "AUX" or "K", that it is OK to transmit. See the diagram below:

QSK-5 Handshake Circuit in Transmit Mode

An Excellent Circuit:
The QSK-5 handshake circuit is an excellent circuit. It won't give the transceiver permission to transmit unless all of the following conditions are met:

1. The QSK-5 is connected to 12V.
2. The QSK-5 is connected to 600V.
3. The T/R input to the QSK-5 has been brought low, placing the QSK-5 in transmit mode.
4. The PIN diode switches in the QSK-5 have actually changed state and are in transmit mode.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu