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

T/R Input and Timing Circuits

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!

Important Timing Considerations:
When a mechanical relay is used for transmit/receive switching in an amplifier, the physical construction of the relay prevents the output of the amplifier from ever being simultaneously connected to the input of the amplifier. However, when an electronic T/R switch is used, it is quite possible to turn on both the transmit and receive switches at the same time. If this happens the results will be catastrophic. If the output of the amplifier is connected to the input for even the tiniest length of time, the amplifier will become a high powered oscillator that is connected to the antenna and the transmitter/transceiver output, resulting in damage to the transmitter/transceiver, amplifier, and/or electronic T/R switch. Thus, the electronic transmit and receive switches must never be on at the same time!

The first step in guaranteeing that the transmit and receive switches are never on at the same time is to design the control circuits so that when one switch is on the other is always always off. However, because PIN Switches can be partially on or off, it is possible, if both switches are activated at the same time, for both to be partially on while the system switches from receive to transmit and vice-versa. If both switches are partially on, the same catastrophic results mentioned above can still occur. To prevent this from happening, the control circuit must be designed so that the first switch turns "Off" completely before before the other switch turns "On". For instance, when going from receive to transmit, the control circuit must guarantee that the receive switch has turned completely "Off" before the transmit switch is turned "On". In other words, when switching from receive to transmit and vice-versa, the switch that is on must turn off very quickly, and there must be a time, however short, during which both switches are completely off, before the other switch is turned on. It cannot be stated strongly enough how important this is!

Receive Mode:
There are two timing circuits in the QSK-5, one for the receive switch and one for the transmit switch. Since they are very similar, only the receive timing circuit will be discussed here. The best way to see how the circuits work is to follow the receive timing circuit from receive mode, to transmit mode, and then back to receive mode.

In the schematic below, the T/R input to the timing circuits is pin D at the lower left. On Ameritron amplifiers this is connected to the "Relay" connector on the amplifier. In receive mode the T/R or Relay input is open or "High". To switch the QSK-5 to transmit, the T/R input must be grounded or brought "Low". The 1N4007 diode connected to the T/R input protects transistor Q5 is case the T/R input is accidently connected to a positive voltage, rather than being brought to ground.

When the T/R input D is open, current flows into the base of transistor Q5 turning it on. The collector of Q5 is then near ground potential, or "Low", and this discharges capacitor C19 through resistor R17, bringing the "-" input of the op-amp IC1B, to a "Low" state. The op-amp IC1B functions as a comparator. When the "-" input is less than the "+" input, as it is in this case, the output of the op-amp is turned on and the output is near the positive supply potential, or "High". The output of the op-amp is connected to the input of the receive PIN Switch driver, turning on the receive PIN Switch.

Receive Timing Circuit in Receive Mode

Receive to Transmit Transition:
When the T/R input is brought "Low" to place the QSK-5 in transmit mode, the current into the base of Q5 is shorted to ground, turning off Q5, causing the collector of Q5 to go "High". When the collector of Q5 goes high, capacitor C19 charges very quickly through diode D13, which is forward biased, effectively bypassing resistor R17, as shown by the arrow in the diagram below. As soon as the potential across C19 exceeds 6V, op-amp IC1B is shut off, and its output goes "Low". This turns off the receive PIN Switch. All of this happens very quickly, so that the receive PIN Switch is shut off immediately when the T/R input is brought low. This gives the timing circuit a "Fast Off" characteristic.

Receive Timing Circuit During Receive to Transmit Transition

Transmit Mode:
When the T/R input D is grounded, no current flows into the base of transistor Q5, and Q5 is turned off. The collector of Q5 is then "High", and capacitor C19 is charged through diode D13. This brings the "-" input of the op-amp IC1B, to a "High" state. Op-amp IC1B functions as a comparator. When the "-" input is higher than the "+" input, as it is in this case, the output of the op-amp is turned off and the output is near ground potential, or "Low". The output of the op-amp is connected to the input of the receive PIN Switch driver, turning off the receive PIN Switch.

Receive Timing Circuit in Transmit Mode

Transmit to Receive Transition:
When the time comes to return the QSK-5 to receive mode, the T/R input is brought "High". This allows current to flow into the base of Q5, turning it on, and bringing its collector "Low". When this happens, capacitor C19 discharges. However, since diode D13 is now reverse biased, C19 must discharge through R17, the 10k resistor, as shown by the arrow in the diagram below. When the potential across C19 drops below 6V, IC1B turns on, bring the op-amp output "High", turning on the receive PIN Switch.

Since C19 must discharge through R17 during this transition, there is a lag of several ms after the T/R input is brought "High" before the receive PIN Switch is turned on. This lag is controlled by the values of resistor R17 and C19. Increasing C19 increases the delay, as does increasing R17. This provides the delay needed to insure that the transmit switch has turned completely off before the receive switch is turned on.

Receive Timing Circuit During Transmit to Receive Transition

In summary, it is the action of diode D13, resistor R17, and capacitor C19 that provides provides the delay that insures that the transmit switch has completely shut off before the receive switch is turned on, as mentioned earlier under Important Timing Considerations.

Transmit Timing Circuit:
The transmit timing circuit is connected to the collector of Q5 just like the receive timing circuit explained above but with the following changes:

1. The "+" and "-" inputs to the op-amp are switched. This switches the polarity of the input of the op-amp, and likewise the polarity of the output. This guarantees that the transmit switch is always off when the receive switch is on and vice-versa.

2. The polarity of the diode across the 10k timing resistor is reversed. Switching the "+" and "-" inputs to the op-amp not only changes the polarity of the output of the op-amp, but also causes the delay to occur before the switch turns off, rather than before it turns on. To correct this, the polarity of the diode across the 10k timing resistor must also be reversed. This makes the delay occur in the proper place, before the switch turns on.

3. The value of the timing capacitor is decreased from 0.022 uf to 0.01 uf. This reduces the delay time before the transmit switch is turned on to insure that the transmit switch is fully on before any RF is transmitted through the switch.

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