Fall 2005 Issue

Above and Beyond, 1296 MHz and Up

 By Chuck Houghton, WB6IGP

This time we will discuss a simple sequencing relay-control switching circuit that I have used for some time. Although the circuit is simple, it controls two 12-volt relays that are the heart of the device. Activation of the switching is done by PTT (push to talk) RF detection from a 2-meter multimode low-power (modified) IF driver. In this case, RF output power is reduced from the normal low-power setting by modification of the 2-meter multimode radio to obtain RF power in the 100- to 200-mw range. An additional relay can be installed to further reduce the power to the desired level by insertion of a relay to add a pad of 3 to 10 dBm in the coax path before it is injected into the mixer circuit of the microwave converter. The idea here is to keep power low to protect the 10-GHz mixer, as it is a precious device and difficult to obtain. All efforts are made to keep RF transmit power low in the IF converter stages of the microwave converter to protect the mixer.

Additional protection is provided by the circuit’s two relays, which are sequence driven by the N-channel FETs and the switching time constant that controls each one of them. Contacts on these two relays are used to control the coaxial relays and associated receive and transmit amplifier components of the microwave converter. When the PTT switch is depressed, it causes the receiving pre-amplifier circuits to be switched out before the transmit amp control circuitry is switched on. The opposite is done by releasing the relay circuitry. When the IF transmit PTT is released, the transmit relay circuitry is fast to release and the receive preamp control switching is slow to release. This is accomplished by using two different time constants in this circuit. They are 1 mFd charged by a 1K-ohm resistor and 1 mFd charged by a 47K-ohm resistor. Charging or discharging a 1-mFd capacitor through a 1K-ohm resistor is fast compared with charging through a 47K-ohm resistor.

Capacitor values can be changed to add or remove capacitance to suit your specific timing requirements. I set up a couple of LEDs tied to the drains of each FET with a series 1.3K-ohm resistor to +12 VDC for the LEDs’ DC power. In this way I could watch the timing of the “A” and “B” relay drivers by observing the LEDs for visual circuit timing of the “A” and “B” relay controls. Relays could have been used here just as well, watching the time each relay operates and releases.

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