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Bandpass Filter Emitter Circuits

Circuit Operation || Curing RF Problems || Replacing the SPST Switches with IR Switching
Commercial Matched Pair Emitter/Receiver

Image showing the IR paths in the Bandpass Filter.


Circuit Operation

When the SPST switch is open, Q1 is turned on and the 40/30 Bandpass filter (BPF) is ready to go. 12 Volts enters the 40/30 BPF output relay through a 100 ohm resistor, passes through an IR LED, which turns on the input relay, and is grounded through Q1.

The drain of Q1 is also connected to the gate of Q2, and as long as Q1 is on, the gate voltage of Q2 is zero, which has Q2 turned off. A 1N914 is also connected to this line to prevent a gate failure from a spike when the relay switches.

When the SPST switch is closed, this puts the gate of Q1 at zero volts, turning off Q1. The positive voltage through the relay and IR LED brings the gate voltage of Q2 up to 12 volts, turning on Q2. Because the impedance of the gate is so high, no current is drawn and the relay/LED cannot turn on.

With Q2 turned on, 12 volts flows through the drain to the source and activates the 20/17 BPF output relay. The ground circuit of the output relay turns on two IR LEDs, which turn on the 10.455 VFO relay, and the input relay to the 20/17 BPF.


Curing RF Problems

All the SPST switch connections have a .01 capacitor to ground. Without this capacitor, the gate lead of the VN0106N3 and the large connection pad area picks up RF from the BPF, amplifies it, and places it on the 12 Volts line.

Since the 100K gate resistor does not pass RF, the gate of the MOSFET will pick up RF. This is also true at the Crystal Filters where a very similar circuit is used. If you are using long wires to a front panel switch, wire a .01 bypass capacitor to ground right at the switch to make sure all RF is bypassed.


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