Computer Automation Technology

High Impedance COR Buffer/Driver circuit

If your in need of a high impedance COR Buffer/Driver, here is a simple circuit that should be helpful in most cases. This circuit can be built on a perforated construction board obtain from your local RadioShack store. It is highly recommended that you use a shielded cable when connecting the COR signal from the repeater to this driver circuit. To proceed , follow these simple six steps:

Step 1
List of parts you'll need for this circuit.

Part Schematic ID Part Identification
RadioShack Part Number
U1 Operational Amplifier,   TL084
R1 100K ohm 1/4 watt resistor
R2 100K ohm 1/4 watt resistor
R3 10K ohm variable resistor
C1 .1uf 50V Capacitor
  8 pin socket for U1
  Perf board to hold circuit components

Step 2
If necessary, remove the DB25 male connector from your CAT controller. Unscrew the two "hold-together" screws to open the connector. See Figure 1.

Cor Fig1

Step 3
Carefully unsolder the wire connected to pin 6 of the connector and bring that wire out the back of the connector. Solder additional wires to pin 6 (CAT COR Input), pin 16 (+12VDC output), and pin 17 (Ground). Refer to figure 2 for the correct pin location.

Cor Fig2 DB25 Male Connector viewing from pin side.

Step 4
Build the circuit on a perforated board following the schematic below. The DB25 wires will connect as follows:

DB25 Pin Number
Connect To Circuit
Connect to U1 pin 7
+12VDC connect to U1 pin 8, C1 and one side of R3
Connect to U1 pin 4, C1 and other side of R3
COR signal from Repeater
Connect to 100K resistor R1

Cor Fig10

Step 5
Carefully re-assemble the DB25 connector and reconnect to your CAT controller.

Step 6
Obtain a high-impedance voltmeter and make the following measurements and calculations:

  1. Measure the voltage at pin 1 of amplifier U1.
  2. Key and Unkey a signal and record the two voltages (with/without COR).
  3. Calculate the center voltage between the two voltages in step 2.
  4. Measure the voltage of the wiper arm of R3.
  5. Adjust R3 until the voltage equals the calculated voltage from step 3.

Once adjusted and setup, the output of U1 pin 7 will provide a strong drive voltage for your CAT controller.