Keying Amplifiers with an Icom Exciter

by Adam Farson, VA7OJ/AB4OJ

First published November 2, 2002 on the Icom Reflectors

Here is some general information on this much-batted-about topic: 

To determine whether or not an interface (buffer) is required in the keying line between an Icom HF transceiver and an amplifier, one should measure the keying voltage and current of the amplifier's keying line with a digital multimeter, as follows: 

1. Measure the open-circuit voltage between the amplifier keying line and chassis.

2. Measure the closed-circuit current between the amplifier keying line and chassis.

3. If the open-circuit voltage exceeds 16V DC, and/or if the current exceeds 0.2A (the current Icom spec is 0.5A, but a safety factor is recommended), an interface is required.

A solid-state interface will minimise the latency associated with the switching time of an open-frame relay. In some cases, a reed-relay interface will be sufficiently fast to keep the latency to a minimum. 

The Icom FAQ site carries an excellent article on this topic, including links to several keying interfaces. Click here for a list of amplifier keying-line specs. Click here for a simple solid-state interface proposed by Icom Service (courtesy DXLab).

Many current tube amplifiers, and all solid-state amplifiers, have a low-level (typically TTL-level) keying line which requires no interface. Some of the Alpha amplifiers have a 28V/7mA keying voltage/current rating; this is well within the 50 VA rating of the OMR-109F relay.

Caution: Heathkit amplifiers, and the Kenwood TL-922 series, have open-circuit keying-line voltages in excess of 100V.

Copyright © 2002 A. Farson VA7OJ/AB4OJ. All rights reserved.

Last updated: 01/05/2003