KI5YN QLF Filter Introduction

KI5YN QLF Filter

Operation Parts list Schematic Assembly Procedure Ordering Info


Product Description

The QLF filter is a microprocessor based interface designed to go between a standard Morse code key and a radio transmitter. The circuit receives a signal from the key, processes it, and re-transmits it to the radio. A microprocessor in the circuit is pre-programmed with a proprietary algorithm which makes a number of measurements and adjustments to the less percise human generated code.


This is an Amateur Radio accessory. It is packaged as a kit for experienced project builders. The builder should be familiar with construction of digital circuits, soldering, and the proper handling of static sensitive parts. The kit may not contain everything you need for a specific situation. You will need to add a 5V power supply, connectors, and a enclosure.


The circuit is based on a Phillips 87C750 8 bit microprocessor, which is running at 16MHZ. There is a 1K EPROM space within the processor that contains the firmware. The chip includes a timer which is used to monitor the incoming signal characteristics. 6 LEDs are included to indicate the state of the circuit. There is also a transistor for driving a speaker, and a 2 transistor circuit for driving a transmitter or a relay.

Firmware - Signal Correction Algorithm

The firmware on the processor is the result of countless hours of effort. Almost every facility on the chip and every ounce of processing power available has been used to achieve a remarkable amount of functionality from this little chip. This firmware has been programmed into the write-once plastic package version of the chip, and the security bit has been set to prevent access to the code.

The processor works in real time, monitoring the dits and dashes as they arrive. Based on parameters of the signals that have arrived in the past, certain assumptions can be made about the signal currently being sent. For example, a very short dit could indicate that the key was let up too fast, or not debounced properly. In this case, the dit is extended to the proper length. Likewise, spaces and dashes are checked as they arrive.

If you are blessed with a perfect fist, your signal will fall within normal tolerances, and the filter will not make any changes. More likely, you will from time to time miss the perfect code timing. When this happens, the circuit kicks in and makes a remarkable effort to correct the error.

The QLF filter makes the following improvements to any inadvertant "QLF" (Q Left Footed) code:

You may be wondering how the circuit knows the "proper" length. The length is dependent on the sending speed, which is determined by previous characters. In order for the circuit to work with different speeds, the algorithm takes into account changes in sending speed, and will gradually make the adjustment to track the intentions of the operator. If you want to force a constant speed, a simple key sequence is used to lock-in a speed and disable the speed tracking function. Another sequence disables the lock.

Last Update 12/01/96