KI5YN QLF Filter Introduction
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:
- debounces the key to prevent scratchy transitions
- completes short dits and dashes.
- insures proper inter-dit/dash spacing
- can lock-in a specific code speed to prevent speed wandering.
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