A new digital mode for HAMs by sv3ora,
that can be implemented in hardware
KISS703 stands for Keep It Stupid Simple mode that occupies a
bandwidth of 703Hz. It is a new mode, designed by me (sv3ora) in my try
to come up with a digital mode that does not require the encoding and
decoding to be done in the brain (like CW) and does not require the
usage of any computer to send and receive messages. More over, it is
composed of a few common discrete components, that can be found in any
reasonably equipped shack. It also is complete, meaning that there is a
way to send the letters, encode them, decode them and display the
decoded letters. The hardware requires about 40mA at 12v, so this is
definitely a mode that suits SOTA/IOTA etc activities, where battery
consumption is important.
The mode works this way.
During transmission, there is a pilot tone at 500Hz. This tone is
present whether you transmit any characters or not. It is used, to let
the receivers know that someone uses the frequency, but also as a means
to correctly set the receivers to the transmitter frequency. When the
transmistter sends a character, the pilot tone is instantly switched
off and replaced by the tone of that character to be sent. After each
character transmission, the pilot tone is switched on again, up to the
next character transmission, where the procedure is repeated. This way,
the receivers can synchronize their frequency to the transmitter pilot
tone frequency, at any point during the transmission of a message and
not just at the beginning or at the end of it.
Apart from the pilot tone, each character in the KISS703 mode, has a
distinct tone assigned to it. There are a total of 37 characters. 26 of
them are used for the letters (case insensitive), 10 are used for the
numbers and 1 is used for the space. Space is actually inplemented as a
character in the KISS703 mode and it has to be transmitted just like
any other character. The tone frequencies assigned to each character
are shown in the schematic image above. Any hardware (or software) that
is to be used for the KISS703, must comply to these frequencies, so we
can all communicate and understand each other on air.
The 703Hz bandwidth has been chosen after careful inspection and
experimentation with the detector part of the schematic. It is a
compromise for stable operation of the circuit and also reasonably
narrow bandwidth of the mode. Specifically, the meter driving circuit,
suffers from thermal stability issues on very low bandwidths. In fact,
the schematic shown, allows for bandwidth settings of up to 80Hz, but
the thermal stability of the driver has a much greater effect in such
low bandwidth and it requires constant adjustment of the pilot
frequency in the decoder, so that you can decode the letters correctly.
At a bandwidth of 703Hz, this effect is much suppressed. Note, that I
have tested differential meter drivers in the circuit with limited
success, so I got back on the original simple idea and just made the
I made the circuit as simple as possible, with relatively few and
definitely common components that exist in any reasonably equiped
shack. On the left side, is the variable square wave oscillator, which
produces the pilot tone and the character tones. A switch, switches
between the preset pilot (when on stand by) and the character tone to
be sent. The character tone is first set by the precalibrated
potentiometer and then the switch is pressed to send that character.
On receive mode, the audio tones pass through a series of amplifiers
that convert sine waves to square waves, so as to keep their amplitude
independent of input voltage variations, and then to the detector and
driver circuits. The detector is a pulse counting frequency meter. It
uses the RC time constant of the 10nF capacitor and the resistors
around it, to convert the pulses to sawtooth with amplitude depending
on frequency. The final diode then converts the sawtooth back to a
measurable DC the meter can use. The meter driver that follows, is just
another amplifier, which allows for increasing the sensitivity of the
meter and hence decreasing the bandwidth of the mode.
of the circuit, requires an audio frequency meter and generator, as
well as opening the meter protective plastic and writing (or printing)
characters to it. Alternatively, the sound blaster of your computer can
be used, with appropriate software. There are plenty of software audio
frequency meters/generators or oscilloscopes on the internet to choose
Before using the circuit for the first time, open the meter protective
cover and then connect the input of the detector circuit (T/R switch to R position) to the souncard output. Set the sound card generator to 500Hz and preset
the Zero potentiometer for minimum meter reading of the pilot tone
frequency. Mark that needle position on the meter with "PILOT". Next
set the sound card generator to 1203Hz and preset the BW trimmer for maximum meter reading on that frequency.
Mark that needle position on the meter with "0" (zero). Go back and
repeat the previous steps to ensure that there are no interactions
between these two settings (there should not be). Then, set the sound card generator to 519Hz and mark
the needle position on the meter with "SPACE". Repeat this last step
for all the remaining characters, by looking at the
characters/frequency table on the schematic. After completing these,
you will have a calibrated detector.
To calibrate the sender, disconnect the circuit from the sound card, put the T/R switch to T position and preset
the pilot tone frequency trimmer, so that the needle reads "PILOT" on
the meter. Next, keep the momentary switch pressed and set the symbols
potentiometer so that the needle marks each corresponding character on
the meter. Mark that character onto the symbols potentiometer scale.
Next time you turn the potentiometer to that position, the appropriate
symbol will be sent. Repeat this procedure for all the remaining
characters. After completing the procedure you will have a calibrated
During operation of the circuit in receive mode and depended on the
temperature variations, you may have to correct the Zero potentiometer
once in a while, so that the pilot tone causes the meter needle to read
"PILOT". Alternatively, you can tune the frequency of your HAM radio
transceiver, to achieve the same result.
There is nothing stopping you from replacing the single calibrated
character potentiometer with a full-size keyboard, with one key and one
trimmer per character. I have successfully done that. Also you could
replace the needle meter display with a few LM3914 and use one LED per
character instead, which will also improve drammatically the speed of
receiving characters. Also, you do not need to use an audio oscillator.
I wanted to make this thing universal, but if you homebrew your own
transmitters, you could use an RF VXO, as long as it can be frequency
varied by the amount needed by the KISS703 and calibrate its tuning
knob for the crystal you use.
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