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.

Mode description

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.

Technical challenges

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 bandwidth wider.


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 a
nd hence decreasing the bandwidth of the mode.


Calibration 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 from.

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 sender.

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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