CTCSS Tone encoder

CTCSS Tone encoder

The circuit is shown in Fig 2.  It’s centred around a purpose-made IC, the FX315, which has been used in professional circles for many years. I have built a number of these units for my own use.  It draws only 1 .5mA in use, so it’s eminently suitable for handhelds, as well as mobile and base station transceivers. There’s no tone alignment needed, so you can just fit and forget it for use in your geographical area.  For multi-tone encode, a small DIP switch can be added to switch between any of the 40 standard CTCSS tone frequencies provided.

 The circuit

The FX315 is a monolithic CMOS integrated circuit tone generator, which has been designed especially for sub-audio tone squelch systems.  It’s made by Consumer Microcircuits Ltd (CML) and, although it’s a ‘specialist’ IC, it is readily available throughout the world. 

An onboard oscillator circuit  is contained within the IC, from which all the CTCSS tone frequencies are generated.  The oscillatorrequires an external 1MHz crystal, so if you have one of these in your junk box you can save yourself some money.  Otherwise, I’d recommend using a low cost 1MHz ceramic resonator, which is available for less than £1 .00 from component suppliers such as Maplin.  I’ve used both crystals and ceramic resonators and either will work fine in the circuit described here.

Apart from setting the tone output level, there’s no alignment needed at all in this project. That’s because the IC uses digital division and filtering techniques to produce a ‘rock stable’ output tone frequency, derived from the 1MHZ oscillator.

To select the required CTCSS tone frequency you simply connect one or more of the IC programming input lines, D0 to D5 to ground, Table 1 gives the required program input links The DO to D5 input lines are internally pulled up to positive logic ‘1’ by the IC, thus if any pin is left unconnected it’ll be automatically linked to logic ‘1’.  Shorting any pin to ground gives logic ‘0’. For example, repeaters use 71.9Hz, so all that’s needed is to short D5 to 0v, I.E. IC pin 6 to DC, to provide a 0 on D5 and 1 on all other lines.

 The construction

Because of the very simple circuit arrangement, making a ‘one-off’ PCB for the project could be regarded as overkill.  The component layout isn’t at all critical, I used Veroboard for my units, although even this isn’t necessary as you could simply solder the components directly to the IC pins if you wish.   This works well if you need to get the finished unit into a small area such as inside a handheld transceiver case. A hint here: bending the IC pin legs out horizontally and then soldering the component leads, suitably trimmed short, to these pins aids construction and achieves a thinner overall assembly, which could fit into spaces where there other-wise wouldn’t be enough depth.

The IC is a CMOS type, so take suitable static precautions when you’re handling it, also make sure you observe the correct polarity of the electrolytic capacitors when soldering them in.

The FX315 requires a 5V supply:

If you connect it to any voltage higher than 7V you risk destroying it.  Many transceivers have a switched +5V line on transmit, which is ideal. If you need to use a higher voltage supply, e.g. the main 13.8V supply rail in your mobile transceiver or, say, the 7.2V or 9.6V nicad voltage in your handheld, you need to add a simple voltage regulator such as a small 78L05 three-pin regulator.  Remember to add suitable decoupling capacitors on the input and output of such an IC to prevent oscillation.   A suitable circuit is shown in here the capacitor values are not at all critical and you can use any suitable type from your junk box.

You’ll need to feed the audio tone output to your transmitter’s audio stages after the microphone audio shaping and amplification circuitry.  You can not lust connect it to your mic socket.  If you do, you will almost certainly find it gets filtered by your rig, the result being no CTCSS tone transmitted at the output!   If your set has an internal connector for an optional CTCSS unit, this is an ideal connection point. Simply connect the tone output line from the unit to the TX tone input line on the connector.  Virtually every ex-PMR rig has facility for a CTCSS unit, and a quick check of the conversion details or a circuit diagram will show you the correct tone injection point – it’s usually marked ‘TX sub-tone’ or similar In the case of the Pye / Philips M290 and MX290 series of equipment, this is an easily-accessible pin on the front PCB-mounted facility connector.

The circuit shown generates the CTCSS tone whenever voltage is applied to it, so you can usefully link it to the +5V TX rail.  If you connect it permanently, i.e. so that it’s active also on receive, in rare cases and depending on your radio’s circuitry, you may possibly find received signals arc modulated with the sub-tone as well it the set’s VCO is modulated with the tone. If you only have a negative-going line on PTT rather than a switched positive voltage line, then to make it CTCSS unit only generate a tone on transmit PTT leave the FX315 pin 13 open circuit, and instead use a switched connection to ground on pin 12 of the FX315, i.e. the PTT line, to initiate tone generation .  I’d advise adding a series diode on this line to prevent any voltage greater than 5V from your PTT line being fed back to the unit.

Level adjustment

After you’ve wired the unit into your transceiver, all that remains is to set the tone output level correctly.   You’ll need to achieve between 10% and 20% of your peak system deviation for the CTCSS tone, i.e. 250 – 500Hz deviation for 12.5kHZ spacing, and 500Hz – 1kHz deviation for 25Khz spacing, 500Hz being a good ‘in-between’ setting for both. The FX315 gives 0dBm output into a 600 ohm load (775mV). and depending upon your transmitter’s circuitry this might already be a suitable level.

Connecting a series resistor of a suitable value, i.e. the variable pre-set VR1 in the Circuit diagram, will reduce this if needed, you should choose the value of this to suit your rig’s circuitry.  You might need to connect this instead as a potentiometer, i.e. with one leg grounded and the output taken from the wiper, in cases where the transmitter has a high input source impedance, although check here that your normal speech deviation isn’t affected Again depending upon your transmitter’s circuitry, you may or may not need a series capacitor, i.e. C5 in the circuit diagram, to isolate any DC component on the output.


For setting the level required, if you have access to a deviation meter then all well and good.  If you have no test equipment then I’d suggest initially setting the output level to the lowest possible, gradually increasing it until you access the repeater satisfactorily on-air.  Then note the trimmer position and increase it somewhat further, say to double the amount from this ‘minimum access’ point, for reliable operation.

CTCSS Circuit diagram & Parts list

  • C1 FX315PJ
  • Cl 1µF 10V electrolytic
  • C2 33p
  • C3 68p
  • C4 0.1µF 16V electrolytic
  • R1 1M
  • RV1 47k typical (10-100k)
  • Xl 1 MHz ceramic resonator or crystal

Download Data sheet Adobe Acrobat reader is needed to read data sheet

1 MHz  crystals available. Harwood trading Doncaster tel ..01302 351766, price is £1.00 each.

CTCSS Data sheet


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