CW/CTCSS sinus tone oscillator. Stable, low distortion, no keying clicks.
pa0nhc print v 20121209. PCB 4.4     (201212 : 20130506 + PDF   200150121 layout.

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This circuit is able to generate one pure sine wave tone : 1750Hz generator, subaudible tone (71.9 - 88.5 Hz), CW practice, roger beep.
Buffered keying : switching level : 3.6V. Suited for mechanical morse key, but also for digital key input (TTL/CMOS).

Simple analog circuit. No special IC, nor software used. No SMD components.
Course frequency range is set by five identical capacitors (18Hz : 1uF to 18kHz : 1nF. With 22nF ca. 820Hz).
Filter bandwidth + and - 10 % @ -3dB. Allows for accurate fine tuning of the tone, up to + / -10%.
Second harmonic distortion ca. -35dB.

Adjustable output. Max 4 Vpp / 1.2 Vrms.
Power: 12V-15V / 11 mA.
Low noise and hum level. Can allways be switched on, 

PCB design provided. Double sided or jumperfree single sided. Small : X x Y =  42.926 x 38.227 mm2
For cheap, standard through-hole components.
Wide traces, practical isolation wideness, for easy soldering.
Good RF-immunity thanks to max. surface mass-planes. PCB is suited for installing inside a transmitter housing.

Available downloads : 2400DPI MASK file, ready for printing + CAM files (GRB / DRL). Optimal for home made, or factory made PCBs.
No wire jumpers needed for a single sided version..
Detailed description.

Download files:

Xray view


Schematic (GIF 600DPI)


(Top + Bottom)

Top silkscreen (PNG 600DPI)



Partslist (DOC)



PCB Xray view.
Green is top copper. No traces there. 
Crossed pads must be soldered on both sides (if possible).


Bottom copper

    How does it work?
When "key" is connected to mass, this keying signal is buffered by IC1b, which switches at 3.6V level. IC1a then oscillates with a squarewave as output. 

Such a keyed squarewave is unusable for modulating a transmitter. The on/of switching by the keying signal introduces a large DC-shift, and a loud "PLOK" in the listeners speaker. A squarewave also contains much harmonics. Tiring to listen to. And causing to wide bandwidth of the transmitted signal. Simple low-pass and high pass filters are not adaquate for cleaning up susch a signal ! 

The double active bandpassfilter in IC1c and IC1d removes not only all frequencies above the fundamental, but also the frequencies below it. The result is a remarkable clean signal without sharpness nor key-clicks. But still fast keyable.

The oscillogram shows a tone of 820Hz, keyed with 35Hz. With tuning capacitor values for C1, C3, C4, C7 and C8 : 22nF
A 10 times lower frequency demands 10 times higher capacitor value and vice versa.

The bandwidth of the filter is so choosen, that fast enough keying is possible (up to 40WPM CW), without "ringing". It also allows some (accurate) de-tuning of the oscillator, up to + - 10% away from the filter center.

If you only want to use this circuit as a simple sounder, the component  demands are less stringent. In such cases you could use all ceramic capacitors (on;y of the X7R type, not Z5U nor Y5V).
P1 then could be a simple trimpot. You still get, although less stable, a nice keyed tone.

1750Hz and CTCSS (sub audilbel tone) filters in repeaters are very selective. These tones must be generated accurately and stable. This calls for a stabilised power supply, and temperature stable metalfilm resistors and FILM capacitors for C1, C3, C4, C7 and C8.

Exept NP0 types, all ceramic capacitors change with applied DC and AC voltage and are aging in time. They are NOT stable. Only useable for decoupling. Not for coupling.
P1 MUST be a CERMET 10-turn trimpot for reasons of exact adjustment and frequency stability. Best use a 1K value or even less. A low value helps for easy and accurate adjustment.

You can use R14+R14a for course setting of the  oscillator frequency (totally approx. 33-34 kOhms (27k + 5k6)).
C1b can be used to adjust the wanted tone frequency into the middle of the adjustment  range of P1.

In several prototypes the circuit showed a slight POSITIVE temperature coŽfficcient (ca. +0.5% for ca. 30C up). C1b could be a POSITIVE temperature coŽfficient ceramic capacitor to compensate it. The used values must be calculated with the used capacitances in mind.

The here wanted filter capacitor values are mostly not standard values. For each of C1, C3, C4, C7 and C8, two places in parallel are reserved on the PCB. You can combine two capacitors for one exact value. 

Tolerances of capacitors can be 10%. It is wise to measure the capacitor combinations before soldering them in. The pairs should be  +/- 1% of the total calculated value.

Using a cheap, modern digital multimeter, suited combinations can easily be selected.


Drill all holes 0.8mm.

Drill the holes for D2 and the connectors 1.2 mm.

Dril the screwholes 3.5mm.

Before assembling parts, connect the mass planes of the PCB to the soldering irons mass, to prevent static damage.

Solder IC2. Solder pin2 on both mass planes. Solder bottomside first.

Solder first C5, then C6. Allow for a little space between the capacitor and the top copper plane. Solder bottomside first. Solder their mass pins on both mass planes.

Solder D2.

Solder IC1. Solder pin 11 on both sides.

Solder D1.

Solder the resistors. Solder the mass connection of R3, R6, R8, R11 on both planes.

Solder the remaining capacitors.

Solder P1, 2, 3.

Solder the PCB connectors.


Pre-adjust the trimpots until :
at P1 you measure 34kOhm between Pin 1 and 2 of IC1,
at P2 you measure 1 kOhm between its mid connection and mass,
P3 is at max position (0 Ohm between its mid connection and C9).

    Setting up:

1 Connect an oscilloscope (10V range) or a multimeter (2Vac range) to "Output".
2 Connect 12-15Vdc power.
3 With a high-impedance (digital) multimeter check that the voltages on IC1-4 is 8.0Vdc and on IC1-6-10-12 is 3.6V dc.
4 Connect "Key" to mass.
5 Tune the frequency with P1 to max output on the scope or meter.
6 Adjust P2  for max. undistorted output, then turn back to 80% of that value.
7 Readjust P1 for the wanted frequency. Up to + or - 10% from the filter center frequency is allowed.
8 Readjust P2 for 80% of max. output.
9 Lock P1 and P2 with wax, glue or nail lacker.
The final output voltage shall from now on be adjusted with P3. Do not change the adjustments of P1 nor P2 anymore.


At the very start (first minutes), the frequency creeps a little bit up : ca. 0.1% . This is due to the warming-up of the IC1 chip. Therefor the circuit should best be under power all the time.

The frequency stability is completely dependent of the quality of the used film Cs , especially C1 (MKS2). 

For 1750Hz the capacitor values can be calculated as ca. (820 / 1750) x 22n = 10.29 nF (use 10nF + 270pF). For C1b use a ceramic capacitor with POSITIVE temperature coŽfficient.
Adjustment to 1750Hz can best be done using a counter with 10s porttime.

    Sub audible tones (for CTCSS).
For 80 Hz, the capacitor value can be calculated as : (820 / 80) x 22n = 225.5 nF.
Using only one capacitor value of 220nF +-1% for C1, C3, C4, C7 and C8, all subaudible tones between 71.9 and 88.5 Hz could be generated with full output and low distortion.

    IMPORTANT: Where to connect a sub audible tone into a transmitter.
 As harmonics in this signal are well filtered, little or no "hum" will be noticed when listening to a transmitter.
            As long as :
- the tone deviation does not exeed 250Hz (which is sufficient to operate a repeater in bad signal conditions)
- the tone is injected BEHIND THE CLIPPER (f.i. via a suitable value seriesresistor, directly soldered at the center connection of the daviation potentiometer).

A subaudible tone may >>NOT<< be injected somewhere in the microphone audio amplifier of a transmitter !
The clipper circuit  WILL generate severe distortion in the modulation AND WILL causing shutdown  (hickups) of the repeaters CTCSScircuit during modulation peaks !

    An accurate adjustment method for a sub audible tone frequency (CTCSS tone).

Because subaudible tone detectors in repeaters have a very narrow bandwidth tone filter, the tolerance for the subaudible tone frequency is very small.

Definitive adjustment should be done when the PCB (transmitter) is warmed up to abt 35C.

- Connect the tone output to the X-input of a oscilloscope.
- Connect the Y-input to the data-output of the receiver (in a good receiver the sub audible tone is filered from the speaker signal, and not measurable there).
- Switch the oscilloscope in X-Y mode and adjust X gain and Y gain and position.
- Tune the receiver to a repeater which sends the desired tone frequency. 
- Tune P1 to a still-standing ellips.
- Test the temperature stabilty by heating the PCB several times up to 50C using a hair dryer, and cooling it down.

The tone frequency is accurate enough, if on the oscilloscope the ellips turns around fully 1 time after a longer peroid than 12s time.