Topsilk

Bottom copper

Prototype

    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.

    Parts.
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.

    But:
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 coëfficcient (ca. +0.5% for ca. 30C up). C1b could be a POSITIVE temperature coëfficient ceramic capacitor to compensate it. The used values must be calculated with the used capacitances in mind.

   Assembling.
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.

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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.

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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:

    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.