Intelligent Tone Module; Overview; Circuit Operation - ozQRP MST3 Construction And Operation Manual

Transistor Q8 is controlled by the ITM to mute the audio amplifier as required. When Q8 is
turned on it pulls pin 4 immediately to ground and effectively shuts off U4 and prevents any
audio being heard in the speaker. When Q8 is turned off audio is passed to the speaker. This is
the normal case for receive state, but also to allow tones to be heard during transmission.
Whenever Q8 is turned off, capacitor C56 charges slowly and provides a smooth click-less
transition.
3.11 I
NTELLIGENT
3.11.1 O
VERVIEW
The Intelligent Tone Module (ITM) is based around an ATtiny85 microcontroller incorporating
a high speed pulse width modulation (PWM) generator. The ITM has a central role in the MST3
and performs the following functions:

Tone generator for CW operation. The keyed tone bursts are shaped to minimise keying
clicks.

Generate a range of test tones.

Send a MST3 ident during voice operation.

Generate user feedback for controls and functions.

Processing of Push To Talk (PTT), CW key closures and Tone pushbutton contacts.

Control of PTT and audio mute signals.

Entry and storage of a call sign.
3.11.2 C
IRCUIT OPERATION
The ITM (U1) is clocked by an internal 8MHz oscillator. There is also an internal PLL circuit that
is used along with a divider to generate a 16MHz clock for the PWM module. When an audio
signal is generated the PWM is continually updated from a 256 value sinewave lookup table.
This equates to a sampling frequency of 62.5KHz. The highest required output frequency is
3KHz, and with such a high sample frequency this is easily obtained with a simple RC low pass
filter formed by R20 and C22. There is some remnant 62.5KHz ripple but this is inaudible in the
receiver and transmitted signal.
The PWM output (pin 6) is driven high and low when generating a tone. When the tone is
turned off the firmware forces the pin into a high impedance state, and the 10K resistors hold
the pin voltage at 2.5V. This means the tone waveform always starts and finishes at 2.5V and
minimizes DC transients that would otherwise cause audible clicks in the tones.
To further minimize the possibility of keying clicks the firmware shapes the start and finish of a
burst of tone by increasing the amplitude at the start of the waveform from zero to maximum
over about a 5mS period. This is done in reverse at the end of the tone burst.
The tones are coupled to the top of the Mic gain control via R14 and C14. The level of tone and
hence the transmitter output can be varied by adjusting the Mic gain control.
Internal tones are supplied to the audio amp via VR2 and C52. During transmit the ITM turns on
Q6 taking one leg of VR2 to ground. This now allows VR2 to adjust the level of sidetone during
transmit.
MST3 Construction and Operation Manual – Issue 1
T M
ONE ODULE
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