4/8 tone CTCSS encoder

Last updated: August 31, 2012


CTCSS encoder	Connections

The encoder is based on a 32 bit DDS generator. The output is a PWM signal. After the low-pas filter this becomes a reasonably clean sine.

[ctcss.png]

Specifications

4 frequencies selectable with DIP switches.
8 frequencies if the RST input is use for IO (not tested, requires HVSP programmer)
Maximum output 1.5 Vtt
DDS frequency ~ 50 dB down for frequencies in the 70 - 80 Hz range
Power consumption 45mA @ 8-15V

Switches

Pin 1 RST:

1: Reset. Always off.
2 and 3: Tone select

S2 S3 Output

on on 71.9 Hz

on off 77.0 Hz

off on 82.5 Hz

off off 88.5 Hz
4: PTT test. Simulates 'PTT down'. Normally off.

Pin 1 I/O (not tested)

1, 2 and 3: Tone select

S1	S2	S3	Output	S1	S2	S3	Output
on	on	on	71.9 Hz	off	on	on	67.0 Hz
on	on	off	77.0 Hz	off	on	off	250.0
on	off	on	82.5 Hz	off	off	on	500.0 Hz
on	off	off	88.5 Hz	off	off	off	1000.0 Hz

4: PTT test. Simulates 'PTT down'. Normally off.

Results


Upper trace: signal after 1st RC section Lower trace: signal after 2nd RC section Photo: PE1GGX	Specification: 71.9 Hz +/- 0.4% ;-) Photo: PE1GGX

Implementation notes

The ATtiny13 8 bit timer is driven by the system clock of 1.8432 MHz. Every 256 clock ticks a Timer Overflow interrupts is generated and the Output Compare Register is updated. This give a 'DDS frequency' of 7200 Hz. This is more than sufficient to generate sines in the 70 - 80 Hz range.

The code contains a table with 8 DDS phase increments (ctcss_inc). PB5, PB4 and PB2 are used as an index in this table. If Pin 1 of the ATtiny13 is used as RST only the first 4 entries are used. PB5 is read as '0'. If Pin 1 is used for I/O all 8 entries are used.

When PTT is high the microcontroller goes into a power-save mode. This is rather useless, because the TCXO draws more than 40 mA. The power save mode saves 0.1 mA (measured). If power usage is an issue, another type of microcontroller that supports an external Xtal and power-save modes (ATtiny25, ATtiny45) should be used.

The Archive contains a program to calculate phase increments (calc_inc.c) and a program to calculate a sine table (sin_table.c)

Programming

The improved STK200 programmer (local copy), connected to a FreeBSD 8 system with avrdude, has been used for this project. This programmer does not support Atmels HVSP prorgramming mode.
The low fuse byte must be set to 0x7A (external CLK input), the high fuse byte remains 0xFF.

Downloads

Archive containing the Xcircuit files (encapsulated postscript) and the software.