Ham Radio Repeater Toneburst Module
In the UK and across Europe there are many repeaters which are primarily intended to improve the range of mobile operators. On 70cm FM they also tend to act as centres of activity where Novices are quite often assured of a contact. These repeaters need an access tone to switch the transmitter from standby before use; this can be either a CTCSS tone (for more details see the RSGB Call Book) or a single frequency tone burst. Repeater Toneburst
The Repeater Toneburst was the first method used and is still fitted to all UK repeaters. The tone, which is at the start of the transmission, is a frequency of 1750Hz lasting for about 0.5 seconds. Most commercial sets come with a toneburst circuit fitted but if you are using an ex-PMR set which has been converted for the amateur bands, then this facility is not usually available.
Circuit Description
The circuit is shown in Fig 1. The tone is generated from a ceramic resonator (XL1) which is used to provide the output from the oscillator is divided by 256 to generate the 1750Hz tone.
The IC (integrated circuit) which is used contains both the oscillator and the dividers. The ceramic resonator is actually designed to operate at 455kHz but in this circuit we want it to oscillate at 448kHz (1750Hz x 256) so the two capacitors C1 and C2 and resistor R1 are used to pull the resonant frequency to the value we need.
In order to get the tone to last for the 0.5 seconds that we need, capacitor C3 and resistor R2 are used to control the reset pin. This is held low until the capacitor is charged sufficiently to force the IC to reset and the output is stopped. When the supply voltage to the toneburst is switched off, diode D1 discharges the capacitor (via resistor R3) ready for the circuit to operate again.
Components:
- XL1: 455kHz ceramic resonator (modified to oscillate at 448kHz)
- IC1: Oscillator/divider IC containing both oscillator and ÷256 divider 74AHC1G4208
- R1: Frequency pulling resistor (works with C1, C2)
- R2, C3: Timing network for 0.5 second burst duration
- R3, D1: Reset circuit for next operation
- S1: Activation switch (+12V)
- C4: Output coupling capacitor
The circuit generates a precise 1750Hz toneburst lasting 0.5 seconds, essential for accessing UK and European amateur radio repeaters.
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