MT8888C

Data Sheet

10

Zarlink Semiconductor Inc.

The lookup table contains codes which are used by the switched capacitor D/A converter to obtain discrete and

highly accurate DC voltage levels. Two identical circuits are employed to produce row

and column tones, which

are then mixed using a low noise summing amplifier. The oscillator described needs no “start-up” time as in other

DTMF generators since the crystal oscillator is running continuously thus providing a high degree of tone burst

accuracy. A bandwidth limiting filter is incorporated and serves to attenuate distortion products above 8 kHz. It can

be seen from Figure 8 that the distortion products are very low in amplitude.

Figure 10 - Spectrum Plot

8.0

Burst Mode

In certain telephony applications it is required that DTMF signals being generated are of a specific duration

determined either by the particular application or by any one of the exchange transmitter specifications currently

existing. Standard DTMF signal timing can be accomplished by making use of the Burst Mode. The transmitter is

capable of issuing symmetric bursts/pauses of predetermined duration. This burst/pause duration is 51 ms

±1ms,

which is a standard interval for autodialer and central office applications. After the burst/pause has been issued, the

appropriate bit is set in the Status Register indicating that the transmitter is ready for more data. The timing

described above is available when DTMF mode has been selected. However, when CP mode (Call Progress mode)

is selected, the burst/pause duration is doubled to 102 ms

±2 ms. Note that when CP mode and Burst mode have

been selected, DTMF tones may be transmitted only and not received. In applications where a non-standard

burst/pause time is desirable, a software timing loop or external timer can be used to provide the timing pulses

when the burst mode is disabled by enabling and disabling the transmitter.

Scaling Information

10 dB/Div

Start Frequency = 0 Hz

Stop Frequency = 3400 Hz

Marker Frequency = 697 Hz and

1209 Hz