MT8880C
Data Sheet
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 10 that the distortion products are very low in amplitude.
Scaling Information
10 dB/Div
Start Frequency = 0 Hz
Stop Frequency = 3400 Hz
Marker Frequency = 697 Hz and
1209 Hz
Figure 10 - Spectrum Plot
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±1 ms
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, a second burst/pause time of 102 ms
±2
ms is available. This extended interval is useful when precise
tone bursts of longer than 51 ms duration and 51 ms pause are desired. 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 duration is required, burst mode must be disabled and the
transmitter gated on and off by an external hardware or software timer.
Single Tone Generation
A single tone mode is available whereby individual tones from the low group or high group can be generated. This
mode can be used for DTMF test equipment applications, acknowledgment tone generation and distortion
measurements. Refer to Control Register B description for details.
Distortion Calculations
The MT8880C is capable of producing precise tone bursts with minimal error in frequency (see Table 1). The
internal summing amplifier is followed by a first-order lowpass switched capacitor filter to minimize harmonic
components and intermodulation products. The total harmonic distortion for a
single tone
can be calculated using
9
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
