MT8889C
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
ACTIVE
INPUT
OUTPUT FREQUENCY (Hz)
SPECIFIED
ACTUAL
%ERROR
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, 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.
L1
L2
L3
L4
H1
H2
H3
H4
697
770
852
941
1209
1336
1477
1633
699.1
766.2
847.4
948.0
1215.9
1331.7
1471.9
1645.0
+0.30
-0.49
-0.54
+0.74
+0.57
-0.32
-0.35
+0.73
Table 2. Actual Frequencies Versus Standard
Requirements
Distortion Calculations
The MT8889C is capable of producing precise tone
bursts with minimal error in frequency (see Table 2).
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 Equation 1, which is the ratio of the
total power of all the extraneous frequencies to the
power of the fundamental frequency expressed as a
percentage.
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.
V
22f
+ V
23f
+ V
24f
+ .... V
2nf
THD (%) = 100
V
fundamental
Equation 1. THD (%) For a Single Tone
7
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