MT8889C datasheet(7/18 Pages) MITEL

7 / 18 page

MT8889C/MT8889C-1

4-113

Figure 10 - Spectrum Plot

Scaling Information

10 dB/Div

Start Frequency = 0 Hz

Stop Frequency = 3400 Hz

Marker Frequency = 697 Hz and

1209 Hz

Burst Mode

In certain telephony applications it is required that

DTMF signals being generated are of a specific

duration

determined

either

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.

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.

Table 2. Actual Frequencies Versus Standard

Requirements

Distortion Calculations

The MT8889C/MT8889C-1 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

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.

Equation 1. THD (%) For a Single Tone

ACTIVE

INPUT

OUTPUT FREQUENCY (Hz)

%ERROR

SPECIFIED

ACTUAL

697

699.1

+0.30

770

766.2

-0.49

852

847.4

-0.54

941

948.0

+0.74

1209

1215.9

+0.57

1336

1331.7

-0.32

1477

1471.9

-0.35

1633

1645.0

+0.73

THD (%) = 100

Vfundamental

V22f + V

3f + V

4f + .... V

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