MV5089

2

OUTPUT FREQUENCY

Table 1 shows the output frequency deviation from the

standard DTMF frequencies when a 3.58MHz crystal is used

as the reference.

The row and column output waveforms are digitally

synthesised using R-2R D-to-A converters (see Fig.3),

resulting in staircase approximations to a sinewave. An opamp

mixes these tones to produce a dual-tone waveform. Single

tone distortion is typically better than 7% and all distortion

components of the mixed dual-tone should be 30dB relative to

the strongest fundamental (column tone).

DISTORTION MEASUREMENTS

THD for the single tone is defined by:

Tone Output

Frequency Using

3.5795545 MHz

Crystal

Standard

DTMF

(Hz)

697

770

852

941

1209

1336

1477

1633

% Deviation

from Standard

+0.62

+0.19

+0.61

-0.63

+0.57

-0.32

-0.35

+0.73

701.3

771.4

857.2

935.1

1215.9

1331.7

1471.9

1645.0

Column

Row

High

Group

Low

Group

Table 1: Output frequency deviation

100 (

2f

3f

4f

nf

100 (

2R

3R

nR

Where V2f --- Vnf are the Fourier components of the waveform.

THD for the dual tone is defined by:

2C

3C

nc

IMD

where

Figure 3: Typical sinewave output (a) Row tones (b) Column tones

COL

ROW

V

OUT

V

OUT

t

t

a)

b)