DTMF RECEIVER

HM 9270C/D

- 8 -

C

V

C

R

V

DD

V

DD

St/GT

ESt

V

DD

t

GTP

GTA

t

=(RC) ln (

V

V

DD

=(RC) ln (

V

V

DD

-

)

)

TST

TST

0.1

µf

V

DD

S

t

GT

/

ES

t

C

R2

R1

V

DD

S

t

GT

/

ES

t

C

R2

R1

FIGURE 7. BASIC STEERING CIRCUIT

Guard Time Adjustment

In many situations not requiring independent selection of receive and pause, the simple steering circuit of Fig. 7 is

applicable. Component values are chosen according to the following formulae:

t

t

The value of t

the device (see table) and t

the receiver. A value for C of 0.1 µF is recommended for most applications, leaving R to be selected by the

designer. For example, a suitable value of R for a t

Different steering arrangements may be used to select independently the guard-times for tone-present (t

tone-absent (t

both tone duration and interdigital pause.

Guard-time adjustment also allows the designer to tailor system parameters such as talk off and noise immunity.

Increasing t

maintain signal condition for long enough to be registered. On the other hand, a relatively short t

REC

with a long t

DO

would be appropriate for extremely noisy environments where fast acquisition time and immunity to drop - outs

would be required. Design information for guard-time adjustment is shown in Fig. 8.

tGTP=(Rp C) In (

tGTP=(Rp C) In (

tGTA=(R1 C) In (

Rp=

tGTA=(R1 C) In (

Rp=

TST

TST

)

V

TST

)

)

V

TST

)

R1+R2

R1R2

R1+R2

R1R2

a) Decreasing t

b) Decreasing t

FIGURE 8. GUARD TIME ADJUSTMENT