HM9270C Ver la hoja de datos (PDF) - ELAN Microelectronics
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HM9270C Datasheet PDF : 10 Pages
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HM 9270C/D
DTMF RECEIVER
0.1µf
V DD
St/GT
ESt
R
S tD
V DD
C
t
GTA
=(RC) ln (
V DD
V TST )
VC
t GTP =(RC) ln (
V DD
V DD - VTST
)
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:
tREC = tDP + tGTP
tID = tDA + tGTA
The value of t is a parameter of the device (see table) and t is the minimum signal duration to be recognized by
DP
REC
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 of 40mS would be 300k.
REC
Different steering arrangements may be used to select independently the guard-times for tone-present (t ) and
GTP
tone-absent (tGTA). This may be necessary to meet system specifications which place both accept and reject limits on
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 improves talk-off performance, since it reduces the probability that tones simulated by speech will
REC
maintain signal condition for long enough to be registered. On the other hand, a relatively short t with a long t
REC
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.
VDD
V
DD
C
C
S t / GT
R1
R2
ES t
S t / GT
R1
R2
ES t
tGTP=(Rp C) In (
VDD
VDD - VTST
)
tGTA=(R1 C) In ( VDD )
R1R2
V
TST
Rp= R1+R2
tGTP=(Rp C) In (
VDD
VDD - VTST
)
tGTA=(R1 C) In ( VDD )
R1R2
V
TST
Rp=
R1+R2
a) Decreasing t (t < t )
GTP GTP
GTA
b) Decreasing t (t > t )
GTP GTP
GTA
FIGURE 8. GUARD TIME ADJUSTMENT
-8-
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