ISO
2
-CMOS
Functional Description
V
DD
MT8870D/MT8870D-1
The
MT8870D/MT8870D-1
monolithic
DTMF
receiver offers small size, low power consumption
and high performance. Its architecture consists of a
bandsplit filter section, which separates the high and
low group tones, followed by a digital counting
section which verifies the frequency and duration of
the received tones before passing the corresponding
code to the output bus.
Filter Section
Separation of the low-group and high group tones is
achieved by applying the DTMF signal to the inputs
of two sixth-order switched capacitor bandpass
filters, the bandwidths of which correspond to the low
and high group frequencies. The filter section also
incorporates notches at 350 and 440 Hz for
exceptional dial tone rejection (see Figure 3). Each
filter output is followed by a single order switched
capacitor filter section which smooths the signals
prior to limiting. Limiting is performed by high-gain
comparators which are provided with hysteresis to
prevent detection of unwanted low-level signals. The
outputs of the comparators provide full rail logic
swings at the frequencies of the incoming DTMF
signals.
Decoder Section
Following the filter section is a decoder employing
digital counting techniques to determine the
frequencies of the incoming tones and to verify that
they correspond to standard DTMF frequencies. A
complex averaging algorithm protects against tone
simulation by extraneous signals such as voice while
0
V
DD
St/GT
ESt
R
StD
MT8870D/
MT8870D-1
C
v
c
t
GTA
=(RC)In(V
DD
/V
TSt
)
t
GTP
=(RC)In[V
DD
/(V
DD
-V
TSt
)]
Figure 4 - Basic Steering Circuit
providing tolerance to small frequency deviations
and variations. This averaging algorithm has been
developed to ensure an optimum combination of
immunity to talk-off and tolerance to the presence of
interfering frequencies (third tones) and noise. When
the detector recognizes the presence of two valid
tones (this is referred to as the “signal condition” in
some industry specifications) the “Early Steering”
(ESt) output will go to an active state. Any
subsequent loss of signal condition will cause ESt to
assume an inactive state (see “Steering Circuit”).
Steering Circuit
Before registration of a decoded tone pair, the
receiver checks for a valid signal duration (referred
to as character recognition condition). This check is
performed by an external RC time constant driven by
ESt. A logic high on ESt causes v
c
(see Figure 4) to
rise as the capacitor discharges. Provided signal
PRECISE
DIAL TONES
X=350 Hz
Y=440 Hz
DTMF TONES
A=697 Hz
B=770 Hz
C=852 Hz
D=941 Hz
E=1209 Hz
F=1336 Hz
G=1477 Hz
H=1633 Hz
10
20
ATTENUATION
(dB)
30
40
50
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1kHz
X
Y
A B
C
D
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G
H
FREQUENCY (Hz)
Figure 3 - Filter Response
4-13
MITEL [ MITEL NETWORKS CORPORATION ]
