MT8870 datasheet(3/12 Pages) MITEL

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ISO2-CMOS

MT8870D/MT8870D-1

4-13

Functional Description

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

350

and

440

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

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

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

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 vc (see Figure 4) to

rise as the capacitor discharges. Provided signal

VDD

St/GT

ESt

StD

MT8870D/

MT8870D-1

tGTA=(RC)In(VDD/VTSt)

tGTP=(RC)In[VDD/(VDD-VTSt)]

Figure 3 - Filter Response

ATTENUATION

(dB)

A B

1kHz

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

FREQUENCY (Hz)

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