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MV8870

Integrated DTMF Receiver

Unspecified 

MV8870/MV8870-1

FUNCTIONAL DESCRIPTION

The MV8870 / MV8870-1 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 tone groups, 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 band-pass 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 Fig.3). Each

filter 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 trequencies of the incoming DTMF signals.

For testing and monitoring, the high and low group filter

and zero crossing detector outputs are made available via

FHT, FH, FLT and FL (HP package only).

DECODER SECTION

Following the filter section is a decoder employing digital

counting techniques to determine the frequencies ot 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 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 recognises the

simultaneous presence of two valid tones (this is referred to as

the ‘Signal Condition’ in some industry specifications) the Early

Steering output (ESt) will go to an active state. Any

subsequent loss of signal condition will cause the ESt pin to go

to its inactive state (see Fig.5).

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 the

voltage at the SVGT pin (VSt/GT) to rise as the capacitor

discharges (see Figs.4 and 5).

Provided signal condition is maintained (ESt remains high)

for the validation period (tGTP), VSUGT reaches the threshold

(VTSt) of the steering logic which allows it to register the tone

pair and strobe the corresponding 4-bit code into the output

latch (see Fig.6). At this point the SVGT pin is activated as an

output and drives VSt/GT to VDD (see Fig.5).

St/GT continues to drive high as long as ESt remains high.

After a short delay (tDP) to allow the output latch to settle, the

delayed steering output pin (StD) goes high to indicate that the

code for a new received tone-pair is available. The contents of

the output latch are output onto the output bus (Q1 to Q4 pins)

when the three-state output enable (TOE) pin is high.

The steering circuit works in reverse to validate the

interdigit pause between signals. Thus as well as rejecting

signals too short to be considered valid, the receiver will

tolerate signal interruptions (drop-out) too short to be

considered a valid pause. This facility, together with the

capability of selecting the steering time constants externally,

allows the designer to tailor performance to meet a wide

variety of system requirements.

VDD

18

VDD

MV8870/ St/GT

MV8870-1 ESt

17

16

StD 15

C

R

tGTA = RC In {VDD ÷ VTSI}

tGTP = RC In {VDD ÷ (VDD - VTSU)}

Figure 4: Basic Steering Circuit

0

10

20

ATTENUATION

(dB)

30

40

50

XY

ABC D

EF G

H

200 400 600

800 1000 1200

FREQUENCY (Hz)

1400 1600

1800

2000

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

2

Figure 3: Filter response

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