MT3170B/71B, MT3270B/71B, MT3370B/71B
Summary of MT3x70/71B Product Family
Device
Type
MT3170B
MT3171B
MT3270B
MT3271B
MT3370B
MT3371B
8 Pin
18 Pin
20 Pin
PWDN
2 Pin
OSC
Ext
CLK
ESt
DStD
Functional Description
The MT3x7xBs are high performance and low power
consumption DTMF receivers.
These devices
provide wide dynamic range DTMF detection and a
serial decoded data output. These devices also
incorporate an energy detection circuit. An input
voiceband signal is applied to the devices via a
series decoupling capacitor. Following the unity gain
buffering, the signal enters the AGC circuit followed
by an anti-aliasing filter. The bandlimited output is
routed to a dial tone filter stage and to the input of
the energy detection circuit. A bandsplit filter is then
used to separate the input DTMF signal into high
and low group tones. The high group and low group
tones are then verified and decoded by the internal
frequency counting and DTMF detection circuitry.
Following the detection stage, the valid DTMF digit is
translated to a 4-bit binary code (via an internal look-
up ROM). Data bits can then be shifted out serially
by applying external clock pulses.
Automatic Gain Control (AGC) Circuit
As the device operates on a single power supply, the
input signal is biased internally at approximately
VDD/2. With large input signal amplitude (between 0
and approximately -30dBm for each tone of the
composite signal), the AGC is activated to prevent
the input signal from being clipped. At low input
level, the AGC remains inactive and the input signal
is passed directly to the hardware DTMF detection
algorithm and to the energy detection circuit.
Filter and Decoder Section
The signal entering the DTMF detection circuitry is
filtered by a notch filter at 350 and 440 Hz for dial
tone rejection. The composite dual-tone signal is
further split into its individual high and low frequency
components by two 6
th
order switched capacitor
bandpass filters. The high group and low group
tones are then smoothed by separate output filters
and squared by high gain limiting comparators. The
resulting squarewave signals are applied to a digital
detection circuit where an averaging algorithm is
employed to determine the valid DTMF signal. For
MT3x70B, upon recognition of a valid frequency from
each tone group, the early steering (ESt) output will
go high, indicating that a DTMF tone has been
detected. Any subsequent loss of DTMF signal
condition will cause the ESt pin to go low. For
MT3x71B, an internal delayed steering counter
validates the early steering signal after a
predetermined guard time which requires no external
components. The delayed steering (DStD) will go
high only when the validation period has elapsed.
Once the DStD output is high, the subsequent loss of
early steering signal due to DTMF signal dropout will
activate the internal counter for a validation of tone
absent guard time. The DStD output will go low only
after this validation period.
Energy Detection
The output signal from the AGC circuit is also
applied to the energy detection circuit. The detection
circuit consists of a threshold comparator and an
active integrator. When the signal level is above the
threshold of the internal comparator (-35dBm), the
energy detector produces an energy present
indication on the SD output. The integrator ensures
the SD output will remain at high even though the
input signal is changing. When the input signal is
removed, the SD output will go low following the
integrator decay time. Short decay time enables the
signal envelope (or cadence) to be generated at the
SD output. An external microcontroller can monitor
this output for specific call progress signals. Since
presence of speech and DTMF signals (above the
threshold limit) can cause the SD output to toggle,
both ESt (DStD) and SD outputs should be
monitored to ensure correct signal identification. As
the energy detector is multiplexed with the digital
serial data output at the SD pin, the detector output
is selected at all times except during the time
between the rising edge of the first pulse and the
falling edge of the fourth pulse applied at the ACK
pin.
4-5
MITEL [ MITEL NETWORKS CORPORATION ]
