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M-8870-01SMTR Datasheet(PDF) 2 Page - List of Unclassifed Manufacturers

Part No. M-8870-01SMTR
Description  DTMF Receiver
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Maker  ETC2 [List of Unclassifed Manufacturers]
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Functional Description
M-8870 operating functions (see Figure 2) include a bandsplit
filter that separates the high and low tones of the received pair,
and a digital decoder that verifies both the frequency and dura-
tion of the received tones before passing the resulting 4-bit code
to the output bus.
Filter
The low and high group tones are separated by applying the
dual-tone signal to the inputs of two 6th order switched capacitor
bandpass filters with bandwidths that correspond to the bands
enclosing the low and high group tones. The filter also incorpo-
rates notches at 350 and 440 Hz, providing excellent dial tone
rejection. Each filter output is followed by a single-order
switched capacitor section that smooths the signals prior to lim-
iting. Signal limiting is performed by high-gain comparators pro-
vided with hysteresis to prevent detection of unwanted low-level
signals and noise. The comparator outputs provide full-rail logic
swings at the frequencies of the incoming tones.
Decoder
The M-8870 decoder uses a digital counting technique to deter-
mine the frequencies of the limited tones and to verify that they
correspond to standard DTMF frequencies. A complex averag-
ing algorithm is used to protect against tone simulation by extra-
neous signals (such as voice) while tolerating small frequency
variations. The algorithm ensures an optimum combination of
immunity to talkoff and tolerance to interfering signals (third
tones) and noise. When the detector recognizes the simulta-
neous presence of two valid tones (known as “signal condition”),
it raises the Early Steering flag (ESt). Any subsequent loss of
signal condition will cause ESt to fall.
Steering Circuit
Before a decoded tone pair is registered, the receiver checks for
a valid signal duration (referred to as “charac-
ter-recognition-condition”). This check is performed by an exter-
nal RC time constant driven by ESt. A logic high on ESt causes
VC (see Figure 2) to rise as the capacitor discharges. Provided
that signal condition is maintained (ESt remains high) for the val-
idation period (tGTF), VC reaches the threshold (VTSt)ofthe
steering logic to register the tone pair, thus latching its corre-
sponding 4-bit code (see Table 3) into the output latch. At this
point, the GT output is activated and drives VC to VDD.GTcon-
tinues to drive high as long as ESt remains high. Finally, after a
short delay to allow the output latch to settle, the “delayed steer-
ing” output flag (StD) goes high, signaling that a received tone
pair has been registered. The contents of the output latch are
made available on the 4-bit output bus by raising the three-state
control input (OE) to a logic high. The steering circuit works in re-
verse to validate the interdigit pause between signals. Thus, as
well as rejecting signals too short to be considered valid, the re-
ceiver will tolerate signal interruptions (dropouts) too short to be
considered a valid pause. This capability, together with the abil-
ity to select the steering time constants externally, allows the de-
signer to tailor performance to meet a wide variety of system
requirements.
Guard Time Adjustment
Where independent selection of signal duration and interdigit
pause are not required, the simple steering circuit of Figure 3 is
applicable. Component values are chosen according to the for-
mula:
tREC =tDP +tGTP
tGTP
≅ 0.67 RC
The value of tDP is a parameter of the device and tREC is the mini-
mum signal duration to be recognized by 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
RforatREC of 40 ms would be 300 k
Ω. A typical circuit using this
steering configuration is shown in Figure 4. The timing require-
ments for most telecommunication applications are satisfied
with this circuit. Different steering arrangements may be used to
select independently the guard times for tone-present (tGTP) and
tone-absent (tGTA). This may be necessary to meet system
specifications that place both accept and reject limits on both
tone duration and interdigit pause.
Page 2
M-8870
Teltone Corporation, 22121-20th Avenue SE, Bothell, WA 98021-4408 U.S.A
Phone: 800-426-3926 or 425-487-1515 Fax: 425-487-2288
40-406-00011, Rev. E
E-mail: info@teltone.com Internet: www.teltone.com
Figure 3 Basic Steering Circuit
Figure 4 Single-Ended Input Configuration




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