-=c
.
Portable
PLUS
Computer
Functional
Description
5-11
The tone dialing
path
requires two frequencies simultaneously: both the TCI
and
TC2 outputs of the
modulator
(A3U3, pins 10
and
9) are used. These two frequencies are mixed at the appropriate ratio
(determined
by
A3R38, A3R39,
and
A3R40), fed through the first filter (A3U8A, A3R37,
and
A3C20),
then ac coupled to the next stage. Here the
path
deviates from the normal data
path
and
goes through
the two filters (A3R22/ A3C18
and
A3R21/
A3C17)
and
to the analog switch A3U9B, which selects
between normal data mode
and
tone dial mode. The final stages of the tone dialing
path
are then
identical to the normal data transmission path.
Note:
The transmitted signal
is
also fed back into the first stage receive filter (via A3R8, A3R9,
and
A3C13). This allows the transmitted signal to be
'subtracted'
out
of the receive signal,
allowing the determination of the true receive data.
The
Receive
Path.
The signal present at the
phone
interface transformer
A3T1
contains both transmit
and
receive components. This signal
is
summed with the transmit signal by the receive
summer
circuit
(A3UI0A,
A3Rll
and
A3R12,
and
A3C14) to remove the transmit signal, leaving only the true re-
ceived signal.
The
true received signal
is
then ac coupled via A3C15 to the analog switch A3U9A,
which selects between normal
and
local analog loopback modes.
The
output
of A3U9A is fed directly to the switched-capacitor filter's 'receive
input'
RI
(A3U12, pin
16). Note that
A3U12 can be configured to sample four different
'bands'
of frequencies, as selected
by
the clock rate fed into the OSCIN input (A3U12, pin 5)
and
the state of the MODE
input
(A3U12, pin
13). This allows dial tone detection, remote ring detection,
and
voice detection, as well as normal
modem
operation.
Following the filtering, the receive signal
is
output on A3U12, pin 7 to the ac coupling capacitor
A3C16. Note that A3C16
and
A3R24 combine to form a high-pass filter. This is necessary because of
some low frequency oscillations present at the
RO
output of A3U12.
The
receive signal is then passed through an amplifier/filter (A3U10B, A3R25, A3R23, A3C21,
A3CR4,
and
A3CR5) called the soft-limiter. The soft-limiter
is
designed to amplify the signal
by
a
factor of 145, clip it into a pseudo-square wave,
and
maintain
an
exact 50% zero-crossing duty cycle.
(This
50%
duty
cycle
is
critical for 300-bps
FSK
operation.) The output of this stage is fed to two
different circuits.
The first is
an
energy-detect circuit.
It
is
designed to provide
the
active high signal NRG whenever
the
input
signal to the receive filters exceeds a pre-determined level (-
42
dBm). This controls the 'carrier
detect threshold' of the modem. The energy detect circuit consists of A3CRl, A3CR3, A3R26-A3R31,
A3C22,
and
A3U15B.
Note:
Approximately 3
dB
of hysteresis
is
built into this circuit
by
A3R30. Once the connec-
tion
is
established, the signal level must
drop
below
-45
dBm before the connection is lost.
The
other
circuit being fed by the soft-limiter
is
a zero-crossing detector consisting of A3R32-A3R36,
A3CR2,
and
A3U15A. The output of this zero-crossing detector circuit
is
a true square-wave signal.
This signal
is
the received data that is fed to
the
demodulator as the
'slicer'
input (A3U2, pin 1).
Note:
The square-wave signal should be
an
exact 50% duty cycle for proper
FSK
operation.
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