LM1893 National Semiconductor, LM1893 Datasheet - Page 6

• Manufacturer Part Number: LM1893
• Description: LM1893/LM2893 Carrier-Current Transceiver
• Manufacturer: National Semiconductor

4 shows the interface circuit now discussed The controller
Application Information
THE DATA PATH
The BI-LINE
carrier-current transceiver (CCT) system of Figure 3 Figure
may select either the transmit (TX) or receive (RX) mode
Serial data from the controller is used to generate a FSK-
modulated 50 to 300 kHz carrier on the line in the TX mode
In the RX mode line signal passes through the coupling
transformer into the PLL-based receiver The recreated seri-
al bit stream drives the controller
With the IC in the TX mode (pin 5 a logic high) baseband
data to 5 kHz drive the modulator’s Data In pin to generate
a switched 0 978I 1 022I control current to drive the low TC
triangle-wave current-controlled oscillator to
tion The tri-wave passes through a differential attenuator
and sine shaper which deliver a current sinusoid through an
automatic level control (ALC) circuit to the gain of 200 cur-
rent output amplifier Drive current from the Carrier I O de-
velops a voltage swing on T
proportional to line impedance then passes through the
step-down transformer and coupling capacitor C
line Progressively smaller line impedances cause reduced
signal swing but never clipping-thus avoiding potential radio
frequency interference When large line impedances threat-
en to allow excessive output swing on pin 10 the ALC
shunts current away from the output amplifier holding the
voltage swing constant and within the amp’s compliance
limit The amplifier is stable with a load of any magnitude or
phase angle
In the RX mode (pin 5 a logic low) the TX sections on the
chip are disabled Carrier signal broad-band noise transient
spikes and power line component impinge of the receiver’s
input highpass filter made up of C
bandpass filter In-band carrier signal band-limited noise
heavily attenuated line frequency component and attenuat-
ed transient energy pass through to produce voltage swing
on the tank swinging about the positive supply to drive the
Carrier I O receiver input The balanced Norton-input limiter
amplifier removes DC offsets attenuates line frequency
performs as a bandpass filter and limits the signal to drive
the PLL phase detector differentially The differential de-
modulated output signal from the phase detector contain-
ing AC and DC data signal noise system DC offsets and a
large
through a 3-stage RC lowpass filter to drive the offset can-
cel circuit differentially The offset cancelling circuit works
by insuring that the (fixed)
data squaring (‘‘slicing’’) comparator is centered around the
0 mV comparator switch point Whenever the comparator
signal plus DC offset and noise moves outside the carefully
matched
circuit it adjusts its DC correction voltage in series with the
differential signal to force the signal back into the window
While the signal is within the
is stored on capacitor C
offset hold capacitor charging during offset cancelling the
DC cancellation is done much more quickly than with an AC
coupling capacitor normally used in place of the offset can-
cel circuit Since impulse noise spikes normally ring the sig-
nal symmetrically around 0 V the fully bilateral offset cancel
topology affords excellent noise rejection The switched cur-
rent output of the comparator drives the impulse noise filter
integrator capacitor that rejects all data pulses of less than
the integrator charge time Noise appears as duty-cycle jitter
at the open collector serial data output
twice-the-carrier-frequency
g
50 mV voltage ‘‘window’’ of the offset cancel
TM
chip serves as a power line interface in the
M
By grace of the highly non-linear
g
g
50 mV signal delivered to the
1
50 mV window the DC offset
’s ( Figure 4 ) resonant tank
C
component
and T
1
g
and the tank
2 2% devia-
C
onto the
passes
6
for equivalent LM2893 pin
Unless otherwise noted all pin references refer to LM1893 but hold true
FIGURE 3 The block diagram of a carrier-current
system using the Bi-Line chip to interface digital
See NS Package Number M20B or N20A
Small Outline
Order Number LM2893M or LM2893N
FIGURE 2 Connection Diagrams
See NS Package Number N18A
controllers via the power line
Order Number LM1893N
Dual-In-Line Package
Top View
Top View
Dual-In-Line Package
TL H 6750 – 41
TL H 6750 – 2
TL H 6750 – 3