LM1893 National Semiconductor, LM1893 Datasheet - Page 17

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

The Coupling Transformer
P
where I
P
R
R
suming Q
l
R
R
Only Q
the
Q
An iterative solution is forced where line pull
guessed to find Q
pull guess a large error requires a new guess Try a BW of
8 7% - that is 4 4% for deviation 1% for TC of F
3 3% for
L
Knowing the core inductance per turn L and L
ber of turns is found
T
T is normally an integer but these transformers require so
few turns that half-turns are specified remembering that the
remaining
loosely coupled
T
giving an L
kHz transformer mirrors these specifications The resonat-
ing capacitor is
C
Z
1
O
O
1
2
Q
Q
Q
QS
Q
L
LN
e
e
ll l
e
e
e
e
e
e
FIGURE 29 Impressed line voltage for a given Z
is found using Z
b
l
e
I
Z
O
2
T
3 dB (half-power) bandwidth by
(18
BW (% of F
N
R
(2 F
e
LN
L
V
1
O
1
Q
L
remains to be found to calculate L
O
L
e
l
N
c
is in amps peak-to-peak at an elevated T
U
1
ll l
a
b
e
R
on the recommended transformers
2
1
F
e
e
2
1
Q
e
Z
49 5
7 07
125 000
Q
I
Z
Q
1
Q
2 2
4 7) 0 06
)
OPP
4
LN
of 0 98
for each of the 3 taps available
2
LN
V
turn is completed on the P C board and is
U 2
P
- giving Q
35
442
L
O 2
1
l
O
20 nH T
49 0 H
1
e
e
e
L
b
O
2(
e
e
)
and L
The secondary turns are calculated
7 00
(7 07)
1
LN
b
l
c
Z
33 1
(
1210
a
e
b
LN
V
1
H Note that the recommended 125
and the value for N found when as-
11 5
ALC
2 2
L
V
e
35
e
0 200 W
l
1
2
ALC
c
e
e
49
13 9
2
L
7 turns
a
1
e
10
e
11 5
I
442
V
O
a
is then used to check the line
1
49 0 H
b
a
1
e
V
turns
9
)
b
a
1
e
695
e
) 2
695
33 nF
(
1
b
e
4 7
e
1
442
a
1210
Q
4
(Continued)
L
V
F
is related to
a
1
TL H 6750– 32
Q
)I
the num-
O
J
must be
O
L
(10)
(11)
(12)
(13)
(15)
(16)
and
(5)
(6)
(7)
(8)
(9)
17
Figure 30 shows a low cost carrier amplitude detection cir-
Line pull
tude of 14
(11) an 11 8% bandwidth is required forcing L
duced to reduce Q That fix was not implemented some
signal attenuation under worst-case drift and
lowed L
conducts a
Line Carrier Detection
While the addition of a carrier detection circuit (for a mute or
squelch function) will only decrease receiver ultimate sensi-
tivity there is sometimes good reason to employ it to free
the controller from watching for RX signal when no carrier is
incoming or to employ it to reduce the probability of line
collisions (when multiple transmitters operate simultaneous-
ly to cause one or more transmissions to fail) Unless the
detector is heavily filtered or uses a high carrier amplitude
threshold there will be false outputs that force the controller
to have Data Out data checking capability just as is required
when using no carrier detector If false triggering is mini-
mized the probability of line collisions is increased due to
the inability to sense low carrier amplitudes and because of
sense delay The property of the LM1893 to change output
state infrequently (although the polarity is undefined) when
in the RX mode with no incoming carrier reduces the desire
to implement carrier detection and preserves the full ulti-
mate sensitivity Also many impulse-noise insensitive trans-
mission schemes like handshaking are easily modified to
recover from line collisions
Regarding this it should be stated that for very complicated
industrial systems with long signal runs and high line noise
levels it is probably wise to use a protocol which is inherent-
ly collision free so that no carrier detect hardware or soft-
ware is needed A token passing protocol is an example of
such a system
cuit
Audio Transmission
The LM1893 is designed to allow analog data transmission
and reception Base-band audio-bandwidth signals FM
modulate the carrier passing through the tuned transformer
(placing a limit on the usable percent modulation) onto the
power line to be linearly demodulated by the receiver PLL
Because the receiver data path beyond the phase detector
will pass only digital signal external audio filtering and am-
plification is required Figure 31 shows a simple audio trans-
mitter and receiver circuit utilizing a carrier detection mute
circuit A single LM339 quad comparator may be used to
build the carrier detect and mute Filter bandwidth is held to
a minimum to minimize noise especially line-related corre-
lated noise
Communication and System
Protocols
The development of communication and system protocols
has historically been the single most time consuming ele-
ment in design of carrier current systems The protocols are
defined as the following
1 Communication protocol a software method of encoding
and decoding data that remains constant for every transmis-
F
Q
was 6 4% - well above the 3 3% estimate Referring to
1
F
is already so small that the 31 gauge winding
Q
and up with any phase angle from
was calculated (reference 3) for a Z
A
RMS
circulating current
b
1
90 to 90
F
to be re-
L
Q
magni-
is al-