LM1893 datasheet(17/24 Pages) NSC

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The Coupling Transformer (Continued)

TLH6750 – 32

FIGURE 29 Impressed line voltage for a given ZL

for each of the 3 taps available

on the recommended transformers

POeIOVOe

IOPP

2(bVALCaVa)

(e(b47aVa)IO

(5)

where IO is in amps peak-to-peak at an elevated TJ

PO e

(18 b 47) 006

0200 W

(6)

ll lZLNl e

VO2

(bVALC a Va)

442 X

(7)

RQ is found using ZLN and the value for N found when as-

suming QU e 35

lZLNl e N2 ZLN e (707)2 139 e 695 X

(8)

RQ e

ll lZLNl

lZLNl

442

695

1210 X

(9)

RQS e

1 a QU2

1210

1 a 352

1 X

(10)

Only QL remains to be found to calculate L1 QL is related to

the b3 dB (half-power) bandwidth by

QL e

BW (% of FO)

(11)

An iterative solution is forced where line pull DFQ must be

guessed to find QL and L1 L1 is then used to check the line

pull guess a large error requires a new guess Try a BW of

87% - that is 44% for deviation 1% for TC of FO and

33% for DFQ - giving QL e 115

L1 e

442

2q c 125 000 c 115

490 mH

(12)

Knowing the core inductance per turn L and L1 the num-

ber of turns is found

T1 e

0L1Le0490mH

20 nHT

turns

(13)

T is normally an integer but these transformers require so

few turns that half-turns are specified remembering that the

remaining

turn is completed on the PC board and is

loosely coupled

The secondary turns are calculated

T2 e

495

707

700 e 7 turns

(15)

giving an L2 of 098 mH Note that the recommended 125

kHz transformer mirrors these specifications The resonat-

ing capacitor is

CQ e

(2qFQ)2 L1

331 c 10b9 e 33 nF

(16)

Line pull DFQ was calculated (reference 3) for a ZL magni-

tude of 14X and up with any phase angle from b90 to 90

FQ was 64% - well above the 33% estimate Referring to

(11) an 118% bandwidth is required forcing L1 to be re-

duced to reduce Q That fix was not implemented some

signal attenuation under worst-case drift and DFQ is al-

lowed L1 is already so small that the 31 gauge winding

conducts a

ARMS circulating current

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

Figure 30 shows a low cost carrier amplitude detection cir-

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

Communication protocol a software method of encoding

and decoding data that remains constant for every transmis-

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