LM1893 datasheet(16/24 Pages) NSC

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Power Line Impedance (Continued)

TLH6750 – 29

TLH6750 – 30

TLH6750 – 31

FIGURE 27 Complex-plane plots of measured 115V 60 Hz line impedance where ZL e RL a jXL

Power Line Attenuation

The wiring in most US buildings is a flat 3 conductor cable

called Amerflex BX or Romex All referenced line imped-

ances refer to hot-to-neutral impedances with a grounded

center conductor The cable has a 100 X characteristic im-

pedance a 125 kHz quarter-wavelength of 600 m (250 m at

300 kHz) and a measured 7 dB attenuation for a 50 m run

with a 10 X termination Generally line loads may be treat-

ed as lumped impedances Instrument line cords exhibit

about 07 mH and 30 pF per meter

Limited tests of CCT link range using this chip show exten-

sive coverage while remaining on one phase of a distribu-

tion transformer (100’s of m) with link failure often occuring

across transformer phases or through transformers unless

coupling networks are utilized Total line attenuation allowed

from full signal to limiting sensitivity is more than 70 dB

Typically signal is coupled across transformer phases by

parasitic winding capacitance typically giving 40 dB attenu-

ation between phased 115 V windings Coupling capacitors

may be installed for improved link operation across phases

Power factor correcting capacitor banks on industrial lines

or filter capacitors across the power lines of some electronic

gear short carrier signal and should be isolated with induc-

tors Increasing range is sometimes accomplished by elect-

ing to install the isolating inductors (

Figure 28 ) and coupling

capacitors as well as by electing to use the boost option

Frequency translating or time division multiplexed repeaters

will also increase range

TLH6750 – 40

FIGURE 28 An isolation network to prevent 1) noise

from some device from polluting the AC line and 2) to

stop some low impedance device (measured at Fo)

from shorting carrier signal Component values given

as an example for Foe125 kHz on

residential power lines

The Coupling Transformer

The design arrived at for T1 is the result of an unhappy

compromise - but a workable one The goals of 1) building

T1 with a stable resonant frequency FQ that is little affect-

ed by the de-tuning effect of the line impedance ZL and of

2) building a tightly line-coupled transformer for transmitted

carrier with loose coupling for transients are somewhat mu-

tually exclusive The tradeoffs are exposed in the following

example for the CCT designer attempting a new boost-ca-

pable or different core transformer design

The compromises are eased by separating the TX output

and RX input in the LM2893 An untuned TX coupling trans-

former with only core coupling (not air-coupled solenoid

windings) would employ a high permeability high magnetic

field low loss square saturating toroidal core The reso-

nant RX path would be isolated from line-pull problems by a

unilateral amplifier that operates at line voltages with much

more than 110 dB of dynamic range or by a capacitively

coupled pulse transformer driving a unilateral amplifier and

filter for increased selectivity See the LM2893-specific ap-

plications section

For a LM1893-style transformer application first choose

the turns ratio N based on an estimated lowest ZL likely

encountered ZLN Figure 29 shows graphically how N af-

fects line signal N should be as large as possible to drive

ZLN with full signal If T1 has an unloaded Q QU of well less

than 35 a guess of N somewhat high should be used and

later checked for accuracy The recommended transformers

have secondary taps giving a choice of Ne707 10 and

141 (nominally) for driving ZLN’s of 14 70 and 35 X re-

spectively (at TJ e 25 C Vae 18V and QU e 35)

The resonating inductance of the tuned primary L1 is

sought Note that while standard transformer design gives a

transformer self-inductance with an impedance at operating

frequency well above load impedance the tuned transform-

er requires a low L1 for adequate QU and minimum line pull

Result relatively poor mutual coupling

L1 e

2qFOQ

(3)

It is known that resonant frequency FQ e FO and some

minimum bandwidth or maximum Q will be required to pass

signal under full load conditions

L1 e

ll lZLNl

2q FOQL

(4)

lZLNl is the reflected ZLN QL is the loaded Q and parallel

resistance RQ models all transformer losses and sets QO

ll lZLNl is found knowing that it absorbs full rated power

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