LM1893 datasheet(9/24 Pages) NSC

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Component Selection (Continued)

be made above 10 pF so that parasitic capacitance is not

dominant

Excessive or unbalanced common-mode-to-

ground capacitance should be avoided A low temperature

coefficient (TC) of capacitance (k100 PPM C) such as a

monolithic NPO ceramic multilayer type preserves low TC

of FO Figure 6 finds a CO value given FO

Resistor RO is used by the IC to generate a VBE R related

current that is multiplied by 2 to produce the 200 mA ICO

control current that sets FO The control current TC ‘‘bucks’’

the VBE related tri-wave amplitude across CO to effect a low

TC of FO Vary RO to trim FO within limits Raising FO more

than 20% above its untrimmed value by means of decreas-

ing RO more than 20% is not recommended Low RO and

so high control current risks ICO saturation and poor TC

under worst-case conditions Raising RO reduces the de-

modulated signal amplitude from the phase detector raising

RO by more than a factor of 2 (1 octave) is not recommended

Since lower TC pots are relatively costly it is recommended

that RO be made up of a 56 k fixed (k100 PPM C) resistor

witha2kX (k250 PPM C) series pot

CA and RA

Components CA and RA control the dynamic characteristics

of the transmitter output envelope Their values are not crit-

ical Use the values given in

Figure 5 CA and RA are func-

tions of loaded T1 tank Q RO fDATA and line impulse

noise Any changes made in CA and RA should be made

based on empirical measurements of a CCT on the line

Roughly CA acts as an ALC pole and RA an ALC zero

At this point the CCT system designer may choose to use

one of the recommended transformers or to design custom

T1 Consult ‘‘The Coupling Transformer’’ section to help

with the design of T1 if a new or boost-capable transformer

is needed The recommended 125 kHz transformer func-

tions with an IO of up to 600 mApp

It is recommended that CCT systems use the recommended

transformers described in

Figure 7 for T1 The 3 transform-

ers are optimized for use in the ranges of 50 – 100 kHz 100 –

200 kHz and 200 – 400 kHz with unloaded Q’s (QU) of about

35 and loaded Q’s (QL) of about 12 Three secondary taps

are supplied with nominal 707 10 and 141 turns ratios (N)

to drive industrial and residential power line impedances of

35 7 and 14X respectively All are inexpensive all have

the same pin-outs for easy exchange in a PC board and all

are small - on the order of 10 mm diameter at the base

Tank resonant frequency FQ must be correct to allow pas-

sage of transmitter signal to the line Use

Figure 8 to find

CQ’s value Trimming FQ to equal FO is done with T1’s trim-

ming slug The inductance of T1 hasaTCof a150 PPM C

which may be cancelled by using a b150 PPM C cap such

as polystyrene Since circulating current in the tank is

ARMS CQ should have a low series resistance (a 1 X series

resistance is too much) Polypropelene caps are excellent

‘‘orange drop’’ mylars are adequate while many other my-

lars are inadequate A 100V rating is needed for transient

protection

TLH6750 – 5

FIGURE 6 Find CO’s value knowing FO

TLH6750 – 10

FIGURE 8 Find CO’s value given FO

Bottom View

TLH6750 – 6

TLH6750 – 7

125 kHz

Toko 707VX-A042YUK

TLH6750 – 8

50 kHz

Toko 707VX-A043YUK

TLH6750 – 9

300 kHz

Toko 161XN-A207YUK

FIGURE 7 The recommended T1 transformers available through

Toko America 1250 Feehanville Drive Mount Prospect IL 60056 (312) 297-0070

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