LM1893 datasheet(12/24 Pages) NSC

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Component Selection (Continued)

CF and RF

These phase-locked loop (PLL) loop filter components re-

move some of the noise and most of the 2FO components

present in the demodulated differential output voltage signal

from the phase detector They affect the PLL capture range

loop bandwidth damping and capture time Because the

PLL has an inherent loop pole due to the integrator action of

the ICO (via CO) the loop pole set by CF and the zero set by

RF gives the loop filter a classical 2nd-order response

TLH6750 – 18

FIGURE 16 The Norton-input limiter amplifier bandpass

filter line-frequency signal attenuation given CL

TLH6750 – 19

FIGURE 17 Find CF given FO Figure 19

gives the maximum data rate

No CF and RF give the most stable PLL with the fastest

response Large CF’s with a too-small RF cause PLL loop

instability leading to poor capture range and poor step re-

sponse or oscillation

Calculation of CF and RF is quite difficult involving not only

the 2nd-order loop step response but also the PLL non-

dominant poles the tuned transformer stepped-frequency

response and the RC lowpass step response (for data rates

approaching 1 kHz) CF and RF values are best found em-

pirically Tolerance is not critical Component values are se-

lected to give the best possible impulse noise rejection

while preserving a g20% capture range and wide stability

margin

Figures 17 and 18 give CF and RF values versus FO

where ‘‘fDATA kk MAX DATA RATE’’ means that fDATA

should be less than the maximum data rate in kHz from

Figure 19 divided by 10

Note that CF and RF are a function of data rate only for high

data rates and are not plotted against data rate - as one

might expect The reason for this is important to understand

if the CCT system designer wishes to find CF and RF empiri-

cally Data signal is loosely speaking passed through the

PLL loop and is therefore potentially attenuated if the loop

bandwidth is on the order of the 3rd harmonic of the data

rate or less Overall loop bandwidth is held as low as possi-

ble for maximum noise rejection while passing the data

Loop bandwidth is roughly proportional to the geometric

mean of the unfiltered loop bandwidth and the filter pole set

by CF Therefore CF is related to data rate Unfortunately

the loop capture range falls to critically low values when

large enough values of CF are used to reduce loop band-

width down to the 100’s of Hz range for low data rates The

obvious way out is to then reduce the unfiltered loop band-

width That bandwidth is approximately proportional to the

value of CO For a fixed FO unfiltered loop bandwidth reduc-

tion requires a larger CO and larger control current With this

chip changing the control current is not allowed So one is

forced to choose a CF RF combination with some minimum

capture range say g20% that is within some guardband

from the point of loop instability Happily impulse noise

tends to last only fractions of a millisecond so that the lack

of low bandwidth loop response with low data rates is not a

heavy penalty As long as there is adequate capture range

the impulse noise filter performs admirably Note that reduc-

ing FO will reduce the no-filter loop bandwidth and indeed

the maximum data rate falls below the limit set by the RC

lowpass filter as FO falls below 100 kHz (Figure 19 )

The tuned transformer characteristics will affect the demod-

ulated data waveform more than CF and RF at low data

rates Tank Q and off-tuning will affect overshoot during the

FSK frequency steps This is a property of tuned circuits

The maximum data rate of

Figure 19 is measured from the

receiver input to the Data Out and does not include the data

bandwidth reducing effects of TI

Capacitor CM stores a voltage corresponding to a correction

factor required to cancel the phase detector differential out-

put DC offsets The stored voltage is

of the DC offset

plus some bias level of about 22 V A large CM value in-

creases the time required to bias-up the receive path at the

beginning of transmission A large CM does filter well and

store its bias voltage long Because of the initial random

charge of CM the receiver must be given a data transition to

charge to the proper bias voltage Therefore reducing CM’s

value to one that may be charged in less than 2 bit-times will

not save biasing time and is not recommended

TLH6750 – 20

FIGURE 18 Find RF given FO with FDATA a parameter

TLH6750 – 21

FIGURE 19 The maximum data rate versus FO using

loop filter components optimized for max noise

performance while retaining a min g20% capture

range (large signal)

Use

Figure 20 to find CM’s value knowing fDATA assuming

the standard 2 bit receive charge time is desired The cap

value and TC are not critical but the capacitor should have

low leakage

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