LA1245 datasheet(3/11 Pages) SANYO

3 / 11 page

LA1245

No.737-3/11

Using the automatic search-stop signal

Signal Meter-driving output circuit is equivalent to Figure. 1, signal meter driving output (abbreviated as VSM) is

narrowed in band width and can be used as an automatic search-stop signal when a narrow band series resonator is

connected to pin 15. VSM can be adjusted with R208 and R211 both in wide band and narrow band since R208 is

inversely proportional to VSM, while R211 is proportional to VSM. R208 is related to the Q of narrow band signal

meter. When the resistance of R208 is increased, the Q will be damped and the band width increased. On the other

hand, R211 used as the output impeadance of VSM and affects the cut-off frequency and time constant of low pass

filter for VSM and the meter drive impedance. The time constant τ and the cut-off frequency fc can be expressed as

follows :

τ = (C

114+C115+CS) (R211//R

in)

fc = 2πτ

A semi-fixed resistor is recommended to be used as R211 to cope with the fluctuation of VSM. Refer to Figure. 3 for

the value of the semi-fixed resistor since this depends upon VSM and R208. Figure. 3 shows the lowest limit of the

semi-fixed resistor in relation to R208 with the parameter of VSM set point, and the value of the semi-fixed resistor

will be equal to or greater than that shown in Figure. 3. For example, when VSM=5V and R208=240Ω, R211 becomes

28k

Ω. Thus, the value of the semi-fixed resistor is determined to be about 30kΩ. When the value of V

SM is too large,

it is limited and saturated to the source voltage so it is recommended to follow the condition of VSM≤VCC–2(V).

When a narrow band serial resonator is used, include the resonant impedance to determine the value of R208.

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