LA1235 Просмотр технического описания (PDF) - SANYO -> Panasonic

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LA1235

FM IF System Applications

SANYO -> Panasonic 

LA1235

• Setting of muting circuit constnats

Drive current to be output at muting drive output pin 12 is 0.75mA typ., but

approximately 0.4mA may be caused by variations in characteristic or

changes in temperature (smaller at higher temperatures). It is desirable to

design the circuit so that the total current to be output from pin 12 at the time

of muting ON does not exceed 0.35mA. When driving the muting circuit of

the LA1235, the muting drive current (input current at pin 5) must be

considered besides this total output current. The muting drive current of the

LA1235 is 0.2mA max.

Thus, the muting constants are obrained as follows. If the muting drive

current is IR207≥0.2mA, R207≤14kΩ occurs and the total current It is shown

by the following expression.

Muting Circuit

It=IR206+IR207=

6.2V

R206

+

4.8V

R206+10kΩ

(10kΩ : Input resistance at pin 5, Refer to the above figure.)

If R207=10kΩ is taken with the variation s in voltage V12 at pin 12 and input resistance 10kΩ at pin 5 considered,

R206≈56.4kΩ is obtained, and then R206=68kΩ and R207=10kΩ are obtained.

• Setting of C112 (Capacitance between pin 12 and ground)

C112 influences S/N and muting attenuation. S/N is improved 0.5 to 2.0dB by changing C112 from 1µF to 100µF.

Muting attenuation becomes as shown in Mute (att) – fm (next page) characteristic. This phenomenon occurs

because the output at pin 7 appears at pin 6 through pin 10 and capacitance C110 between pin 7 and ground also exerts

influence. The relation between muting attenuation andC110 is such that if C110=2.2µF and C112=220µF, attenuation

at modulation frequency 100Hz becomes –80dB.

AF Output Circuit

No.873-5/12

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