• MRC circuit
LA1780M
S-meter
FM
S-meter
Noise amplifier
fc = The MPF + AMP 70 kHz
Required external transistor: 2SC536 or equivalent
Reason: Since the LA1780M MRC is a simplified
circuit, an OMRC level shifter is required.
To SNC
Pin 28
• If there is no AC noise on pin 32:
V24 = V27 – VBE
↑
QMRC
V27 will be about 2.5 V for an antenna input of 60 dBµ or higher.
• Since the MRC noise amplifier gain is fixed, the MRC circuit is adjusted by reducing the AC input level.
• The MRC circuit attack and release times are determined by the capacitor C27 on pin 27.
Attack: 7 µA × C27
Release: 500 Ω × C27
5. FM Soft Muting
Smaller R47
Larger R47
Larger
R47
Smaller R47
By comparing the pin 47 mute on adjustment voltage to the S-meter voltage (V24), the muting on point can be adjusted.
No. 5724-24/38