Ordering number : EN660A
Monolithic Linear IC
FM IF System
Being developed for FM IF System, LA1231 is a highly unit: mm
integrated circuit in which almost all functions are 3006B-DIP16
contained concerning the FM tuner IF stage. The block
diagram of equivalent circuit is shown below. Usual
peripheral circuits are shown also in examples of
The IF amplifier and limiter stages are composed of 6-
stage double-end type differential amplifier, which is
superior on its AMR. The signal meter driving stage which
is located in parallel therewith consists of a 3-stage level
detector circuit and a drive circuit in order to extend the
The FM detection stage composed of a double-balance type
quadrature detecting circuit is attached with a low frequency preamplifier and muting control circuit.
The muting drive stage is composed of the following 3 circuits.
1) A level detection circuit which detects the S/N ratio of carrier wave when the input is weak.
2) A circuit which detects the dc output of FM detector's 'S' curve when detuning is made.
3) A driving circuit.
Thus, this muting driving stage can reduce foreign factors such as the interstation noises, the shock noise caused from
muting when detuning is made, and so forth.
Besides, a voltage-inverting circuit is built in LA1231N so that the muting may be enabled at any appropriate input signal
level. The output of this voltage-inverting circuit is connected with the muting drive output terminal. Thus, the muting can
be placed to the ON or OFF status when a control voltage is applied from other section to the input terminal of the voltage-
inverting circuit. For the control voltage, the output of the signal meter driver is appropriate. Both the AFC output stage
and tuning meter driving stage are of the current-driven type. So, not only the sensitivity of AGC but also the muting band
(when detuning is made) can be controlled by an outer resistor.
Being utilized for disabling the FM IF amplifier when AM reception is made, the IF amplification/stop circuit can decrease
the shock noise caused at the conversion between FM and AM reception modes.
• IF amplification, Limiter.
• Quadrature detection.
• AF preamplifier.
• Muting at weak.
• Muting at the detuning.
• Signal meter drive output.
• AFC tuning meter drive output.
• Delay AGC output.
• Inverting circuit for muting drive voltage. • IF amplifier stop circuit.
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