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LA1235 FM IF System Applications

Overview
The LA1235 is a high integrated IC developed for use in high S/N, low distortion FM IF system applications. This IC features S/N=88dB, distortion factor=0.015% and has almost all functions required for FM tuner IF stage. The IF amplifier and limiter stage consist of 6 stages of double ended differential amplifier having an excellent AMR, and this stage is followd by the signal meter driver which consists of 4 stages of level detector, thereby creating extended linearity up to strong input. The FM detector stage consists of a double balanced quadrature detector to which a low frequency preamplifire and a muting controller are attached. The muting drive stage consists of an OR circuit for weak signal muting drive output which detects signal intensity and detuning muting drive output which detects S curve DC output and enables the prevention of noise at the time of weak signal and detuning. Further, the weak signal muting drive output circuit contains a Schmitt circuit having hysteresis and enables the prevention of muting malfunction due to amplitude component at the time of weak signal. The AFC output and tuning meter drive stage is of current drive type which makes it possible to adjust AFC sensitivity and muting band width by means of an external resistor, and the built-in tuning meter null (short) circuit foreces the tuning meter to be [0] when th IF amplifier stops working. The IF amplifier stop circuit, being a circuit to stop the FM IF amlifier at the time of AM reception, makes it possible to decrease shock noise due to FM-AM receiving mode switchover.
LA1235 Block Diagram
Functions
&127; IF amplifier, Limiter.
&127; Quadrature detection.
&127; AF preamplifier.
&127; Signal intesity muting drive output.
&127; Detuning muting drive output.
&127; AF signal muting circuit.
&127; Signal meter drive output.
&127; AFC, tuning meter drive output.
&127; IF amplifier stop circuit.
&127; Tunig meter null circuit.

Features
&127; High S/N (88dB typ.).
&127; Low distortion (0.015% typ.).
&127; Weak signal muting dirve output having hysteresis.
&127; Tuning mete null (short) circuit.
&127; Signal meter dirve output having wide dynamic range.
&127; High limiting sensitivity.
&127; Built-in constant-voltage regulated circuit (Operating voltage : 10 to 14V).



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