The U2860B-M includes two identical sound IF channels. Each consists of a limiter
amplifier, PLL FM demodulator and AF amplifier. Additionally, this circuit contains a
squelch function, mute switch and internal voltage regulation.
The intercarrier signals are fed through external ceramic bandpass filters to a 7-stage
limiter amplifier. This guarantees high input sensitivity and excellent AM suppression.
The alignment-free Phase Locked Loop (PLL) demodulator covers a wide frequency
range of 4.5 MHz up to 6.5 MHz with low-noise performance. The linear voltage to
frequency characteristic results in low harmonic distortion. The free-running frequency
of the internal VCO circuit is about 5.5 MHz. For this frequency, the input sensitivity and
VCO locking is optimal. An additional external resistor at pin 10 allows a frequency shift
of ±1 MHz via an internal offset current. With this option, it is possible to shift the opti-
mum conditions to the upper frequency (6.5 MHz) or to the lower frequency (4.5 MHz).
The offset current acts simultaneously on both VCO circuits. If no resistor is connected,
the offset current is disabled.
The demodulated signals are amplified to 500 mVrms with low output impedance at the
audio outputs (pin 6 and pin 7). AC decoupling at pin 8 and pin 9 of the audio amplifiers
leads to high common mode rejection.
For channel 2 the audio output amplifier and VCO2 is muted automatically (squelch)
when the second sound carrier is not present. This avoids a wrong identification for
stereo and dual sound in the stereo decoder. Therefore, with mono sound, there is no
output signal at pin 6. The automatic squelch function can be disabled by switching pin 4
Simultaneous muting of both circuits is possible by switching pin 3 to ground.
The internal bandgap reference ensures constant performance independent of supply
voltage and temperature.