Bose AV28 Manual page 22

The composite audio signal is filtered by C124 and fed back into the LA1837 on pin 22. The value
of C124 affects FM stereo separation performance. Stereo MPX decoding is also performed by
U101 and the decoded left and right output signals are produced on pins 16 and 17. The pilot PLL
VCO is completely internal to the LA1837 detector IC, not requiring an external 456 kHz ceramic
resonator as in the older LA1836 and LA1851-based designs. The pilot PLL loop filter is formed by
C120, R118, and C122 on pin 14.
Capacitors C132 and C133 and the internal resistance of the LA1837 set the FM de-emphasis. For
a US unit the capacitor values are set to produce 75-uSec de-emphasis, and for Europe/Japan
they are set to produce 50-uSec de-emphasis. MPX filters, T104 and T105, reject the residual 19
kHz pilot tone and 38 kHz sub-channel demodulation components.
The FM and AM S-meter signals, which are at pin 11 and 12 of the LA1837 respectively, are
analog voltage levels that are proportional to the FM IF/AM RF input signal levels. These signals
are used to control the FM stop level, FM force-mono level, and AM stop level. The nominal FM
stop/force-mono levels are:
Parameter
FM Seek Stop
FM Force Mono
Both S-meter signals are connected to a 2-input 6-bit analog to digital converter that resides in the
LC72144 PLL IC, U103. During factory final test booth tuner alignment the appropriate test signal
levels are injected into the UUT and the resultant ADC values for stop and force-mono levels are
stored in the Flash memory of the main board. Level scaling is applied to both the AM and FM S-
meter signals so that these signals do not saturate the dynamic range of the 3.3 V full scale A/D
converter.
6.2 AM Tuner
The signal from the external AM loop antenna enters through the 2.5 mm AM jack, J300, and is fed
to the AM front end module, T101. This module contains the varactor-tuned RF and Local Oscillator
(LO) tracking circuit. This part is pre-tuned by the manufacturer for proper alignment in this circuit
with AM antenna, and is further adjusted during factory alignment, if necessary. The RF tuned
output appears on pin 12 and is fed to the AM buffer FET transistor Q103. This buffered output is
sent to pin 27 of U101 which contains the AM RF amplifier, mixer, IF amplifier, AM detector, and
AM S-meter circuitry. The 450 kHz AM IF output signal that appears on pin 2 is filtered by the IF
filter, T102, and fed back into the IC on pin 4. The AM IF signal is demodulated by the LA1837 and
the audio output is sent to pins 16 and 17, which are the left and right outputs.
The AM seek stop processing and factory alignment is performed in a similar fashion to FM mode
processing. The S-meter voltage that corresponds to the desired AM stop level is stored during
factory final booth tuner alignment in the main board Flash memory.
The nominal AM stop level is 56 dBuV/m at 1080 kHz.
Theory of Operation
Frequency
98.1 MHz
98.1 MHz
22
Level
30 dBf
40 dBf