Bose 3-2-1GS Series II Handbuch zur Fehlersuche - Seite 16
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THEORY OF OPERATION
6.3 FM Tuner
6.3.1 FM Tuner Front End Circuitry
The FM RF signal from the antenna is input via the F or PAL-type input connector J2001 [sheet
1, B8] and goes to the FM front-end module. The antenna supplied with the console is the
standard quarter-wavelength dipole antenna.
The FM front-end [sheet 1, B/C8] contains a tuned RF amplifier, FM local oscillator and a mixer.
The 10.7 MHz IF output signal (pin 7 of the module) passes through a 10.7 MHz ceramic filter,
CF2000 [C7], an FM IF amplifier Q2001 [C6] and then through a second ceramic filter, CF2001
[C6]. Transistor Q2001 and related circuitry form the FM IF amplifier produces about 15 dB of
voltage gain and provides the proper impedance matching for the ceramic filters. These FM IF
filter stages reject unwanted FM stations and noise.
The software that controls the FM tuner has provisions for an IF offset to optimize tuner perfor-
mance for a given range of IF filters. The possible values of IF offset are -25kHz, 0, and
+25kHz, with the available offsets determined by twice the reference frequency (2 x 12.5kHz).
The software measures ("counts") the IF frequency, and this offset is added to the count. In
tuner alignment, the value that minimizes THD at 98.1 MHz for an un-modulated signal is
chosen and stored before other stop levels are set.
6.3.2 FM Tuner Output Circuitry
The output signal from CF2001 is fed to the LA1837 AM/FM detector IC, U2000 [C4] at pin1.
This device contains the FM IF limiter, FM detector, FM stereo MPX decoder, and the S-meter
circuitry used for seek processing. The FM IF input signal to the LA1837 goes through several
gain/limiter stages and then to a single-tuned, coil-based discriminator circuit. The discriminator
coil, T2001 [C5], is adjusted for minimum second harmonic audio distortion. The recovered FM
composite signal appears on pin 23 of U2000.
The composite audio signal is filtered by C2018 [C2] and fed back into pin 22 of U2000. The
value of C2018 is chosen to optimize FM stereo separation. The stereo MPX decoding is also
performed by U2000 and the decoded left and right signals are output on pins 20 and 21. The
pilot PLL VCO is completely internal to the LA1837 detector IC, not requiring an external 456
kHz ceramic resonator as in older designs. The pilot PLL loop filter is formed by C2014, R2016,
and C2016 on pin 14.
FM de-emphasis for the left audio channel is set by C2026, R2022 and the output impedance of
pin 21 of U2000 (3.3k). FM de-emphasis for the right audio channel is set by C2027, R2023 and
the output impedance of pin 20 of U2000 (3.3k). For a US unit the capacitor values are set to
produce 75µS de-emphasis, and for Europe/Japan they are set to produce 50µS de-emphasis.
The resultant de-emphasized and amplified audio signal appears on pins 16 and 17. Signals
above the audio band, including the 38 kHz sub-channel demodulation components are cut off
at the bass module by the input filters in A/D converters and the audio DSP is used to create a
notch filter at 19 kHz to reject the 19 kHz pilot tone, thus removing the need for external MPX
filters.
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