Bose AV28 Manual page 23

6.3 Phase-locked Loop Tuning
The AM and FM local oscillators are controlled by the LC72144 PLL IC, U103. Control from the
main board selects the AM or FM band and the particular frequency. The 7.2 MHz crystal, Y102, is
connected across an inverting amplifier inside U103 to form an accurate and stable crystal oscilla-
tor. The 7.2 MHz oscillator is divided down to produce a 12.5 kHz reference frequency in FM mode
and 10 kHz reference frequency in AM mode. U103 divides down the AM or FM LO and compares
it to the appropriate reference frequency, generating an error signal on pin 21. This error signal is
integrated by Q208, Q207 and associated components, producing the tuning voltage at the collec-
tor of Q207. C150, C146, R146, and R217 control the gain and pole-zero locations of this active
lead-lag filter
The AM tuning voltage is further filtered by R108 and C106 and is fed to pin 14 of the AM front end,
T101. The tuning voltage varies the capacitance of the varactor diodes, which in turn tunes the AM
antenna and the AM LO. Similarly, in FM mode, the tuning voltage is filtered by R148 and C151 and
fed to the FM front end. As in the AM case, the tuning voltage is fed to varactors which tune the LO
frequency and RF filtering.
Both the PLL IC and the LC72722 PLL IC are controlled via a CCB bus interface. This is a bidirec-
tional interface that sends control information, such as band select and frequency control, to the
PLL from the main board. This bus also sends status information, such as digitized S-meter value
and FM mono/stereo status, from the PLL to the main board controller
6.4 RDS Operation
This feature is only implemented in the European version of the AV28 media center.
The LC72722 Radio Data System (RDS) decoder IC, U102, is a single-chip system IC that imple-
ments the signal processing required by both the European RDS standard and US RDBS system.
RDS/RDBS systems can send digital information over the airwaves along with the standard FM
signal by adding a digitally modulated 57 kHz subcarrier to the normal FM composite signal. The
LC72722 includes a bandpass filter, demodulator, synchronization, and error correction circuits.
The input (pin 2) to the RDS IC comes from Q104, which buffers the FM composite signal at pin 23
of the LA1837. The timebase for the decoder is a crystal oscillator formed by the 4.332 MHz
crystal, Y101, the inverter internal to the IC across pins 12 and 13, and the two shunt capacitors,
C138 and C139. Control of the RDS IC is achieved by using the same CCB bus interface used for
the PLL IC.
6.5 Tuner Software
Software that controls the AM and FM tuner resides on the Main PCB.
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.
The software measures ("counts") the IF frequency, and this offset is added to the count. In the
final console test, the value that minimizes THD at 98.1 MHz for an un-modulated 50dBF signal is
chosen and stored before other stop levels are set.
It should be noted that the stop levels for the tuner are also stored on the main board; the implica-
tion of this is that every time a different tuner board is mated with a main board, the stop levels
must be reset due to board-to-board component variations.
Theory of Operation
23