Bose 3-2-1 Manuel de dépannage - Page 18

Parcourez en ligne ou téléchargez le pdf Manuel de dépannage pour {nom_de_la_catégorie} Bose 3-2-1. Bose 3-2-1 28 pages. Home entertainment system
Également pour Bose 3-2-1 : Manuel du propriétaire (44 pages), Manuel d'installation rapide (1 pages)

Imprimer la page

THEORY OF OPERATION

6.4 AM Tuner

The signal from the external AM loop antenna enters through the 2.5 mm AM jack, J2000 [sheet

1, D8], and is fed to the AM front end module, T2000 [D6]. The antenna input is balanced by

placing C2000 [D7] between one side of the input and ground, reducing electric field interfer-

ence. The front end module contains the varactor-tuned RF and Local Oscillator (LO) tracking

circuits. This part is pre-tuned by the manufacturer for proper alignment with AM antenna

PT199824-002, and is further adjusted during factory alignment, if necessary. The RF tuned

output is fed to the AM buffer FET transistor Q2002 [D6] and the buffered output is sent to pin

27 of U2000 [C3] 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, T2001 [C5], and fed back into the IC on pin 4. The AM IF signal is demodulated by U2000

and the audio output is sent to pins 20 and 21, to pass through the low pass filter used in FM for

de-emphasis.

In order to avoid having harmonics of the main board switching power supplies interfere with the

AM tuner the switching frequency is controlled by F_SYNCH, a clock output from U3 [console

schematic sheet 14, C4] on the main board. The signal Supply_Freq_Sel (via lookup table

based on AM station) is used to vary the output of F_SYNCH between two frequencies that will

avoid the selected channel frequency and its image the best.

The AM seek stop processing and factory alignment is performed in a similar fashion to FM

mode processing. The nominal AM stop level is 56 dBµV/m @ 1080 kHz.

6.5 Phase-locked Loop Tuning

The AM and FM local oscillators are controlled by the PLL IC, U2074 [sheet 2, C3]. The micro-

processor selects the AM or FM band and the particular frequency. The 7.2 MHz clock refer-

ence is generated by the microprocessor which is the stable crystal oscillator frequency divided

by 2. The 7.2 MHz oscillator is divided down to produce a 12.5 kHz reference frequency in FM

mode and a 10 kHz (9 kHz for European and Japan units) reference frequency in AM mode.

U2074 [sheet 2, C/D3] divides down the AM or FM LO input, compares it to the appropriate

reference frequency and generates an error signal that is output on pin 19.

This error signal is integrated and amplified by an active lead-lag filter formed using an internal

FET inside U2074 and associated components connected to pins 19, 20 and 21. C2096, C2097,

R2079, R2080 and R2081 control the gain and pole-zero locations of the filter. The values of

these components are chosen to ensure stability of the PLL while providing sufficient speed,

moderate overshoot, and symmetric up/down settling time. The resulting signal output at pin 21

is used as a tuning voltage and is fed back to the AM and FM front-ends.

The AM tuning voltage is further filtered by R2078 and C2095 and is fed back to the common

node of the varactors inside the AM front end, T2004. The tuning voltage varies the capaci-

tance of the varactor diodes, which in turn simultaneously tunes both the AM antenna and the

AM LO. In FM mode, the tuning voltage is filtered by R2077 [sheet 2, D1] and the input capaci-

tance of pin 5 of the FM front-end (0.047uF) [sheet 1, B/C8]. As in the AM case, the tuning

voltage is fed to varactors which tune the LO frequency and RF filtering.