Revox B261 Serviceanleitung Seite 39

STUDtrR
]IEVOX
8261 SA
sEcTroN 3/4
STEREO DECODER
PCB
I.726.760
DOUELE
TUNED
19
kHz-CIRCUIT
76 kHz
OSCILNTOR
19 kHz BLOCKING
CIRCUIT
DEEMPHASIS
15
l5
kHz
LOW
PASS FILTER
The 38 kHz subcarrier
is generated in a PLL from the 19
kHz
pj-1ot tone. From the 76 kHz oscillator, the signal is taken
through a pulse shaper to a frequency divider
(:2).
The divided
38 kHz frequency controLs the
MPX
switching demodulator.
The
signal is taken through a second frequency divider
(:2) to the
phase comparator. The pilot tone is filtered out of the
IUPX
srgnal in a wide-band, phase-stable 19 kHz band-pass and also
taken to the phase comparator. If the two j-nput signals of the
phase comparator do not coincide with respect to frequency
and
phase, the error voltage of this stage follows up the 16
kHz
oscillator
through the loop filter and the tuning circuit.
The
MPX
signal is taken through a second path to the 19 kHz
band
rejection filter
where the piLot tone is eliminated.
The
signal
is now split
into the main channel through the de-emphasis
network and the auxiliary
channel through the 38 kIIz circuit.
For stations with weak reception, the
SN
ratio
can be increased
at the expense of cross tal-k with the aid of the
SEPARATION
switch. The main channel suppties the'aggregate
signal through
an amplrfier. The
differential. signal is developed in the
switching demodulator from the auxiliary
channel and taken to
the matrix.
After two 15 kHz low-passes which suppress the
residual
MPX
si-gnals, the AF signals are input to the audio
section.
The pilot
tone is branched off in the first circuit of the
19 kHz band-pass filter and taken to a switching demodulator
which operates in parallel with the phase comparator.
The
subsequent switching stage supplies the pilot
present signal
to the microcomputer control unit.