Sony CRF-230B - Page 6

| CRF-230B
1-3 TECHNICAL FEATURES
The SONY Model CRF-230B is a high-perform-
aruc: radio receiver having many features found in
cOmrmunication receivers. Among them are the
fo lbwing:
e 4 total of 23 bands covering the broadcast fre-
quencies of any country in the world. Included
we 2 fm bands, 19 sw bands, a mw band and
alw band.
e (ET (Field Effect Transistor) front ends in fm
and sw bands provide superior sensitivity, image
rejection, and stability.
e fasy-to-tune sw bands due to a dual-conversion
system providing a uniform 600 kHz tuning range
on each band.
e ?roduct detector for receiving ssb signals.
e An fm i-f circuit employing ceramic filters. An
am i-f circuit employing two ceramic filters
with two-step adjustable selectivity.
e Individual tuning knobs for sw2~ 19 bands,
swl/mw/lw bands and fm bands. Preset-tuning
of three stations is available.
e The power supply will operate from household
current in any country of the world (via built-in
voltage selector), internal battery, or car battery.
e Stable, noise-free fm tuning by means of AFC
and a muting system.
e ANL (Automatic Noise Limiter) to minimize
noise.
e Up to 4 watts of undistorted audio power output
with two built-in speakers or external speakers.
TO MUTING
CIRCUIT
1-4. CIRCUIT EXPLANATIONS
AFC (Automatic Frequency Control)
Available for Both FM Bands
In the Model CRF-230B, the local oscillator
frequency is above the signal frequency for the FM2
band, but below the signal frequency for the FM1
band. Because of this, oscillator drift produces a
detector dc output-voltage change of different
polarity on the FM1 band than on the FM2 band.
Therefore, dc control voltages taken directly from
the detector output can not be used to stabilize the
frequency of the local oscillator. To get around this
problem, a comparator circuit with selectable output
polarity is used (Fig. 1-2). The detector output is
devided by equal-value resistors R232 and R233 to
provide two voltages identical in magnitude but
opposite in polarity. These voltages are compared
against a reference voltage of approximately 1.5 volts
produced by the forward voltage drop across D204
and D205. The resultant voltages are applied to the
bases of transistor Q211 and Q212. The each
collector voltage is fed through AFC switch $202
to variable-capacitance diode (DO1 for FM1 band
and D101 for FM2 band). The variable-capacitance
diode is voltage sensitive, so its capacitance (and
hence the oscillator frequency) depends on the mag-
nitude of the control voltage. Load resistors R254,
R255 and also R258, R259 are connected in a
balanced bridge and the contact voltage at the
junction of R260 and R261 is used as a fixed bias
voltage (0.7V) for the variable-capacitance diode
when AFC switch S202 is set to OFF.
IF T204 nee a ie
c circa (DIO!)
TO AFC
CIRCUIT (DON
Fig. 1-2