Sony CRF-230B - Page 9

CRF-230B
Product Detector for Single-Sideband Reception:
Input Level (dB)
30
A product detector is a type of heterodyne
detector. Single-sideband signals can be recovered by
passing them through nonlinear device after being
mixed with a carrier identical in frequency to that
used during modulation at the transmitter. That is,
these two signals, sideband and carrier, are converted
into two beat signals, upper and lower, against the
carrier frequency by heterodyne action. The upper
beat signal is eliminated by passing through the filter
circuit and the lower beat signal is fed to the next
stage as audio signals.
In the model CRF-230B, the detector utilizes
the square-law characteristic (output current pro-
portional to the square of the effective value of the
input voltage) of a diode for the nonlinear device.
To minimize distortion, two diodes D502 and
D503 are connected in reverse each other and applied
the signals respectively positive in phase. That is
- ia ae - < because the range of square-law curve of one diode is
narrow causing distorted detection for strong input
signal.
The BFO injection voltage used for carrier re-
insertion is comparatively high (about 0.8 volt is
optimum) to set the operating point of the detector
within a linear portion of the diodes’ characteristic.
This results to minimize distortion of the recovered
40
50
60
70
BO
Fig. 1-5 — Overall i-f response curve
ANL (Automatic Noise Limiter):
This limiter in the a-m section clips any noise audio signal.
pulses accompanying the signal to a level no longer
than the signal amplitude. The clipping level is auto- IFT504 —
matically adjusted to match the variations in signal ras T —1¢ 7 ‘ pa Lis
level. The collector voltage of i-f amplifier Q502 for- if signal ad
7 Wy ~—we- detected
Rsi7 | Rs20 | signal
Cae Csi9
ward biases diodes D504 and D505 through resistors
R509 and R521, while the output voltage of detector
D501 provides a reverse-bias voltage (Fig. 1-6). These
two
bias
voltages
adjust
the
clipping
level
of
diodes
habit
D504
and
D505
to
match
the
average
signal
level.
|
Lov 2i a
Fig. 1-7 Production detector
D504 | 0505
detected clipped
signal signal
Fig. 1-6 | Automatic noise limiter circuit