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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
|
Lov
2i
a
Fig. 1-7
Production detector
D504 | 0505
detected
clipped
signal
signal
Fig. 1-6
| Automatic noise limiter circuit
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