Akai VS-2EGN - Page 90

10-2-3 RF TUNING AMPLIFIER
The RF amplifier is a tuned amplifier having a double
tuned circuit in its output stage. The frequency charac-
teristic, gain, noise factor, and cross modulation of the
tuner are almost completely determined here. Import-
ant points to note are as follows:
1) Cross modulation distortion
The received signal is modulated by other distortion
components, This is called cross modulation distortion,
This is generated from third order distortion components
occurring in the RF transistor. This can be improved by
using an FET. with the optimum bias.
RO
TO AGC
2) Noise figure (NF)
NF is indicative of the noise level generated by a transis-
tor. The smaller the NF, the better the S/N ratio on the
transistor output side. Since the NF is almost complete-
ly determined by the RF stage, it is necessary to use a
transistor with a better NF and to obtain a sufficient
gain at the same time.
3) Frequency response of tuned circuit.
Since the frequency characteristic of the tuner is almost
completely determined by this circuit, sufficient band-
width and flatness between fp (video carrier) and fs
(audio carrier) are essential. When the response of this
circuit is poor, the frequency response after video detec-
tion becomes worse, picture quality is affected, and the
selectivity falls.
INPUT
LOW CH lav
C35
if if 4t-——> OUTPUT
aT og
¢:
r
TUNING VOLTAGE <<
HIGH CH 12V <<
4) Fig. 7-137 shows the RF tuned amplifier.
@ Referring to Fig. 7-137, TR4 is an RF FET ampli-
fier with a grounded input on the first gate.
An AGC voltage is applied to the second gate. By chang-
ing the bias, the gain is changed. Amplification the high-
est when the output circuit is tuned, and only a specifi-
ed frequency band is amplified, which depends on the
characteristic of the tuned circuit.
PRIMARY SIDE<—
Fig. 7-137 RF tuned amplifier
© HF related components in the tuned circuit.
Fig. 7-138 shows mutual inductance coupled double
tuned circuit. The primary and the secondary tuned cir-
cuits are coupled by mutual inductance. The two tun-
ed circuits are symmetrical. When receiving a HIGH-CH
signal, D11 and D12 conduct so that C26, R15, Ld2,
Lm1, Lb2, R18 and C32 are shorted, and both the tun-
ing capacitance and inductance become smaller. R15
and R18 are damping resistors which are only connected
when LOW-CH signals are being received.
—-= SECONDARY SIDE
INPUT ©- if if ~O OUTPUT
s
& 8
4
= lg
3 is
{
7 l Z
|
EARTH (AT HIGH CH) EARTH( AT LOW CH)
Fig. 7-138 HF related components in the tuned circuit
Service Manual VS-2EGN
87