Akai VS-2EGN

10-2-2 INPUT CIRCUIT
The input circuit consists of a single tuned circuit, and
is provided to give good matching with the antenna as
well as selectivity.
The following symptons indicate poor matching.
1) Ghose on the screen
If matching is poor, signal reflections occur at the an-
tenna cable, which cuase delayed signals to enter the
filter, causing ghosting. The delay depends on antenna
cable length. VSWR (voltage standing wave ratio) is
used for indicating the matching degree matching.
VSWR is expressed using the characteristic impedance of
: from FILTER
a line (Zo: which for a tuner is normally 75Q) and the
network input impedance (ZR) as follows:
I+ Ae)
VSWR = ZR + Zo
1 BES)
ZR + Zo
Matched condition: VSWR = 1 when ZR=Zo
2) When VSWR becomes poor, matching loss increases,
and S/N performance on the screen becomes worse.
3) The VS-2EGN tuner input circuit is as shown in
Fig. 7-127.
TRA
CIRCUIT
AT THE TIME OF
LOW CH 12V
RI
8
SS
AT THE TIME OF HIGH CH I2V
Fig. 7-127 Tuner input circuit
D9,D10 ... . Switching diode
DS si eile aed Vari-cap diode
@_ Fig. 7-128 shows those parts of the circuit related to
HF signals, rewritten from Fig. 7-127.
® Figs. 7-129 and 7-130 show the circuits effective
during LOW-CH reception and HIGH-CH reception res-
pectively.
INPUT
gil:
Soe
i
Z Z ee
Fig. 7-129 Circuit during LOW-CH reception
RD: Damping resistor
Co: FET (TR4) input capacitance
*Channel is changed at Vc5.
Service Manual VS-2EGN
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INPUT
RU+R2/23
lt Le
Fig. 7-128 HF signal related circuits
INPUT
85