Akai VS-2EGN - Page 93

10-2-5 MIXER CIRCUIT
1) The mixer circuit is for obtaining sum and differ-
ence components by applying the received channel sig-
nal and the local oscillator frequency to the semiconduc-
tor which is a non-linear element.
2) Key requirements of the mixer circuit
The voltage to be supplied from the local oscillator shall
be sufficiently large compared to the receiving signal
yoltage. When it is insufficient, the oscillation frequency
may vary when there is a large signal input.
R29
lav
}——> IF OUTPUT
Fig. 7-147 Mixer circuit
In Fig. 7-147, when a voltage of a large amplitude is fed
to the base of the transistor TR5 from the local oscil-
lator through the injection capacitor C78, the transis-
tor operation is non-linear. When the received signal ap-
plied to the mixer is very small, sum and difference com-
ponents are generated, amplified, and picked up as the
collector output. The desired frequency band only
emerges from the subsequent IF tuned circuit.
10-2-6 IF OUTPUT CIRCUIT
1) The IF output circuit is a single tuned circuit for
selecting fjp=fo—fp from fotfp generated by the mixer
circuit.
2) Fig. 7-148 shows the IF output circuit (mixer tran-
sistor is included).
In Fig. 7-148, TRS and TR6 are cascade-connected to
avoid a low impedance as viewed from the mixer in-
put at the base of TRS. The output side of TR6 has a
single tuned circuit, and the output impedance is 75Q.
lav
Fig. 7-148 IF output circuit
3) Fig. 7-149 shows the equivalent circuit of the IF
output circuit. Fig. 7-150 shows IF response.
IN
OUT
c40
Fig. 7-149
fo
fe fp
fs 3dB
BW
Fig. 7-150
it
Resonance
frequency
fo
=
On
Jur
LIPT(C39
+
C40)
—3 dB band width is as follows:
fo
BW =TR TR=
where w=2nfo
R=value when damping resistor is made by (R26+
R29)
This IF response becomes the overall response of the
tuner.
wLIFT
R
‘Service Manual VS-2EGN
90