Akai VS-2EGN - Page 95

11. DEMODULATOR FUNCTIONS
AND CIRCUITS
11-1 DEMODULATOR FUNCTION
The demodulator described here is a front end tuner,
and it amplifies the IF signal, and detects the video and
audio signals separately.
11-2, DEMODULATOR CONFIGURATION
The demodulator consists of the following:
1) Filter circuit for determining the band width of the
IF characteristic.
2) IF amplifier
3) Video signal detector
4) Audio signal detector
5) RF AGC voltage generator
6) AFT control voltage generator
7) Tuner band switching circuit
11-3 CIRCUITS
Circuits will be described with reference to Fig. 7-113.
11-3-1 IF FILTER CIRCUIT
Fig. 7-156 shows the IF filter circuit.
1) In Fig. 7-156, L1 and R44 form the input imped-
ance for the tuner. L1 and R44 are in parallel as the
load, together with some parasitic capacitance, and the
impedance is about 702. Coil L1 is inserted, because
the IF output characteristic is affected when the load
impedance for the tuner becomes capacitive.
2) C2 is a de block and HPF, and C2 is HF FPF. TR1
4) Fig. 7-158 shows the frequency characteristic of the
SAW filter.
‘SAW FILTER
Fig. 7-156 IF filter circuit
CFI
<RIN (saw), R OUT—>
Fig. 7-157 Equivalent circuit of SAW filter
is an amplifier for compensating for the loss of the SAW
(Surface Acoustic Wave) filter. Its gain is about 25 dB.
3)
The
SAW
filter
is
a
solid
state
element
providing
the desired frequency characteristic, and it requires no
adjustment.
Fig. 7-156 shows the equivalent circuit relating to the
SAW filter.
In Fig. 7-157, L2 is a coil for cancelling the SAW input
capacitance while L3 is a coil for cancelling the output
capacitance. Thus the internal capacitance of the SAW
filter is cancelled, and impedance is made a pure resist-
ance.
Input and output impedances viewed from the SAW fil-
ter are determined by R5 and R7, and mismatching loss
is determined thereby. (Insertion loss is 22—23 dB)
Fig. 7-158 SAW filter frequency characteristic
It is desired that the response between fp and fc is flat,
but there -is normally about 5 dB level difference bet-
ween. fp/fc and the peak. In addition, there is an audio
‘Service Manual VS-2EGN
92
trap, where the attenuation is about 15 dB.
fp’ and fs’ are video and audio traps of adjoining chan-
nels, where the attenuation is 40 dB or more.