Aiwa CS-880 H, Руководство по обслуживанию

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CS-880H,HG,U,UC
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[ AIWA
Description of Circuitry
1. Block Diagram o f Synthesizer Tuner
ROD ANTENNA
EXT. O - lf.
ANTENNA
TERM INALS
O-
Fig. 1
2. Outline o f PLL Frequency Synthesizer
The P L L (phase-locked loop) requency synthesizer is a cirucit
which uses the extremely stable frequency of a crystal oscillator
as the reference signal to produce the frequencies desired. For
instance, to pick up a station broadcasting on a frequency of
100 MHz, a local oscillation frequency (fD : output frequency of
voltage-controlled oscillator) supplied to the mixer of 110.7 MHz
(100+ 10.7) is required. This particular unit adopts a prescaler
which employs a pulse swallow system to divide the frequency,
and send it to the programmable counter inside the controller
IC. The output frequency fn then enters the phase comparator.
The frequency of the extremely stable 4 5 MHz crystal oscillator
is counted down (1/180) at the same time and the reference
frequency fref of 25 kHz is sent to the phase comparator. The
phases of fn and fref are compared and the difference between
the two is detected. If there is no difference, the loop is locked;
if there is a difference, the control voltage passes through the
low-pass filter, it is fed out to the VCO and the VCO is con­
trolled until fn is made equivalent to 25 kHz.
The reference frequency fref for AM reception is 9 kHz (or 10
kHz). The VCO frequency signal is sent directly to the pro­
grammable counter.
CRYSTAL
OSCILLATOR 4.5MH1
V. C.O :V O L T « E CONTROLLED OSCILLATOR
Fig. 2
2-1. Operation During FM Reception
The pulse swallow system is first outlined.
The relationship between fosc and fref is expressed as:
^osc= N ^ ^ r e f............................................................ (1)
If N is assumed to be P notation:
fosc = (ni +Pn2 + P2 n3 + ........+ Pn —1 nn) fref
= P (n, /P + n2 + Pn3 + . . . . + Pn _ 2 nn) fref
If, now, the part including the second digit and above is made Np:
fosc = p (ni/ p + NP) fref
This is modulated to become:
fosc = <ni + PNp + Pn, — Pnt ) fref
= [(Np — n ,)P + n, (P + 1 )] f ref ..................... (2)
The above represents the principle of the pulse swallow system.
In order to achieve the relationship expressed in formula (2) by
physical means, this unit has a prescaler with two frequency
division ratios, 1/16and 1/17. In formula (1 ), this corresponds to
P = 16. Actual operation is as follows: when the signal produced
by dividing fosc by (P + 1 ) is counted down n, times at the first
programmable divider digit and nx becomes 0, the P-divided
signal is counted down (Np — n, ) times equivalent to the number
of the first digit subtracted from the number of the second and
higher digits of the programmable divider, and the cycle ends.
This cycle is performed with fref equal to 25 kHz.
When fs = 100 MHz is received:
f|F is 10.7 MHz and so therefore fosc = 100 + 10.7 = 110.7 MHz
From formula (1 ): N = ^ = 4428
25 KHz
If this figure is re-expressed in the sexadecimal notation, and
made to correspond with 114C formula (2):
Np = 114, n, = C
Therefore, fref x [(114 — C) x 10 + C x 11 ] = fosc
If this is re-expressed in the decimal notation:
25 kHz x [(162 + 161 + 4 - 12) x 16+ 12 x 17] = 110.7 MHz
What happens is that the prescaler divides the frequency by 1/17
for the first 12 counts and then by 1/16 until 264 counts, and
this switching operation is repeated. The swallow counter is
locked at 12 and the programmable counter is locked at 264.
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