Akai AT-2650

Vi.
OPERATING PRINCIPLES OF PLL CIRCUIT
EMPLOYED
IN
STEREO
DEMODULATION
CIRCUIT
INPUT SIGNAL fs
VvcO OUT
LEFT CHANNEL
SIGNAL (L)
RIGHT CHANNEL
SIGNAL (R)
DIFFERENCE
SUB CARRIER
To separate the FM stereo broadcast signal received to
date into left and right signals, a 19 kHz pilot signal was
successively multiplier to form a 38 kHz signal and
ste
reo separation was effected from this. However, with
this multiplier system, change in coils due to wear
occurred and adjustment points were numerous, etc.
Therefore, this model employs a newly developed PLL
cir
SW
1.
cuit which produces an exceedingly accurate 38 kHz
itching signal.
PLL CIRCUIT OPERATION
PLL circuit is a kind of feedback circuit and is com-
prised of a Phase Comparator (PC), a Low Pass Filter
(LPF), a Direct Current Amplifier (DC Amp) and a
Voltage Control Oscillator (VCO) as shown in Fig.
13. The PC compares input signal Fs and VCO
oscillator output and generates the difference in
signal voltage Ve(t) proportionately to this phase
deviation. This Ve(t) passes LPF and the DC Amp
and becomes control voltage. This control voltage
supplied to VCO and VCO oscillation frequency is
DC controlled. When there is no input signal Fs,
because there is also no Ve(t), control voltage Vd(t)
becomes zero, and VCO maintains a *Free-running
oscillation frequency. When a signal enters, VCO
oscillation frequency Fo is controlled to narrow the
difference between Fs by means of feedback as
described above, and the PLL circuit assumes a
synchronous condition. This is referred to as input
signal lock. (When the difference between Fo and Fs
is too large, the differential signal frequency becomes
MAIN CHANNEL SIGNAL
MODULATOR
Fig. 13 PLL Circuit
ANTENNA
WIDE BAND
FREQUENCY TRANSMITTER
MODULATOR
SUB CHANNEL SIGNAL
Fig. 14 FM Broadcasting System Diagram
high and is reduced at the LPF. However, because the
VCO control voltage does not change, PLL will not
stay within the *lock range).
Because of the ability of the signal interference
removing LPF to accumulate the previous voltage
when the PLL deviates from within the lock range
due to certain interference, the original condition is
quickly reinstated.
* Free running frequency: Oscillating frequency
when there is no input signal.
* Lock range: At the condition in which the VCO
oscillation frequency is locked to the input
signal, the lock range is the oscillating fre-
quency in which when the input signal
changes, the PL:. maintains its input signal
lock condition.
Accordingly, in case Fs is changed inside the
PLL lock range, VCO oscillation frequency
always follows this, and ano frequency devia-
tion and no phase difference signal is ob-
tained. In other words, VCO oscillation
frequency can be locked to Fs.
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