Sony ICF-6800W - PLL Circuit Locking by VCO2

ICF-6800W |
6) PLL circuitry is locked by VCO2
An important factor involved in the locking of
the PLL circuitry is VCO2.
If the oscillating frequency of VCO2is not locked,
the PLL circuitry cannot be locked either.
VCO2 is oscillating in increments of 1 MHz, from
28 MHz, 29MHz..... up to 37MHz, and if
it is not oscillating correctly at these frequencies,
the PLL circuitry will be unstable.
VCO2 applies the locking as indicated in the
illustrations shown in Fig. 13 and Fig. 14 below.
When the frequency is shifted upwards from the
lower end, the locking takes place as shown
in Fig. 13.
LOCKING
RANGES
Fig. 13
When the frequency is shifted downwards from
the upper end, the locking takes place as shown
in Fig. 14. .
LOCKING
RANGES y
fy PS
; f
ce
Fig. 14
The ranging of locking that takes place in this
manner is referred to as the locking range or
capture range.
The respective ranges are as shown below.
CAPTURE RANGE
APPROX. +50kHz
|
‘
!
fy
‘ LOCKING RANGE |
APPROX. +200kHz4
Fig. 15
The center frequency fo signifies the different
frequencies of 28MHz, 29MHz..... up to
37 MHz which are spaced at intervals of 1 MHz.
A sweep circuit is provided, and so even if there
should be some shift of the frequency as shown
above, as long as it is within the locking range,
it will be locked at the fo.
(If there is no sweep circuit, then it will not lock
at the fo unless within the capture range.)
Fig. 16 shows a block diagram of VCO2.,
PHASE
DETECTOR
Fig. 16
Within fl to £3, VCO2 will be locked at a certain
specific frequency fo by the output from the
phase comparator circuit; however, if the oscilla-
ting frequency of VCO2 is at a frequency between
fl and f4, or f2 and f3, a voltage will be fed to the
variable capacitance diode so that it will come
within the capture range by the sweep circuit,
and be locked at fo.
If VCO2 should be oscillating at a frequency
outside either end of this locking range, the
locking function will not take place.
SWEEP
CIRCUIT