Kenwood TH-28A Service Manual page 7

TH-28A/E
CIRCUIT DESCRIPTION
PLL Circuit
* PLL
A 8kHz or 6.25kHz reference frequency is obtained
by dividing 12.8MHz reference oscillation frequency
X201 in IC201. A comparison frequency is obtained
when the VCO output is amplified by Q202 then di-
vided in 1C201 (pulse swallow system-based PLL IC).
AS, 10, 12.5, 15, 20, or 25kHz PLL synthesizer is im-
plemented by phase-comparing the reference fre-
quency and comparison frequency obtained when ref-
erence oscillation frequency X201 is divided.
+5V
X58-3870 : VCO
+ VCO (X58-3870-XX)
The desired frequency is produced directly by a Col-
pitts oscillator circuit consisting of FET Q2. The VCO
contro! voltage is applied to varicap diodes D1 and D2
to change the oscillation frequencycy. The TX pin is:
made high during reception. Q1 and D4 are then turned
on to change over the oscillation frequency.
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UNLOCK -
DETECTION
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MIC AMP
Fig. 8 PLL and VCO circuits:
¢ Unlock detector circuit
When the PLL circuit is in the unlock state, the pulse
that is output to the UL pin (pin 7) of 1C201 is wave
'from shaped by D202, C210, C211 and R212. The UL
pin is then made high. The voltage at the UL pin is
monitored by the microprocessor to control the trans-
mission or reception selection timing.
IC201
q
oO
PLL IC cpio I
Fig.9 Unlock detector circuit