Radio Shack HTX-400, Service Manual

Page: 5 / 42

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2. DESCRIPTION OF FREQUENCY DETERMINING
AND STABILIZING CIRCUIT
2.1 INTRODUCTION
THE FREQUENCIES FOR TRANSMITTER AND RECEIVER LOCAL FREQUENCIES ARE ALL
DRIVED FROM A SINGLE 15.2 [MHz] VCTCXO BY MEANS OF A PHASE LOCKED LOOP.
THE FIRST LOCAL OSCILLATOR FREQUENCIES ARE 389.15 [MHz] TO 439.15 [MHz].
THE SECOND LOCAL FREQUENCY IS FIXED AT 30.4 [MHz] TO GENERATE SECOND IF 450[KHz].
DURING TRANSMISSION, THE VCO OF THE PLL OPERATES 420.000 [MHz] TO 470.000[MHz].
Q301 Q104 TO TRANSMITTER
VCO BUFFER AMP
420.000 [MHz] 420.000 [MHz]
¡ -
470.000 [MHz]
¡ -
470.000 [MHz]
THE VCO OPERATING FREQUENCY FOR THE RECEIVER IS 389.15 [MHz] TO 439.15 [MHz].
IT IS INJECTED THROUGH THE BUFFER AMP Q302 INTO THE FIRST TR MIXER Q202
2.2 DESCRIPTIONS OF EACH BLOCK
(1) INTRODUCTION
THE SYNTHESIZER CONSISTS OF THE FOLLOWING
COMPONENTS : PLL IC (IC501), VCTCXO (MODULE1), VCO, VARICAP DIODE (D302)
IC501 IS A CMOS LSI THAT INCLUDES PRESCALLER. Q304, L303, L302, D302, D301, C322,
C319, C316, C315 ARE CLAPP OSCILLATOR CIRCUIT TO OPERATE AS A VCO OF IC501.
Q301 IS A SWITCHING TRANSISTOR TO CONNECT OR DISCONNECT THE TUNING
CAPACITOR IN THE VCO OSCILLATOR TANK CIRCUIT FOR TRANSMITTER OR RECEIVER.
Q104 AND Q105 WORK AS A BUFFER AMP FOR RX LOCAL FREQUENCIES AND TX CARRIER
GENERATING FREQUENCIES.
(2) REFERENCE FREQUENCY
THE CRYSTAL, VCTCXO MODULE1(15.2 [MHz] ) AND OTHER COMPONENTS AT PIN 1 AND 16
OF IC501 CAN MAKE A REFERENCE FREQUENCY OSCILLATOR WITH INTERNAL AMPLIFIER.
(3) VCO
Q302 AND SURROUNDING PARTS FORM A CLAPP OSCILLATOR WHICH WORKS AS A VCO OF
IC501 WITH APPROPRIATE CONTROL VOLTAGE ON D302, THE VCO CAN OSCILLATE OVER
THE REQUIRED RANGE OF 389.15 [MHz] TO 470.00 [MHz].