Radio Shack HTX-400 Service Manual page 6

(4) PROGRAMMABLE DIVIDER AND ITS CONTROL
THE PROGRAMMABLE INPUTS FOR EACH CHANNEL ARE SET INSIDE IC801.
EACH INPUT SIGNAL TO CONTROL THE PLL IC501 IS DONE WITH THE PROVIDED (CPU IC
801) KEY MATRIX INPUT PINS - PIN25 TO PIN28 AND FOR EACH KEY MATRIX INPUT, AN
INTERNAL CODE CONVERT EPROM PROVIDES THE APPROPRIATE BINARY CONTROL TO IC501
FOR THAT CHANNEL.
AN ADDITIONAL BIT IS REQUIRED AT PIN 42 OF IC801 TO ALLOW THE ROM TO RECOGNIZE
WHETHER THE STATUS IS TX OR RX.
DURING TRANSMISSION, THE PUSH TO TALK SWITCH MAKES PIN 42 OF CPU IC HI.
THE OUTPUT OF IC501 IS FED TO THE PHASE DETECTOR TO COMPARE IT WITH THE 6.25
[KHz] REFERENCE FREQUENCY INSIDE.
(5) PHASE DETECTOR AND VCO CONTROL
THE PHASE DETECTOR IS A DIGITAL PHASE COMPARATOR WHICH COMPARES THE PHASE
OF THE REFERENCE SIGNAL WITH PROGRAMMABLE DIVIDER OUTPUT SQUARE WAVES AND
DEVELOPS A SERIES OF PULSES WHOSE DC LEVEL DEPENDS ON THE PHASE ERROR OF
EACH SIGNAL.
THE PHASE DETECTOR PULSE OUTPUT IS FED TO AN ACTIVE LOW PASS FILTER, AND FED
TO VARICAP D302 WHICH CONTROLS THE VCO FREQUENCY.
(6) TRANSMITTER / RECEIVER BUFFER AMP
OUTPUT SIGNAL OF Q302 IS FED INTO THE BUFFER AMP Q105, Q104 TO INCREASE THE
STRENGTH OF TX CARRIER FREQUENCY AND 1' st LOCAL FREQUENCIES.
(7) SWITCHING OF TUNING CAPACITOR IN VCO
THE VCO CIRCUIT MUST TUNE WITH A WIDE RANGE OF FREQUENCIES 420.000¡ - 4 70.000
[MHz] FOR TRANSMITTER AND 389.150¡ - 4 39.150 [MHz] FOR RECEIVER.
TO COMPLY ABOVE RANGE OF VCO, THE TUNING CAPACITANCE SHOULD BE SWITCHED
FOR TRANSMISSION OR RECEPTION.
THE TUNING CIRCUIT CONSISTS OF Q304, L303, L302, D302, D301, C322, C319, C316, C315
WHEN THE VCO IS WORKING AS A RX, Q305 BECOMES OFF.
SO, D303 AND C317 MAKE TUNING FUNCTION WHEN TX Q301 BECOMES ON.
SO, L302 AND THE PARALLEL CAPACITANCE OF D302, C322 AND C319 MAKE TUNING
FUNCTION.
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