Radio Shack HTX-400 Service Manual page 7

(8) RECEIVER LOCAL OSCILLATOR OUTPUTS
1' st MIXER : THE OUTPUT SIGNALS OF Q104 IS INJECTED TO THE SOURCES OF 1st MIXER
Q203 IN THE 1' st IF MIXER SECTION.
2' nd MIXER : THE OUTPUT OF 30.4 [MHz] OSCILLATOR CIRCUIT WITH X401 IS INJECTED
INTO IC401. INCOMING IF SIGNAL AND 30.85 [MHz] SIGNAL ARE MIXED INSIDE THE IF IC TO
EXTRACT 2' nd IF SIGNAL 450 [KHz].
FM SIGNALS ARE RECOVERED WITH ENVELOPE DETECTOR.
2.3 FREQUENCY STABILITY
LET : Fo=CRYSTAL OSCILLATOR FREQUENCY
Fr=PHASE DETECTOR REFERENCE FREQUENCY
Fvco=VCO FREQUENCY
Ft=TRANSMIT FREQUENCY
THEN : Fr=Fo/ 2048
AND UNDER LOCKED CONDITIONS : Fr = Fvco / N
Where , "N" IS THE DIVIDE RATIO OF PROGRAMMABLE DIVIDER.
THEN : Fvco = N X Fr
FROM WHICH IT CAN BE SEEN, THE PERCENTAGE ERROR IN Ft IS THE SAME AS THE
PERCENTAGE ERROR IN Fo.
THE STABILITY OF THE CRYSTAL OSCILLATOR IS DETERMINED PRIMARILY BY THE CRYSTAL
ITSELF AND HAVING PASSIVE COMPONENTS OF THE OSCILLATOR.
THE CHOICE OF CRYSTAL AND COMPONENTS IS SUCH THAT THE REQUIRED FREQUENCY
STABILITY IS MAINTAINED OVER THE REQUIRED VOLTAGE AND TEMPERATURE RANGE.
2.4 DESCRIPTION OF OTHER CIRCUITS
TRANSMITTER
(1) RF AMPLIFICATION
THE OUTPUT OF BUFFER AMP Q104 IS FED THROUGH TUNING C126 TO THE BASE OF PRE
DRIVER AMP Q103.
THE OUTPUT IS THEN SUPPLIED THROUGH CAPACITOR C123 TO RF DRIVER AMP Q102.
THE OUTPUT OF Q102 IS SPLITTED WITH TUNING CIRCUIT L111, C119 AND GOES TO THE
BASE OF FINAL RF AMP Q101.
THE OUTPUT OF Q101 IS SUPPLIED TO THE ANTENNA THROUGH L-C TUNING CIRCUIT.
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