President Harrison CB-Radio
DESCRIPTION OF FREQUENCY DETERMINING
AND STABILING CIRUIT
INTRODUCTION
THE FREQUENCIES FOR TRANSMITTER AND RECEIVER LOCAL FREQUENCIES ARE ALL DRIVED FROM A SINGLE 3.6 (MHz] CRYSTAL BY MEANS OF A PHASE LOCKED LOOP.
THE FIRST LOCAL OSCILLATOR FREQUENCIES ARE 16.270MHz (CH.1) TO 16.710MHz (CH.40).
THE SECOND LOCAL FREQUENCY IS FIXED AT 10.240MHz TO GENERATE SECOND IF 455KHz.
DURING TRANSMIT, THE VCO OF THE PLL OPERATES 13.4825MHz](CH.1) TO 13.7025MHz (CH.40).
THE VCO FREQUENCY GOES TO THE DOUBLER CIRCUIT Q107,T9,T10 WHICH DOUBLES THE
FREQUENCY TO GENERATE 26.965MHz (CH.1) TO 27.405MHz (CH.40).
VCO (Q101) --------------------> DOUBLER (Q601) ----------------> TO TRANSMITTER
13.4825 - 13.7025MHz ---------> 26.9650 - 27.405MHz ------------- (FOR CETP BAND)
THE VCO OPERATING FREQUENCY FOR THE RECEIVER IS 16.270MHz (CH.1) TO 16.710MHz
(CH.40). AS THE FIRST LOCAL OSCILLATOR, INJECTED THROUGH THE BUFFER AMP Q107 INTO
THE FIRST TR MIXER Q4.
DESCRIPTIONS OF EACH BLOCK
INTRODUCTION
THE SYNTHESIZER IS IMPLEMENTED WITH THE FOLLOWING COMPONENTS : PLL IC (IC901), X-TAL (XT901), VCO, VARICAP DIODE (D101).
IC901 IS A CMOS LSI THAT INCLUDES MOST OF PLL BLOCK AND LCD DRIVER, THE Q101,T100, C126, C125, C108, C112, C111, VARICAP DIODE D101 ARE CLAPP OSCILLATOR CIRCUITTO OPERATE AS A VCO OF THE IC901.
Q104 IS A SWITCHING TRANSISTOR TO CONNECTOR DISCONNECT THE TUNING CAPACITOR IN THE VCO OSCILLATOR TANK CIRCUIT FOR TRANSMITTER OR RECEIVER.
Q107 WORKS AS A BUFFER AMP FOR RX LOCAL FREQUENCIES (16MHz) AND TX CARRIER GENERATING FREQUENCIES (13MHz).
REFERENCE FREQUENCY
THE CRYSTAL, XT901(3.600MHz) AND OTHER COMPONENTS AT PIN. 1 AND 80 OF IC901 CAN MAKE A REFERENCE FREQUENCY OSCILLATOR WITH INTERNAL AMPLIFIER.
VCO
Q101 AND SURROUNDING PARTS ARE CONSISTING A CLAPP OSCILLATOR WORKS AS A VCO OF
CPU WITH APPROPRIATE CONTROL VOLTAGE ON D101, THE VCO CAN BE OSCILLATE OVER THE
REQUIRED RANGE OF 13.4825MHz TO16.7100 MHz
PROGRAMMABLE DIVIDER AND ITS CONTROL
THE PROGRAMMABLE INPUTS FOR EACH CHANNELS ARE SETTED IC INSIDE. EACH INPUT SIGNAL
TO CONTROL THE PLL IC IS DONE WITH THE PROVIDED KEY MATRIX INPUT PINS - T1 (PIN-15) TO
T5 (PIN.18) AND K0 (PIN.10) TO K3 (PIN.14). FOR EACH KEY MATRIX INPUT , AN INTERNAL CODE
CONVERT ERROM PROVIDES THE APPROPRIATE BINARY CONTROL TO THE PROGRAMMABLE
DIVIDER FOR THAT CHANNEL.
SINCE THE BINARY NUMBER NECESSARY TO CHANGE DURING TRANSMIT AND RECEIVE, AN
ADDITIONAL BIT IS REQUIRED AT PIN.66 OF IC901 TO ALLOW THE ROM TO RECOGNIZING THE
STATUS IS TX OR RX.
DURING TRANSMIT, THE PUSH TO TALK SWITCH MAKES PIN.66 OF PLL IC IS HIGH DIGITALLY
UNDER TRANSMIT MODE.
THE PROGRAMMABLE DIVIDER OUTPUT IS FED TO THE PHASE DETECTOR FOR COMPARING WITH
THE 2.5kHz REFERENCE FREQUENCY INSIDE THE IC5.
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 D101 CONTROL THE VCO FREQUENCY.
TRANSMITTER / RECEIVER BUFFER AMP
OUTPUT SIGNAL OF Q101 IS FED INTO THE BUFFER AMP Q107 TO INCREASE THE STRENGTH OF TX CARRiER FREQUENCY AND 1st LOCAL FREQUENCIES.
TRANSMITTER DOUBLER
THE OUTPUT SIGNAL OF Q107 GOES TO AN AMPLIFIER WITH TUNING CIRCUIT Q601,T601,T602
WHICH DOUBLES INCOMING 13MHz SIGNALS.
SWITCHING OF TUNING CAPACITOR IN VCO
THE VCO CIRCUIT MUST TUNE WITH A WIDE RANGE OF FREQUENCIES 13.4825 - 13.7025MHz FOR
TRANSMITTER AND 16.27 - 16.71MHz FOR RECEIVER.
TO COMPLY ABOVE RANGE OF VCO, THE TUNING CAPACITANCE SHOULD SWITCHED FOR
TRANSMISSION OR RECEPTION.
THE TUNING CIRCUIT CONSISTS WITH T100,C126,C125,C112 WHEN THE VCO IS WORKING AS
A RECEIVER , QlO4 BECOMES TURN OFF.
SO, T100 AND C112 MAKES TUNING FUNCTION WHEN TRANSMITTING Q104 BECOMES ON.
SO, T100 AND THE PARALLEL CAPACITANCE OF C125 AND C126 MAKE TUNING FUNCTION.
RECEIVER LOCAL OSCILLATOR OUTPUTS
1st MIXER : THE SECONDARY OUTPUT SIGNALS OF T503 IS INJECTED TO THE SOURCES OF 1st MIXER Q502,505 IN THE 1st IF MIXER SECTION.
2nd MIXER : THE OUTPUT OF 10.24MHz OSCILLATOR CIRCUIT WITH XT201 IS INJECTED INTO THE IF MODULE (IC201). INCOMING IF SIGNAL AND 10.24MHz SIGNAL ARE MIXED INSIDE THE IF IC TO
EXTRACT 2nd IF SIGNAL 455kHz.
FREQUENCY STABILITY
LET :
Fo=CRYSTAL OSCILLATOR FREQUENCY
FR=PHASE DETECTOR REFERENCE FREQUENCY
Fvco=VCO FREQUENCY
Ft=TRANSMIT FREQUENCY
THEN :
Fr=Fo/1800
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 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.
DESCRIPTION OF OTHER CIRCUITS
TRANSMITTER
RF AMPLIFICATION
THE OUTPUT OF DOUBLER AMP Q601 IS FED THROUGH TUNING IFT T601 AND T602 TO THE BASE OF PRE DRIVER AMP Q609.
THE OUTPUT IS THEN SUPPLIED THROUGH CAPACITOR C634 TO RF DRIVER AMP Q602.
THE OUTPUT OF Q602 IS SPLITTED WITH TUNING CIRCUIT L607 C608,C616 GOES TO THE BASE OF
FINAL RF AMP Q603.
THE OUTPUT OF Q603 IS SUPPLIED TO THE ANTENNA THROUGH L-C TUNING CIRCUIT.
CIRCUIT FOR SUPPRESSION OF SPURIOUS RADIATlON
THE TUNING CIRCUIT BETWEEN THE OUTPUT OF FINAL AMP Q603 AND ANTENNA, 4-STAGE ''PHI''
NETWORK T604,C607 C620,L604,C621,L602,C609,C605,C606,L605 SERVES AS A SPURIOUS RADIATION SUPPRESSOR.
THIS NETWORK ALSO SERVES TO MATCH THE IMPEDANCE BETWEEN TX POWER AMP Q20 AND THE ANTENNA.
MAXIMUM MODULATION CONTROL
FM
MODULATION SIGNALS ARE FILTERED WITH RC NETWORK AND GOES TO THE MIC AMP IC850
TO MAKE NOMINAL SIGNAL LEVEL TO ACHIEVE WANTED MODULATION.
TO CONTROL INCOMING AUDIO SIGNAL, DIODE LIMITTINHG CIRCUIT D850, CORRESPONDING
ALC CIRCUIT CONTROLS WITH PROPORTIONAL TO SAMPLED AUDIO OUTPUT LEVEL.
ADJUST VR850 SHALL NOT EXCEED 2.2kHz DEVIATION UNDER 1250Hz AF 20dB UP FROM 1.5kHz
DEV LEVEL INPUT.
AM
MODULATION SIGNALS ARE FILTERED WITH RC NETWORK AND GOES TO THE AUDIO AMP IC401
TO MAKE NOMINAL SIGNAL LEVEL TO ACHIEVE WANTED MODULATION.
TO CONTROL INCOMING AUDIO SIGNAL Q402,Q405 AND CORRESPONDING ALC CIRCUIT
CONTROLS WITH PROPORTIONAL TO SAMPLED AUDIO OUTPUT LEVEL.
ADJUST RV401 SHALL NOT EXCEED +/- 90% MODULATION UNDER 1kHz AF 20dB UP FROM 50%
MOD LEVEL INPUT.
RECEIVER
(1) RF AMP Q501AMPLIFY RF SIGNAL FROM ANTENNA.
THE AMPLIFIED RF SIGNAL IS FED THROUGH TUNING COILS T501,502,503,504 TO THE GATE OF
MIXER Q502,505, ALSO VCO SOURCE'S FREQUENCY LOWER 10.695MHz THAN THE FREQUENCY OF
EACH CHANNEL IS SUPPLIED TO MIXER.
(2) THE SOURCE OF 10.695MHz FREQUENCY PASSED TUNING COIL T503 IS FED THROUGH MCF501 TO Q201.
(3) THE 455kHz FREQUENCY SIGNAL PRODUCED FROM IC201 BY MIXING THE OUTPUT SOURCE OF
FLl 10.695MHz FILTER AND THE SOURCE OF 10.240MHz FREQUENCY
FROM CRYSTAL OSCILLATOR XT201.
(4) INTERNAL AM AND FM DETECTOR OF ICl DETECT AM & FM AUDIO SIGNAL
(5) A.G.C (AUTOMATIC GAIN CONTROL) SIGNAL OF ICl FED TO IC2,Q105,Q104
CONTROL RECEIVING SENSITIVITY.
(6) AM AUDIO OUTPUT
DETECTED AUDIO SIGNAL IS FED THROUGH C222,R207,C206,C209,RV401 TO
LOW FREQUENCY AUDIOAMP FOR DRIVING SPEAKER.
(7) FM AUDIO OUTPUT
DETECTED AND AMPLIFIED AUDIO SIGNAL IS FED THROUGH HIGH CUT FILTER
C221,R209,C210,C202,RV401 TO SPEAKER.