Cherokee Radios CBS2100
Documentation Project
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THEORY OF OPERATION

INTRODUCTION

The CBS-2100 40 channels Citizen Band Base Station is constructed with a Phase Locked Loop frequency synthesizer to generate suitable RF signals for the double conversion receiver and the AM/SSB transmitter circuits. High level modulation is used in AM mode while low-level modulation is utilized in the SSB operation. A diode circuit performs AM detection and a transistor product detector demodulates the received SSB signal.

The CBS-2100 Block Diagram shall be used in associate with the following circuit description.

FREQUENCY GENERATION

A. PLL CIRCUITRY

PLL is an abbreviation of the Phase-Locked-Loop. An MC45106 PLL integrated circuit, working together with a transistor Voltage Controlled Oscillator (VCO), a microprocessor controlled channel selector, and a down conversion mixer circuit TA-7310 to form a closed loop feedback circuit to generate suitable RF frequencies required for a 40 channel 27MHz CB operation.

TR24 is a Crystal Oscillator to generate the reference frequency of 10.24MHz. This signal is fed into IC707's reference frequency counter (pin 3) to obtain a phase comparison reference frequency of 10KHZ to its phase detector.

TR25 and Varicap Diodes D94 and D30 formed a Voltage Controlled Oscillator. The control voltage is obtained from the phase detector output of the PLL circuit to operate in a frequency range of 16.27 - 16.71 MHz. The output of the VCO is buffered by TR26 to fed the receiver and transmitter circuit.

IC10 TA7310P consists of an amplifier, an oscillator and a mixer. The oscillator is a crystal oscillator operated in a frequency trimmed by inductors and a varicap. Different inductors are connected into the oscillator circuit through switching diodes (D34 - D36) to provide different offset frequency settings in AM (14.010MHz), LSB (14.0075MHz) and USB (14.0125MHz). The output frequencies could be further trimmed by VR507 and varicap D124 for SSB Clarifier operation.

Part of the VCO output is coupled to the buffer amplifier in IC 10 (pin 7). It is then mixed with the 14.010MHz oscillator output to provide the PLL programmable divider input frequency of 2.26 - 2.70MHz at pin 6. The signal is fed to pin 2 of IC3 through a low pass filter circuit.

A microprocessor controlled channel switch circuit is employed to provide the division ratio to IC707 (pins 10 - 18) with correspondence to the channel number (1 - 40) selected by the user. The PLL input frequency (2.26 - 2.70MHz) is divided by the division ratio set by the channel selector to phase compare with the 10KHZ reference frequency. Any phase difference will result in a change of DC voltage output to the VCO. When the VCO frequency reaches the desired value, the DC output to the VCO stabilizes and the loop is in 'locked" condition.

The Lock Detector output (pin 8) of IC3 is used to inhibit the transmitter PTT and receiver audio circuits to ensure the radio always operate on the correct channel frequencies.

Both the PLL and CPU Integrated Circuits are molded in epoxy and are not serviceable by the users

B. 10.695MHZ SECOND OFFSET OSCILLATOR

TR-30 is a crystal oscillator operates at a center frequency of 10.695MHz. The output of the circuit is trimmed by different inductors during AM, LSB and USB operation. On AM mode, D57 is biased on during transmit, and off while receiving. On USB or LSB mode D58 and D59 turns on respectively on Tx and Rx mode

This circuit provides 10.695MHz off-set frequency to L44 of the Tx mixer (IC9), which mixes with PLL VCO output to provide correct transmitting spot frequencies in AM operation. In SSB mode, D58 and D59 switches L27 and L28 into the circuit to provide canier signal of 10.6925MHz for USB and 10.6975MHz for LSB operation.

The signal is then routed to the Balanced Modulator (IC3) and Crystal Filter FL3 to generate low level SSB modulated signal for further processing by the Tx mixer (IC9). A portion of the signal is sent to the product detector (TR-15) for SSB demodulation.

C. FREQUENCY CALCULATIONS

The VCO frequency is obtained from the following calculation;

f vco = f off-set + Nfr

Where
f vco = VCO frequency.
N = Programming Code for divider output.
fr = Reference Frequency step, 10 KHz (0.01 MHz)

i.e. At channel #1 and AM mode, the corresponding N code is 226
f vco = 14.010 + 226 x 0.01 = 16.270 MHz

Since the mixer output determined by two factors: The second offset oscillator output (dependent on AM/SSB mode selector switch) and the PLL / VCO output.

1) Transmitter Operation
The VCO output is mixed with the second off-set oscillator signal and applied to the Tx mixer IC9 through band pass filters L43 and L44.

-For channel 1 AM mode, the output of Tx mixer is 16.270 + 10.695 MHz = 26.965 MHz (Ch 1 Tx frequency).
-For channel 1 LSS mode, the output of Tx mixer is 16.2675 + 10.6975 MHz = 26.965 MHz (Ch 1 LSB Carrier Frequency)
-For channel 1 USB mode, the output of Tx mixer is 16.2725 + 10.6925 MHz = 26.965 MHz (Ch 1 USB Carrier Frequency)

2) Receiver Operation

The output frequencies of PLL and offset oscillators remained the same for Rx operation.

For Channel 1 AM mode, the receiving frequency is 26.965 MHz

- Signal at 1st Local Oscillator Mixer is 26.965 - 16.270 MHz = 10.695 MHz (First IF)
- Signal at 2nd Local Oscillator Mixer is 10.695 - 10.240 MHz = 455 KHz (Second IF)
- Signal is detected by diodes at 455 KHz to recover audio

For channel 1 LSB mode, the receiving frequency is 26.964 MHz (using 1KHz modulating signal)

- Signal.at 1st Local Oscillator Mixer is 26.964 - 16.2675 MHz = 10.696.5 MHz (SSB IF)
- Signal at Product Defector is 10.6975 - 10.6965 MHz = I KHz (Audio)

For channel 1 USB mode, the receiving frequency is 26.966 MHz (using 1KHz modulating signal)

- Signal at 1st Local Oscillator Mixer is 26.966 - 16.2725 MHz = 10.693.5 MHz (SSB IF)
- Signal at Product Detector. is 10.6935 - 10.6925 MHz = 1KHz (Audio)