Cherokee Radios CBS2100
Documentation Project
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CIRCUIT DESCRIPTION

A. Power Supplies Circuit

The CBS-2100 Base Station is designed to operation on AC as well as DC power sources.

When AC Power is used, the unit's internal AC power supply module will be used to convert 115VAC to 13.8V DC at the DC output plug. When AC is not available, just disconnect the DC output plug and replace it with an external 13.8V battery power source.

The output voltage of the AC power supply is fixed and there is no set up or adjustment points in the unit.

13.8V DC supplied from the DC Power connector at the back of the radio are protected against reversed connection by D93, and filtered by an AF choke T1. This supplies voltage directly to the audio power amplifier IC8, AM Modulator TR51 and the 8V regulator TR41.

Two transistor switches TR38 and TR37 turns on and off under the control of IC4 (pin7) to supply voltage to the transrnifter and receiver circuits. The Tx LED will grow in red on transmit.

B. Channel Selector

The channel selector circuit is divided into two parts. The pulsed rotary encoder switch SW701 with the LED display and its driver circuits in one board. The CPU IC701, PLL IC707 and its associated driver circuits are located in another PCB and they are molded in epoxy. Only 40 Citizen Band Channels are mask programmed into the CPU. No extra or spare leads on the CPU/PLL are accessible by the users.

When the channel switch (SW701) is turned clockwise, it sends a pulse to pin 3 or IC701. The CPU will find in its memory look-up table the correct PLL programmable divider division ratio and increment the channel number by 1, the action will revert to decrement when the channel switch is turned anti-clockwise. The data output are latched by IC704 and IC705 to drive the channel display LEDs, and latched by IC706 as the PLL division ratio for use by IC707.

C. Frequency Synthesizer

Please refer to the previous sections for detailed description.

D. AM Modulation, Power Control and ALC

Input from the microphone is controlled by the Mic-Gain potentiometer VR505 and amplified by IC4. The amplified signal is used to drive the AM Modulator circuit consists of TR49 & 51.

There are two signals at the base of TR49, the DC voltage controlled by VR13 (RF Power Level Adjust), and the AF level from pin 1 of IC4. The DC bias controls the operating point of the Darlington pair transistors TR50 & 51 to provide power supply to the RF Power transistor's TR43 & 44, which controls the RF Carrier Power output of the radio. The AF signal is amplified and superimposed on the DC bias to create high level AM Modulation in TR43 & 44.

The maximum modulation is regulated by the Automatic Level Control (ALC) circuit consists of TR53, TR34 and TR32.

TR53 is a comparator circuit with set point adjusted by VR14. When TR53 conducts, the signal is half wave rectified by D83 to drive the current amplifier TR34, which in turn drives TR32 on and shunt down the Mic Audio signal at the input of IC4. The RC circuit (C135, R176 - 178) at the base of TR32 ensures the smooth operation of the ALC. When the modulator over modulates, AC voltages at emitter of TR53 will increase, and TR53 will conduct more during the negative cycle of the modulation signal, which increases the collector current of TR53 and turn TR43/TR32 on. The audio signal is shunted, input to the modulator decreases until it reaches the Modulation Limit set point of VR14.

E. SSB Modulation, Power Control and ALC

After being amplified by IC4, audio signals from the microphone are connected to the balanced modulator IC3 (pin 1). Signal from the second offset oscillator is presented to pin 3 of IC3 to produce the double side band suppressed carrier (DSBSC) signal. VR7 is adjusted for minimum carrier output.

The DSBSC signal is routed by the switching diodes to the 10.695MHz crystal filter, the unwanted sideband is filtered off, and the processed SSB signal is sent to L44/IC9 for mixing with the VCO output (at L43) to obtain the final transmitting frequency.

The high level modulator TR50 &51 are bypassed by TR52 during SSB operation. The RF Power amplifiers are biased to class B operation (VR10 -11) to preserve the envelope of the low-level modulated SSB signal. Since the SSB RF output is directly proportional to the level of audio signal driving the balanced modulator, the RF output is sampled by TR47, which provides negative feedback to the audio ALC circuit (TR34 & 53) to limit the rnaamurn SSB power output.

F. Receiver Circuits

RF signal from the antenna is coupled to the receiver circuit by C43, the signal passes through a pair of PIN fast switching diodes (D15/16), which are biased with AGC and a fixed voltages to protect the receiver front end from transmitter output and other excessively strong signals.

The signal is then amplified by a low noise MOSFET (TR51), passes a bandpass filter (L7 - L8) and mixed with the first local oscillator signal in TR18. The resultant first IF signal of 10.695MHz goes through a series of filter circuits, including those used for Noise Blanker operation, and come to the second mixer.

For AM operation, the IF signal is mixed with 10.240MHz second local oscillator frequency to product 455KHz second IF. The second IF signal is filtered and amplified by two stages of amplifiers (TR9, TR10 and 11), and then coupled to a diode detector (D23 & 24) for demodulation.

For SSS operation, the IF signal goes through the narrow band 10.695MHz crystal filter, amplified by two stages of IF amplifiers (TR19 - 22) and then coupled to a product detector (TR15) for demodulation.

The demodulated audio signal is amplified by TR15 and routed to the audio power amplifier IC8 via the volume control VR502. The audio amplifier is muted by voltages from D80 when Tx is active.

The output of AM and SSB IF amplifier's are also connected to an IC amplifier circuit (IC1) to provide Signal Meter DC voltages (pins 5 - 7), and the operation of Squelch Circuit (pins 8 - 10 & TR16). IC1 also provide AGC voltages (pins 1 - 3 & pins 12 - 14) to the receiver front end and the PIN protection diode D16.

The Noise Blanker taps noise signal from the first IF filter network (L10 - L12), amplify the signal by TR1 - TR3, demodulate it by D1 and D2. The detected noise level is further amplified by TR5 & TR6 to control TR7, which shunts the signal output level in the IF filter network (L12). The IF gain for the noise signal is therefore reduced.

G. Transmitter Circuit

The RF signals generated from the PLL and the second offset oscillator / balanced mixer are assembled at RF Mixer IC9 to provide the final transmitting signal. The signal passes through a bandpass filter (L40 & 42) to two stages of RF pre-driver's (TR45 & 46), RF driver TR44, the final stage amplifier TR43, a low pass filter network and then to the antenna.

In AM operation, modulated voltages only applies to the collectors of TR43 and TR44, while SSB signals passes through all of the RF amplifier chain (TR43 - 46). Please refer to the previous sections for detail description of frequency generation and modulation.

The output signal is picked up by D86 & D87 to give relative RF output power meter indication, and a Standing Wave Ratio (SWR) sampling circuit consists of D401 and D402 provides the SWR meter indication feature.

H. "Clear Drive" Compander Circuit

The CBS-2100 radio is equipped with a compander circuit, which will be marketed under the trademark of "Clear Drive". The feature could be enabled or turned off by a front panel switch labeled as 'CDS'

The Compander circuit consists of IC901 is employed. The circuit can be divided into three parts:

Compressor
Low Pass Audio Filter
Expander

Transmit audio from the microphone is routed to pin 8 of IC901for amplification, and then to the internal Compressor circuit for audio processing. Output from the Compressor (pin 3) is routed through an active three pole low pass filter (pins 1& 2) to the Mic Gain control. All the excessive high frequency signals are suppressed by the filter before transmission.

The reference level of the Compressor is set by R905 to around 5mV microphone input. R901 is inserted into the circuit to obtain suitable modulation level. Received audio from the detector is sent to pin 15 for amplification, and then to the internal Expander circuit for audio processing. The reference level is set by R908 to around 12dB SINAD, the output is taken from pin 19 and is connected to the Volume control on the front panel.

Pin 12 of IC901 provides the bypass feature. When a high level is present, both the Compressor and the Expander are operative. When pin 12 is connected to ground potential, both the Compressor and the Expander are bypassed - All the audio level passes through IC901 without getting any level processing.

In order to increase the dramatic effects of Clear Drive on transmit, CBS-2100 is equipped with dual stage Clear Drive configuration called Super CDS, The user could activate Super CDS by turning VR102 from OFF position clockwise (Auto-CDS) to adjust the compression ratio of the compander. When VR102 is at OFF position, the Clear Drive internal circuits automatically fix the compression ratio. Super CDS is a transmit audio enhancement circuit and has no effects over CDS for the receiver circuit.

I. Frequency Counter Circuit

A frequency counter is included in the CBS-2100 to facilitate the Single Side Band operation. The counter is built around a mask programmed Z80 type CPU.

The frequency generated by the PLL synthesizer is taped from the 10.695MHz offset frequency generator IC10 through L22. The signal, which is 10.695MHz below the TX or the RX operating frequencies is fed to IC603 - a divided by 40 prescaler system to match the lower frequency counting rate that the CPU is operating on.

In additional to the frequency counting application program, there is a look up table in the internal masked memory of the CPU to add 10.695MHz back to the counted frequency so that the correct 27MHz frequencies could be displayed.

The CPU output is sent to a demuliplexer/driver combination of IC604 and IC601 to display the frequency information on a 6 digit 7 segment LED display on the front panel.

There is no internal set up or adjustment required for this frequency counter.