Motorola Cdm And Pro Series Detailed 68p81091c63 O Manual

Theory of Operation2-27
2.7.1 Front-End Band-Pass Filters and Pre-Amplifier
The received signal from the radio’s antenna connector is first routed through the harmonic filter and 
antenna switch, which are par t of the RF power amplifier circuits, before being applied to the receiver 
pre-selector filter (C3001, C3002, D3001 and associated components). The 2-pole pre-selector filter 
tuned by the dual varactor diode D3001 pre-selects the incoming signal (RXIN) from the antenna 
switch to reduce spurious...

2-28Theory of Operation
2.8 Transmitter Power Amplifier (PA) 45 W
The radio’s 45 W PA is a four-stage amplifier used to amplify the output from the VCOBIC to the radio 
transmit level. The line-up consists of three stages which utilize LDMOS and VMOS technology, 
followed by a final stage using a bipolar device. The gain of the first stage (U3401) is adjustable, 
controlled by pin 4 of PCIC (U3501) via Q3501 and Q3502 (VCONT). It is followed by an LDMOS 
pre-driver stage (Q3421), a VMOS driver stage...

Theory of Operation2-29
2.8.3 Driver Stage
The following stage is an enhancement-mode N-Channel MOSFET device (Q3431) providing a gain 
of 10 dB. This device also requires a positive gate bias and a quiescent current flow for proper 
operation. The voltage of the line MOSBIAS_2 is set in transmit mode by the ASFIC and fed to the 
gate of Q3431 via the resistive network R3404, R3406, and R3431-5. This bias voltage is also tuned 
in the factory. If the transistor is replaced, the bias voltage must be tuned...

2-30Theory of Operation
2.8.8 Power Control 
The transmitter uses the power control IC (PCIC, U3501) to control the power output of the radio. A 
portion of the forward and reflected RF power from the transmitter is sampled by the directional 
coupler, rectified and summed, to provide a dc voltage to the RFIN por t of the PCIC (pin 1) which is 
propor tional to the sampled RF power. 
The ASFIC contains a digital to analog converter (DAC) which provides a reference voltage of the 
control loop to the PCIC...

Theory of Operation2-31
Figure 2-13.   VHF Synthesizer Block Diagram
A voltage of 5V applied to the super filter input (U3201 pin 30) supplies an output voltage of 4.5 Vdc 
(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R3363) and the synthesizer 
charge pump resistor network (R3251, R3252). The synthesizer supply voltage is provided by the 5V 
regulator U3211.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin 
VCP (U3201-47), a...

2-32Theory of Operation
2.9.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U3301), the TX 
and RX tank circuits, the external RX buffer stages, and the modulation circuits.
Figure 2-14.   VHF VCO Block Diagram
The VCOBIC together with the Fractional-N synthesizer (U3201) generates the required frequencies 
in both the transmit and receive modes. The TRB line (U3301, pin 19) determines which tank circuits 
and internal buffers are to be...

Theory of Operation2-33
The external RX buffers (Q3301 and Q3302) are enabled by a high at U3301, pin 7 (RX_SWITCH) 
via transistor switch Q3303. In the TX mode, the modulation signal (VCOMOD) from the LVFRAC-N 
synthesizer IC (U3201 pin 41) is applied to varactor diode D3362, which modulates the TX VCO 
frequency via capacitor C3362. Varactor D3362 is biased for linearity from the VSF.
2.9.4 Synthesizer Operation
The complete synthesizer subsystem consists of the low voltage FRAC-N (LVFRACN), reference...

2-34Theory of Operation
2.10 Control Head (PRO3100, CDM750)
ThE Control Head Contains the internal speaker, the on/off/volume knob, the microphone connector, 
several buttons to operate the radio and several indicator Light Emitting Diodes (LED) to inform the 
user about the radio status. To control the LED’s and to communicate with the host radio the control 
head uses the Motorola 68HC11E9 µP.
2.10.1 Power Supplies
The power supply to the control head is taken from the host radio’s FLT A+ voltage via...

Theory of Operation2-35
The µP determines the used keypad type and the control head ID by reading the levels at por ts PC0 
– PC7. Connections JU0852/3/4 are provided by the individual keypads.
The MODB / MODA input of the µP must be at a logic 1 to start executing correctly. The XIRQ and 
the IRQ pins should also be at a logic 1.
Voltage sense device U0832 provides a reset output that goes to 0 volts if the regulated 5 volts goes 
below 4.5 volts. This is used to reset the controller to prevent improper...

2-36Theory of Operation
2.10.7 Microphone Connector Signals
Signals BUS+, PTT IRDEC, HOOK, MIC, HANDSET AUDIO, FLT A+, +5V, and two A/D converter 
inputs are available at the microphone connector J0811. Signal BUS+ (J0811-7) connects to the 
SBEP bus for communication with the CPS or the Universal Tuner. Line MIC (J0811-5) feeds the 
audio from the microphone to the radio’s controller via connector J0801-4. The Line HANDSET 
AUDIO (J0811-8) feeds the receiver audio from the controller (J0801-6) to a...