Voltage Controlled Oscillator (Vco) - Motorola CDM Series Detailed Service Manual

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Section 6: 2-8

Voltage Controlled Oscillator (VCO)

The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U5301), the TX

and RX tank circuits, the external RX buffer stages, and the modulation circuitry.

(U5201 Pin 28)

The VCOBIC together with Fractional-N synthesizer (U5201) generates the required frequencies in

both transmit and receive modes. The TRB line (U5301 pin 19) determines which tank circuits and

internal buffers are to be enabled. A high level on TRB enables TX tank and TX output (pin 10), and

a low enables RX tank and RX output (pin 8). A sample of the signal from the enabled output is

routed from U5301 pin 12 (PRESC_OUT), via a low-pass filter, to pin 32 of U5201 (PREIN).

A steering line voltage (VCTRL) between 3.0V and 10.0V at varactor diode CR5311 will tune the full

TX frequency range (TXINJ) from 450 MHz to 512 (or 527) MHz, and at varactor diodes CR5301,

CR5302 and CR5303 will tune the full RX frequency range (RXINJ) from 405 MHz to 482 MHz. The

tank circuits uses the Hartley configuration for wider bandwidth. For the RX tank circuit, an external

transistor Q5301 is used in conjunction with the internal transistor for better side-band noise.

Similarly, an external transistor Q5311 is used in the TX tank circuit.

6881091C63-F

UHF Band 2 (450–512/527 MHz) 1–25 W Frequency Synthesis

Switching Network

Vcc-Superfilter

Collector/RF in

(U5201 Pin 28)

Figure 6-4 UHF VCO Block Diagram

AUX3 (U5201 Pin 2)

Prescaler Out

Pins 9,11,17

U5201 Pin 32

LO RF INJECTION

(U5201 Pin28)

TX RF Injection

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