User Manual Part 1

The transmit PLL phase locks a transmit oscillator that is operating at an output frequency of

138 to 174 MHz. The RF signal into the PLL chip is created by mixing the transmit frequency

with the receive PLL frequency to generate a mix frequency of 374.4 MHz. This provides low

frequency modulation of the VCO by modulating the transmit PLL reference frequency.

Reference PLL

The reference PLL phase locks the receive PLL reference oscillator to the transmit PLL

reference oscillator with a loop bandwidth of less than 10 Hz. This PLL ensures that the center

frequency of both reference oscillators are the same. It also limits the modulation of the receive

PLL reference oscillator by the low frequency modulation applied to the transmit PLL reference

oscillator.

PLL IC

Two CX72301 sigma-delta modulated PLL chips (U29 & U46) are used for the PLLs

described above. This PLL chip provides good phase noise capabilities to reduce adjacent channel

interference and quick switching between the receive and transmit modes.

Reference Oscillators

One 16.8 MHz oscillator (Y1) is used as the frequency reference for the receive PLL and

also for the receiver backend IC.

The other 16.8 MHz oscillator (Y2) is used as the frequency reference to the transmit PLL.

The center frequency of this oscillator is corrected using a DC tuning voltage from the digital

board in the receive mode and it is modulated with voice or data in the transmit mode.

The receive PLL reference oscillator is phase locked to the transmit PLL reference

oscillator as discussed above.

Analog Switches and PLL Loop Filters

An analog switch (U17) provides faster switching of signals during channel changes by

varying the time constant of the PLL loop filter.

5.2.1.3 TRANSMITTER

Modulation

A “dual-port” modulation scheme is used to provide the DC coupling of the signal required

for data modulation applications. In this scheme, modulation applied to the transmit PLL

frequency reference provides low-frequency modulation, and modulation applied to the transmit

PLL transmit VCO (U47) provides high-frequency modulation. Signals for both modulation ports

are provided by DACs on the digital board.

Power Amplifier