RF2945 datasheet(10/24 Pages) RFMD

10 / 24 page

11-224

RF2945

Rev A10 000919

PLL Synthesizer

The RF2945 evaluation board uses an LMX2316 PLL

IC from National Semiconductor. This PLL IC may be

programmed from the software available from National

Semiconductor

(codeloader

www.national.com/

appinfo/wireless/). An external reference oscillator is

required for the PLL IC allowing for the evaluation of

different reference frequencies or step sizes. The

National Semiconductor software also has a calculator

for determining the R and C component values for a

given loop bandwidth.

The RF2945 is controlled by RX ENABL and TX

ENABL which are decoded to put the RF2945 into one

of four states. It may be put into a PLL-only mode with

TX ENABL and RX ENABL both high. This condition is

used to provide time for the synthesizer to turn on and

obtain lock before turning on the receiver or transmit-

ter. Note that LVL ADJ needs to be held low for PLL-

only mode. Sometimes, it is desirabletoramp upthe

power amplifier to minimize load pulling on the VCO.

To do this with the RF2945, first put the RF2945 into

PLL mode by putting TX ENABL and RX ENABL high.

Then,rampupLVL ADJtoturnonthe transmitterand

PA. The rate at which LVL ADJ is allowed to ramp up is

dependent on the PLL loop bandwidth. VCC pushing

also affects the VCO frequency. A good low pass filter

on VCC will minimize the VCC pushing effects.

For applications requiring fast switching speeds or

turn-on times, and low data rate loop filter bandwidths,

the LMX2316 may be configured to drive the loop filter

in a fast switching mode. Please refer to literature on

the LMX2316 for more information.

RF2945 Datasheet(PDF) 10 Page - RF Micro Devices