RF2915 datasheet(10/18 Pages) RFMD

10 / 18 page

11-120

RF2915

Rev A7 001011

The quad tank of the discriminator may be imple-

mented with ceramic discriminator available from a

couple of sources. This design works well for wideband

applications where temperature range is limited. The

temperature coefficient of ceramic discriminators may

be in the order of +50 ppm/°C. The alternative to the

ceramic discriminator is the LC tank, which provides a

broadband discriminator more useful for high data

rates.

PLL Synthesizer

The RF2915 evaluation board uses an LMX2315 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 RF2915 is controlled by RX ENABL and TX

ENABL which are decoded to put the RF2915 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 RF2915, first put the RF2915 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.

RF2915 Datasheet(PDF) 10 Page - RF Micro Devices