EVAL-ADF7020DBZ2 datasheet(17/48 Pages) AD

17 / 48 page

Data Sheet

ADF7020

Rev. D | Page 17 of 48

VCO Bias Current

VCO bias current can be adjusted using Bit R1_DB19 to

Bit R1_DB16. To ensure VCO oscillation, the minimum bias

current setting under all conditions is 0xA.

VCO

LOOP FILTER

MUX

VCO SELECT BIT

TO PA

VCO BIAS

R1_DB[16:19]

220µF

CVCO PIN

÷2

TO N

DIVIDER

Figure 24. Voltage-Controlled Oscillator (VCO)

CHOOSING CHANNELS FOR BEST SYSTEM

PERFORMANCE

The fractional-N PLL allows the selection of any channel within

868 MHz to 956 MHz (and 433 MHz using divide-by-2) to a

resolution of <300 Hz. This also facilitates frequency-hopping

systems.

Careful selection of the XTAL frequency is important to achieve

best spurious and blocking performance. The architecture of

fractional-N causes some level of the nearest integer channel to

couple directly to the RF output. This phenomenon is often

referred to as integer boundary spurious. If the desired RF channel

and the nearest integer channel are separated by a frequency of

less than the PLL loop bandwidth (LBW), the integer boundary

spurs are not attenuated by the loop.

Integer boundary spurs can be significantly reduced in ampli-

tude by choosing XTAL values that place the wanted RF

channel away from integer multiples of the PFD.

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