ADF7020 datasheet(13/40 Pages) AD

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Preliminary Technical Data

ADF7020

Rev. PrH | Page 13 of 40

VCO Bias Current

VCO bias current can be adjusted using Bits R1_DB19 to

R1_DB16. To ensure VCO oscillation, the minimum bias

current setting under typical conditions is 2.5 mA.

VCO

LOOP FILTER

MUX

VCO SELECT BIT

TO PA AND

N DIVIDER

VCO BIAS

R1_DB (16:19)

220

µF

CVCO PIN

÷2

Figure 10. Voltage Controlled Oscillator (VCO)

CHOOSING CHANNELS FOR BEST SYSTEM

PERFORMANCE

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

868 MHz to 928 MHz (and 433MHz using divide-by-2) to a

resolution of <100 Hz. This also facilitates frequency hopping

systems.

Careful selection of the RF transmit channels must be made to

achieve best spurious performance. The architecture of

fractional-N results in some level of the nearest integer channel

moving through the loop to the RF output. These “beat-note”

spurs are not attenuated by the loop, if the desired RF channel

and the nearest integer channel are separated by a frequency of

less than the LBW.

The occurrence of beat-note spurs is rare, because the integer

frequencies are at multiples of the reference, which is typically

>10 MHz.

Beat-note spurs can be significantly reduced in amplitude by

avoiding very small or very large values in the fractional

1 MHz away from an integer frequency, a 100 kHz loop filter

can reduce the level to <−45 dBc. When using an external VCO,

the fast lock (bleed) function reduces the spurs to <−60 dBc for

the same conditions.

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