ADF7021 datasheet(14/44 Pages) AD

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ADF7021

Preliminary Technical Data

Rev. PrI | Page 14 of 44

VOLTAGE CONTROLLED OSCILLATOR (VCO)

To minimize spurious emissions, the on-chip VCO operates

from 1724 MHz to 1880 MHz. The VCO signal is then divided

by 2 to give the required frequency for the transmitter and the

required LO frequency for the receiver.

The VCO should be recentered, depending on the required

frequency of operation, by programming the VCO adjust bits

R1_DB[23:24]. The VCO is enabled by Bit R1_DB17.

A further frequency divide-by-2 is included after the PLL to

allow operation from 432 MHz to 470 MHz using the internal

VCO inductor and 80 MHz to 325 MHz using the external VCO

inductor. This divide-by-2 is enabled by setting R1_DB18 to 1.

The VCO needs an external 22 nF capacitor between the VCO

and the regulator to reduce internal noise.

VCO

LOOP FILTER

VCO BIAS

R1_DB(19:22)

220µF

CVCO PIN

DIVIDE BY 2

R1_DB18

÷2

MUX

N DIVIDER

TO PA

Figure 12. Voltage Controlled Oscillator (VCO)

The ADF7021 on-chip VCO can use either an internal or

external tank inductor. The possible frequency ranges are given

in Table 5.

Table 5. RF Output Frequency Ranges for Internal/External

VCO Inductor and Divide-by-2 Enabled/Disabled

RF Frequency

Output (MHz)

Int/Ext VCO Inductor

R1_DB25

Divide-by-2

R1_DB18

862 to 940

431 to 470

135 to 650

80 to 325

Internal VCO Inductor

To select the internal VCO inductor, set R1_DB25 to Logic 0,

which is the default setting.

VCO bias current can be adjusted using Bits R1_DB[19:22]. To

ensure VCO oscillation, the minimum bias current setting under

all conditions when using the internal VCO inductor is 0xA.

The VCO should be recentered, depending on the required

frequency of operation, by programming the VCO adjust bits

R1_DB[23:24].

Using the External VCO Inductor

When using an external inductor, the center frequency of the

VCO is set by the internal varactor capacitance and the

combined inductance of the external chip inductor, bond wire,

and PCB track. A plot of the VCO operating frequency vs. total

external inductance (chip inductor + PCB track) is shown in

Figure 13. The inductance for a PCB track using FR4 material is

approximately 0.57 nH/mm. This should be subtracted from the

total value to determine the correct chip inductor value.

200

250

300

350

400

450

500

550

600

650

700

TOTAL EXTERNAL INDUCTANCE (nH)

750

FMIN (MHz)

FMAX (MHz)

Figure 13. VCO Operating Frequency vs. Total External Inductance

VCO bias current can be adjusted using Bits R1_DB[19:22]. To

minimize current consumption and to ensure reliable

operation, the bias current setting should be as indicated in

Table 6.

Table 6. Recommended VCO Bias Currents for External

VCO Inductor

RF Frequency Output (f)

VCO Bias R1_DB[19:22]

f < 200 MHz

0001

200 MHz < f < 450 MHz

0010

f > 450 MHz

0011

CHOOSING CHANNELS FOR BEST SYSTEM

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 register

using the frequency doubler.

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