ADF7021 datasheet(24/64 Pages) AD

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ADF7021

Data Sheet

Rev. B | Page 24 of 64

VOLTAGE CONTROLLED OSCILLATOR (VCO)

The ADF7021 contains two VCO cores. The first VCO, the

internal inductor VCO, uses an internal LC tank and supports

862 MHz to 950 MHz and 431 MHz to 475 MHz operating

bands. The second VCO, the external inductor VCO, uses an

external inductor as part of its LC tank and supports the RF

operating band of 80 MHz to 650 MHz.

To minimize spurious emissions, both VCOs operate at twice

the RF frequency. The VCO signal is then divided by 2 inside

the synthesizer loop, giving the required frequency for the

transmitter and the required local oscillator (LO) frequency for

the receiver. A further divide-by-2 (RF_DIVIDE_BY_2) is

performed outside the synthesizer loop to allow operation in

the 431 MHz to 475 MHz band (internal inductor VCO) and

80 MHz to 325 MHz band (external inductor VCO).

The VCO needs an external 22 nF capacitor between the CVCO

pin and the regulator (CREG1) 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 36. Voltage Controlled Oscillator (VCO)

Internal Inductor VCO

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

which is the default setting.

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

ensure VCO oscillation, the minimum bias current setting under

all conditions when using the internal inductor VCO is 0x8.

The VCO should be re-centered, depending on the required

frequency of operation, by programming the VCO_ADJUST

bits (R1_DB[23:24]). This is detailed in Table 9.

External Inductor VCO

When using the external inductor VCO, 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.

The external inductor is connected between the L2 and L1 pins.

A plot of the VCO operating frequency vs. total external

inductance (chip inductor + PCB track) is shown in Figure 37.

200

250

300

350

400

450

500

550

600

650

700

TOTAL EXTERNAL INDUCTANCE (nH)

750

fMIN (MHz)

fMAX (MHz)

Figure 37. Direct RF Output vs. Total External Inductance

The inductance for a PCB track using FR4 material is approxi-

mately 0.57 nH/mm. This should be subtracted from the total

value to determine the correct chip inductor value.

Typically, a particular inductor value allows the ADF7021 to

function over a range of ±6% of the RF operating frequency.

When the RF_DIVIDE_BY_2 bit (R1_DB18) is selected, this

range becomes ±3%. At 400 MHz, for example, an operating

range of ±24 MHz (that is, 376 MHz to 424 MHz) with a single

inductor (VCO range centered at 400 MHz) can be expected.

The VCO tuning voltage can be checked for a particular RF

output frequency by measuring the voltage on the VCOIN pin

when the part is fully powered up in transmit or receive mode.

The VCO tuning range is 0.2 V to 2 V. The external inductor

value should be chosen to ensure that the VCO is operating

as close as possible to the center of this tuning range. This is

particularly important for RF frequencies <200 MHz, where

the VCO gain is reduced and a tuning range of <±6 MHz exists.

The VCO operating frequency range can be adjusted by

programming the VCO_ADJUST bits (R1_DB[23:24]). This

typically allows the VCO operating range to be shifted up or

down by a maximum of 1% of the RF frequency.

To select the external inductor VCO, set R1_DB25 to Logic 1.

The VCO_BIAS_CURRENT should be set depending on the

frequency of operation (as indicated in Table 9).

Table 9. RF Output Frequency Ranges for Internal and External Inductor VCOs and Required Register Settings

RF Frequency

Output (MHz)

VCO to

Be Used

RF Divide

by 2

(VCO_INDUCTOR)

R1_DB25

(RF_DIVIDE_BY_2)

R1_DB18

(VCO_ADJUST)

R1_DB[23:24]

(VCO_BIAS)

R1_DB[19:22]

900 to 950

Internal L

862 to 900

Internal L

450 to 470

Internal L

Yes

431 to 450

Internal L

Yes

450 to 650

External L

200 to 450

External L

80 to 200

External L

Yes

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