EVAL-ADF7012DBZ1 datasheet(17/28 Pages) AD

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ADF7012

Rev. A | Page 17 of 28

THEORY OF OPERATION

CHOOSING THE EXTERNAL INDUCTOR VALUE

The ADF7012 allows operation at many different frequencies by

choosing the external VCO inductor to give the correct output

frequency. Figure 36 shows both the minimum and maximum

frequency vs. the inductor value. These are measurements based

on 0603 CS type inductors from Coilcraft, and are intended as

guidelines in choosing the inductor because board layout and

inductor type varies between applications.

The inductor value should be chosen so that the VCO is cen-

tered at the correct frequency. When locked, the VCO tuning

voltage can be between 0.2 V and 2.1 V. This voltage can be

measured at Pin 18 (VCOIN). To ensure operation over

temperature and from part to part, an inductor should be

chosen so that the tuning voltage is ~1 V at the desired output

frequency.

MIN (meas)

MAX (meas)

MIN (eqn)

MAX (eqn)

INDUCTANCE (nH)

1200

1000

900

1100

700

800

500

400

600

300

010

515

Figure 36. Output Frequency vs. External Inductor Value

IBIAS = 2.0 mA.

For frequencies between 270 MHz and 550 MHz, it is recom-

mended to operate the VCO at twice the desired output

frequency and use the divide-by-2 option. This ensures reliable

operation over temperature and supply.

For frequencies between 130 MHz and 270 MHz, it is recom-

mended to operate the VCO at four times the desired output

frequency and use the divide-by-4 option.

For frequencies below 130 MHz, it is best to use the divide-

by-8 option. It is not necessary to use the VCO divider for

frequencies above 550 MHz.

ADIsimSRD Design Studio is a design tool which can perform

the frequency calculations for the ADF7012, and is available at

www.analog.com.

CHOOSING THE CRYSTAL/PFD VALUE

The choice of crystal value is an important one. The PFD

frequency must be the same as the crystal value or an integer

division of it. The PFD determines the phase noise, spurious

levels and location, deviation frequency, and the data rate in

the case of GFSK. The following sections describe some factors

to consider when choosing the crystal value.

Standard Crystal Values

Standard crystal values are 3.6864 MHz, 4 MHz, 4.096 MHz,

4.9152 MHz, 7.3728 MHz, 9.8304 MHz, 10 MHz, 11.0592 MHz,

12 MHz, and 14.4792 MHz. Crystals with these values are

usually available in stock and cost less than crystals with

nonstandard values.

Reference Spurious Levels

Reference spurious levels (spurs) occur at multiples of the

PFD frequency. The reference spur closest to the carrier is

usually highest with the spur further out being attenuated by

the loop filter. The level of reference spur is lower for lower

PFD frequencies. In designs with high output power where

spurious levels are the main concern, a lower PFD frequency

(<5 MHz) may be desirable.

Beat Note Spurs

Beat note spurs are spurs occurring for very small or very large

values in the fractional register. These are quickly attenuated by

the loop filter. Selection of the PFD therefore determines their

location, and ensures that they have negligible effect on the

transmitter spectrum.

Phase Noise

The phase noise of a frequency synthesizer improves by 3 dB

for every doubling of the PFD frequency. Because ACP is

related to the phase noise, the PFD may be increased to reduce

the ACP in the system. PFD frequencies of < 5 MHz typically

deliver sufficient phase noise performance for most systems.

Deviation Frequency

The deviation frequency is adjustable in steps of

)

(

PFD

STEP

(10)

To get the exact deviation frequency required, ensure FSTEP is a

factor of the desired deviation.

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