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ADF7012

Rev. 0 | 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 it is between the

minimum and maximum value.

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

recommended 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

recommended 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.

ADIsimPLL is a PLL design tool which can perform the

frequency calculations for the ADF7012, and is available at

www.analog.com/pll.

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 that

should be considered 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

These 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 3dB 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 < 5MHz 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.

TIPS ON DESIGNING THE LOOP FILTER

The loop filter design is crucial in ensuring stable operation of

the transmitter, meeting Adjacent Channel Power (ACP)

specifications, and meeting spurious requirements for the

relevant regulations. ADIsimPLL is a free tool available to aid

the design of loop filters. The user enters the desired frequency

range, the reference crystal and PFD values, and the desired

loop bandwidth. ADIsimPLL gives a good starting point for the

filter, and the filter can be further optimized based on the

criteria below.

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