EVAL-ADF7012EB1 Analog Devices Inc, EVAL-ADF7012EB1 Datasheet - Page 17

• Manufacturer Part Number: EVAL-ADF7012EB1
• Description: BOARD EVALUATION FOR EB1 ADF7012
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADF7012EB1

• Module/board Type: Evaluation Board
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: ADF7020 928MHz
• Lead Free Status / RoHS Status: Not Compliant, Contains lead / RoHS non-compliant

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 (VCO
temperature and from part to part, an inductor should be
chosen so that the tuning voltage is ~1 V at the desired output
frequency.
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.
1200
1100
1000
900
800
700
600
500
400
300
Figure 36. Output Frequency vs. External Inductor Value
0
5
10
IN
I
BIAS
). To ensure operation over
INDUCTANCE (nH)
= 2.0 mA.
15
20
25
MIN (meas)
MAX (meas)
MIN (eqn)
MAX (eqn)
30
35
Rev. A | Page 17 of 28
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
To get the exact deviation frequency required, ensure F
factor of the desired deviation.
F
STEP
(
Hz
)
=
F
2
PFD
14
ADF7012
STEP
is a
(10)