Datasheet

AD9122
Rev. B | Page 45 of 60
PLL SETTINGS
Three settings for the PLL circuitry should be programmed to
their nominal values. The PLL values shown in Table 2 3 are the
recommended settings for these parameters.
Table 23. PLL Settings
PLL Control Register
Register
Address Bits
Optimal
Setting
PLL Loop Bandwidth[1:0] 0x0C [7:6] 11
PLL Charge Pump Current[4:0] 0x0C [4:0] 10001
PLL Cross-Control Enable 0x0D 4 1
CONFIGURING THE VCO TUNING BAND
The PLL VCO has a valid operating range from approximately
1.0 GHz to 2.1 GHz covered in 63 overlapping frequency bands.
For any desired VCO output frequency, there may be several
valid PLL band select values. The frequency bands of a typical
device are shown in Figure 65. Device-to-device variations and
operating temperature affect the actual band frequency range.
Therefore, it is required that the optimal PLL band select value
be determined for each individual device.
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
1000 220020001800160014001200
PLL BAND
VCO FREQUENCY (MHz)
08281-035
Figure 65. PLL Lock Range over Temperature for a Typical Device
Automatic VCO Band Select
The device has an automatic VCO band select feature on chip.
Using the automatic VCO band select feature is a simple and
reliable method of configuring the VCO frequency band. This
feature is enabled by starting the PLL in manual mode, then
placing the PLL in auto band select mode. This is done by
setting Register 0x0A to a value of 0xCF, then to a value of
0xA0. When these values are written, the device executes an
automated routine that determines the optimal VCO band
setting for the device. The setting selected by the device ensures
that the PLL remains locked over the full −40°C to +85°C
operating temperature range of the device without further
adjustment. (The PLL remains locked over the full temperature
range even if the temperature during initialization is at one of
the temperature extremes.)
Manual VCO Band Select
The device also has a manual band select mode (PLL manual
enable, Register 0x0A, Bit 6 = 1) that allows the user to select
the VCO tuning band. In manual mode, the VCO band is set
directly with the value written to the manual VCO band bits
(Register 0x0A, Bits[5:0]). To properly select the VCO band,
follow these steps:
1.
Put the device in manual band select mode by setting
Register 0x0A, Bit 6 = 1.
2.
Sweep the VCO band over a range of bands that results in
the PLL being locked.
3.
For each band, verify that the PLL is locked and read the PLL
using the VCO control voltage bits (Register 0x0E, Bits[3:0]).
4.
Select the band that results in the control voltage being
closest to the center of the range, that is, 1001 or 1000 (see
Table 24). The resulting VCO band should be the optimal
setting for the device. Write this value to the manual VCO
band bits (Register 0x0A, Bits[5:0]).
5.
If desired, an indication of where the VCO is within the
operating frequency band can be determined by querying the
VCO control voltage. Table 24 shows how to interpret the
PLL VCO control voltage value (Register 0x0E, Bits[3:0]).
Table 24. VCO Control Voltage Range Indications
VCO Control Voltage
(Register 0x0E, Bits[3:0]) Indication
1111 Move to higher VCO band
1110
1101
VCO is operating in the higher end
of the frequency band
1100
1011
1010
1001
VCO is operating within an optimal
region of the frequency band
1000
0111
0110
0101
VCO is operating in the lower end
of the frequency band
0100
0011
0010
0001 Move to lower VCO band
0000