Datasheet

AD9125
Rev. 0 | Page 41 of 56
PLL Settings
There are three settings for the PLL circuitry that should be
programmed to their nominal values. Table 24 lists the
recommended PLL settings for these parameters.
Table 24. PLL Settings
PLL SPI Control
Address
Register
Bits
Optimal
Setting
PLL Loop Bandwidth[2:0] 0x0C [7:5] 110
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.
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 writing 0x80 to Register 0x0A. When this
value is 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.)
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
1000 220020001800160014001200
PLL BAND
VCO FREQUENCY (MHz)
09016-035
Figure 65. PLL Lock Range over Temperature for a Typical Device
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. When in manual mode, the VCO band
is set directly with the value written to the manual VCO band,
(Register 0x0A, Bits[5:0]). To properly select the VCO band,
follow these steps:
1.
Put the device in manual band select mode.
2.
Sweep the VCO band over a range of bands that result in
the PLL being locked.
3.
For each band, verify that the PLL is locked and read the
PLL using the VCO control voltage (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, 0000 or 1000 (see
Table 25 for more details). The resulting VCO band should
be the optimal setting for the device. Write this band to the
manual VCO band (Register 0x0A, Bits[5:0]) value.
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 25 shows how to interpret
the PLL VCO control voltage (Register 0x0E, Bits[3:0]) value.
Table 25. VCO Control Voltage Range Indications
VCO Control Voltage Indication
1111 Move to a 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 a lower VCO band
0000