Datasheet

AD9518-1 Data Sheet
Rev. C | Page 22 of 64
THEORY OF OPERATION
OPERATIONAL CONFIGURATIONS
The AD9518 can be configured in several ways. These
configurations must be set up by loading the control registers
(see Table 42 and Table 43 through Table 49). Each section or
function must be individually programmed by setting the
appropriate bits in the corresponding control register or registers.
High Frequency Clock Distribution—CLK or External
VCO > 1600 MHz
The AD9518 power-up default configuration has the PLL
powered off and the routing of the input set so that the CLK/
CLK
input is connected to the distribution section through the VCO
divider (divide-by-2/divide-by-3/divide-by-4/ divide-by-5/divide-
by-6). This is a distribution-only mode that allows for an external
input up to 2.4 GHz (see Table 3). The maximum frequency that
can be applied to the channel dividers is 1600 MHz; therefore,
higher input frequencies must be divided down before reaching
the channel dividers. This input routing can also be used for lower
input frequencies, but the minimum divide is 2 before the channel
dividers.
When the PLL is enabled, this routing also allows the use of the
PLL with an external VCO or VCXO with a frequency of less
than 2400 MHz. In this configuration, the internal VCO is not
used and is powered off. The external VCO/VCXO feeds
directly into the prescaler.
The register settings shown in Table 20 are the default values of
these registers at power-up or after a reset operation. If the
contents of the registers are altered by prior programming after
power-up or reset, these registers can also be set intentionally to
these values.
After the appropriate register values are programmed,
Register 0x232 must be set to 0x01 for the values to take effect.
Table 20. Default Settings of Some PLL Registers
Register Function
0x010[1:0] = 01b PLL asynchronous power-down (PLL off ).
0x1E0[2:0] = 010b
Set VCO divider = 4.
0x1E1[0] = 0b Use the VCO divider.
0x1E1[1] = 0b CLK selected as the source.
When using the internal PLL with an external VCO, the PLL
must be turned on.
Table 21. Settings When Using an External VCO
Register Function
0x010[1:0] = 00b PLL normal operation (PLL on).
0x010 to 0x01D PLL settings. Select and enable a
reference input; set R, N (P, A, B), PFD
polarity, and I
CP
, according to the intended
loop configuration.
0x1E1[1] = 0b CLK selected as the source.
An external VCO requires an external loop filter that must be
connected between CP and the tuning pin of the VCO. This
loop filter determines the loop bandwidth and stability of the
PLL. Make sure to select the proper PFD polarity for the VCO
being used.
Table 22. Setting the PFD Polarity
Register Function
0x010[7] = 0b PFD polarity positive (higher control
voltage produces higher frequency).
0x010[7] = 1b PFD polarity negative (higher control
voltage produces lower frequency).