AD9520-5
Data Sheet
Rev. A | Page 28 of 76
Mode 2—High Frequency Clock Distribution; CLK or
External VCO > 1600 MHz
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-1/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 2400 MHz (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.
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 external VCO/VCXO feeds
directly into the prescaler.
The register settings shown in
Table 22 are the default values of
these registers at power-up or after a reset operation.
Table 22. Default Register Settings for Clock Distribution Mode
Register
Description
0x010[1:0] = 01b
PLL asynchronous power-down (PLL off )
0x1E0[2:0] = 000b
Set VCO divider = 2
0x1E1[0] = 0b
Use the VCO divider
When using the internal PLL with an external VCO, the PLL
must be turned on.
Table 23. Settings When Using an External VCO
Register
Description
0x010[1:0] = 00b
PLL normal operation (PLL on)
0x010 to 0x01E
PLL settings; select and enable a reference
input; set R, N (P, A, B), PFD polarity, and ICP
according to the intended loop
configuration
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 24. Setting the PFD Polarity
Register
Description
0x010[7] = 0b
PFD polarity positive (higher control
voltage produces higher frequency)
0x010[7] = 1b
PFD polarity negative (higher control
voltage produces lower frequency)