Data Sheet
AD9520-3
Rev. A | Page 45 of 80
The channel dividers feeding the output drivers contain one 2-to-
32 frequency divider. This divider provides for division-by-1 to
division-by-32. Division-by-1 is accomplished by bypassing the
divider. The dividers also provide for a programmable duty cycle,
with optional duty-cycle correction when the divide ratio is odd.
A phase offset or delay in increments of the input clock cycle is
selectable. The channel dividers operate with a signal of up to
1600 MHz at their inputs across all channel divider ratios. The
features and settings of the dividers are selected by programming
the appropriate setup and control registers (s
ee Table 50 through
VCO Divider
The VCO divider provides frequency division between the
internal VCO or the external CLK input and the clock
distribution channel dividers. The VCO divider can be set
to divide by 1, 2, 3, 4, 5, or 6 (see
Table 57, Register 0x1E0[2:0]).
However, when the VCO divider is set to 1, none of the channel
output dividers can be bypassed.
The VCO divider can also be set to static, which is useful for
applications where the only desired output frequency is the
VCO frequency. Making the VCO divider static increases the
wide band spurious-free dynamic range (SFDR). If the VCO
divider is static during VCO calibration, there is no output
signal. Therefore, it is important to calibrate the VCO with the
VCO divider set to a nonstatic value during VCO calibration,
and then set the VCO divider to static when VCO calibration is
complete.
The recommended alternative to achieving the same SFDR
performance is to set the VCO divider to 1 and enable VCO direct
mode. This allows the user to program the EEPROM with the
desired values and does not require further action after the VCO
calibration is complete.
Channel Dividers
A channel divider drives each group of three LVPECL outputs.
There are four channel dividers (0, 1, 2, and 3) driving 12 LVPECL
outputs (OUT0 to OUT11
). Table 35 lists the bit locations used
for setting the division and other functions of these dividers. The
division is set by the M and N values. The divider can be
bypassed (equivalent to divide-by-1, divider circuit is powered
down) by setting the bypass bit. The duty-cycle correction can
be enabled or disabled according to the setting of the disable
Divider x DCC bits.
Table 35. Setting DX for the Output Dividers
Divider
Low Cycles,
MValue Bits
High Cycles,
N Value Bits
Bypass
Bits
Disable
Divider x
DCC Bits
0
0x190[7:4]
0x190[3:0]
0x191[7]
0x192[0]
1
0x193[7:4]
0x193[3:0]
0x194[7]
0x195[0]
2
0x196[7:4]
0x196[3:0]
0x197[7]
0x198[0]
3
0x199[7:4]
0x199[3:0]
0x19A[7]
0x19B[0]
Channel Divider Maximum Frequency
The maximum frequency at which all features of the channel
divider are guaranteed to work is 1.6 GHz; this is the number that
appears elsewhere in the datasheet. The maximum frequency at
which all features of the channel divider are guaranteed to work
is 1.6 GHz; this is the number that appears elsewhere in the data
sheet. However, if the divide-by-3 and divide-by-17 settings are
avoided, the maximum channel divider input frequency is 2 GHz.
Channel Frequency Division (0, 1, 2, or 3)
For each channel (where the channel number (x) is 0, 1, 2, or 3),
the frequency division, DX, is set by the values of M and N
(four bits each, representing Decimal 0 to Decimal 15), where
Number of Low Cycles = M + 1
Number of High Cycles = N + 1
The high and low cycles are the cycles of the clock signal that
are currently routed to the input of the channel dividers (VCO
divider out or CLK).
When a divider is bypassed, DX = 1.
Otherwise, DX = (N + 1) + (M + 1) = N + M + 2. This allows
each channel divider to divide by any integer from 2 to 32.
Duty Cycle and Duty-Cycle Correction
The duty cycle of the clock signal at the output of a channel is
a result of some or all of the following conditions:
M and N values for the channel
DCC enabled/disabled
VCO divider enabled/bypassed
CLK input duty cycle (note that the internal VCO has
a 50% duty cycle)
The DCC function is enabled, by default, for each channel divider.
However, the DCC function can be disabled individually for
each channel divider by setting the disable Divider x DCC bit
for that channel.
Certain M and N values for a channel divider result in a non-
50% duty cycle. A non-50% duty cycle can also result in an
even division, if M ≠ N. The duty-cycle correction function
automatically corrects non-50% duty cycles at the channel
divider output to 50% duty cycle.
Duty-cycle correction requires the following channel divider
conditions:
An even division must be set as M = N
An odd division must be set as M = N + 1
When not bypassed or corrected by the DCC function, the duty
cycle of each channel divider output is the numerical value of
(N + 1)/(N + M + 2), expressed as a percent.