Datasheet

Data Sheet AD9520-3
Rev. A | Page 43 of 80
DIVIDE BY 1,
2, 3, 4, 5, OR 6
LF
CLK/CLK
R
DIVIDER
R
DELAY
N
DIVIDER
N
DELAY
PFD
CP
LOOP
FILTER
MUX1
REG 0x01E[1] = 1
01
REFIN/
REFIN
MUX3
REG 0x01E[2]
ZERO DELAY
INTERNAL FEEDBACK PATH
EXTERNAL FEEDBACK PATH
ZERO DELAY FEEDBACK CLOCK
CHANNEL DIVIDER 0
CHANNEL DIVIDER 1
CHANNEL DIVIDER 2
CHANNEL DIVIDER 3
OUT0 TO OUT2
OUT3 TO OUT5
OUT6 TO OUT8
OUT9 TO OUT11
AD9520
07216-053
Figure 49. Zero Delay Function
External Zero Delay Mode
The external zero delay function of the AD9520 is achieved by
feeding one clock output back to the CLK input and ultimately
back to the PLL N divider. In Figure 49, the change in signal
routing for external zero delay mode is shown in red.
Set Register 0x01E[2:1] = 11b to select external zero delay mode.
In external zero delay mode, one of the twelve output clocks
(OUT0 to OUT11) can be routed back to the PLL (N divider)
through the CLK/
CLK
pins and through MUX3 and MUX1.
This feedback path is shown in red in Figure 49.
For VCO calibration to work correctly, the user must specify which
channel divider is used for external zero delay mode. Channel
Divider 0 is the default. Change the value in Register 0x01E[4:3]
to select Channel Divider 1, Channel Divider 2, or Channel
Divider 3 for zero delay feedback.
The PLL synchronizes the phase/edge of the feedback output clock
with the phase/edge of the reference input. Because the channel
dividers are synchronized to each other, the clock outputs are
synchronous with the reference input. Both the R delay and the
N delay inside the PLL can be programmed to compensate for
the propagation delay from the PLL components to minimize the
phase offset between the feedback clock and the reference input.
CLOCK DISTRIBUTION
A clock channel consists of three LVPECL clock outputs or six
CMOS clock outputs that share a common divider. A clock
output consists of the drivers that connect to the output pins.
The clock outputs have either LVPECL or CMOS at the pins.
The AD9520 has four clock channels. Each channel has its own
programmable divider that divides the clock frequency applied
to its input. The channel dividers can divide by any integer from
1 to 32.
The AD9520 features a VCO divider that divides the VCO output
by 1, 2, 3, 4, 5, or 6 before going to the individual channel dividers.
The VCO divider has two purposes. The first is to limit the
maximum input frequency of the channel dividers to 1.6 GHz.
The other is to allow the AD9520 to generate even lower
frequencies than would be possible with only a simple post divider.
External clock signals connected to the CLK input can also use
the VCO divider.
The channel dividers allow for a selection of various duty cycles,
depending on the currently set division. That is, for any specific
division, D, the output of the divider can be set to high for N + 1
input clock cycles and low for M + 1 input clock cycles (where
D = N + M + 2). For example, a divide-by-5 can be high for one
divider input cycle and low for four cycles, or a divide-by-5 can
be high for three divider input cycles and low for two cycles.
Other combinations are also possible.
The channel dividers include a duty-cycle correction function
that can be disabled. In contrast to the selectable duty cycle
just described, this function can correct a non-50% duty cycle
caused by an odd division. However, this requires that the
division be set by M = N + 1.