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| 型號： | AD9518-2A/PCBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 28/64頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVALUATION FOR AD9518-2A |
| 設計資源： | AD9518 Schematics AD9518 Gerber Files AD9518-2 BOM |
| 標準包裝： | 1 |
| 主要目的： | 計時，時鐘發(fā)生器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9518-2A |
| 主要屬性： | 2 輸入，6 輸出，2.2GHz VCO |
| 次要屬性： | LVPECL 輸出邏輯 |
| 已供物品： | 板 |
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AD9518-2
Data Sheet
Rev. C | Page 34 of 64
Table 28. Example Time to Complete a VCO Calibration
with Different fREFIN Frequencies
fREFIN (MHz)
R Divider
PFD
Time to Calibrate VCO
100
1
100 MHz
88 s
10
1 MHz
8.8 ms
10
100
100 kHz
88 ms
VCO calibration must be manually initiated. This allows for
flexibility in deciding what order to program registers and when
to initiate a calibration, instead of having it happen every time
certain PLL registers have their values change. For example, this
allows for the VCO frequency to be changed by small amounts
without having an automatic calibration occur each time; this
should be done with caution and only when the user knows that
the VCO control voltage is not going to exceed the nominal best
performance limits. For example, a few 100 kHz steps are fine,
but a few MHz might not be. In addition, because the calibration
procedure results in rapid changes in the VCO frequency, the
distribution section is automatically placed in SYNC until the
calibration is finished. Therefore, this temporary loss of outputs
must be expected.
A VCO calibration should be initiated under the following
conditions:
After changing any of the PLL R, P, B, and A divider
settings, or after a change in the PLL reference clock
frequency. This, in effect, means any time a PLL register
or reference clock is changed such that a different VCO
frequency results.
Whenever system calibration is desired. The VCO is
designed to operate properly over extremes of temperatures
even when it is first calibrated at the opposite extreme.
However, a VCO calibration can be initiated at any time,
if desired.
CLOCK DISTRIBUTION
A clock channel consists of a pair of outputs that share a
common divider. A clock output consists of the drivers that
connect to the output pins. The clock outputs have LVPECL
signal levels at the pins.
The AD9518 has three channels, each with two LVPECL
outputs, for a total of six LVPECL outputs.
Each channel has its own programmable divider that divides
the clock frequency that is applied to its input. The channel
dividers can divide by any integer from 2 to 32, or the divider
can be bypassed to achieve a divide-by-one.
If the user wishes to use the channel dividers, the VCO divider
must be used after the on-chip VCO. This is because the internal
VCO frequency is above the maximum channel divider input
frequency (1600 MHz). The VCO divider can be set to divide by
2, 3, 4, 5, or 6. External clock signals connected to the CLK
input also require the VCO divider if the frequency of the signal
is greater than 1600 MHz.
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.
In addition, the channel dividers allow a coarse phase offset or
delay to be set. Depending on the division selected, the output
can be delayed by up to 31 input clock cycles. The divider
outputs can also be set to start high or start low.
Internal VCO or External CLK as Clock Source
The clock distribution of the AD9518 has two clock input sources:
an internal VCO or an external clock connected to the CLK/CLK
pins. Either the internal VCO or CLK must be chosen as the
source of the clock signal to distribute. When the internal VCO
is selected as the source, the VCO divider must be used. When
CLK is selected as the source, it is not necessary to use the VCO
divider if the CLK frequency is less than the maximum channel
divider input frequency (1600 MHz); otherwise, the VCO divider
must be used to reduce the frequency to one that is acceptable
by the channel dividers. Table 29 shows how the VCO, CLK,
and VCO divider are selected. Register 0x1E1[1:0] selects the
channel divider source and determines whether the VCO divider
is used. It is not possible to select the VCO without using the
VCO divider.
Table 29. Selecting VCO or CLK as Source for Channel
Divider, and Whether VCO Divider Is Used
Register 0x1E1
Channel Divider Source
VCO Divider
Bit 1
Bit 0
0
CLK
Used
0
1
CLK
Not used
1
0
VCO
Used
1
Not allowed
CLK or VCO Direct to LVPECL Outputs
It is possible to connect either the internal VCO or the CLK
(whichever is selected as the input to the VCO divider) directly
to the LVPECL outputs, OUT0 to OUT5. This configuration
can pass frequencies up to the maximum frequency of the VCO
directly to the LVPECL outputs. The LVPECL outputs may not
be able to provide a full voltage swing at the highest frequencies.
To connect the LVPECL outputs directly to the internal VCO or
CLK, the VCO divider must be selected as the source to the
distribution section, even if no channel uses it.
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