The AD9520 on-chip VCO must be calibrated to ensure proper
operation over process and temperature. The VCO calibration
is controlled by a calibration controller running off a divided
REFIN clock. The calibration requires that the PLL be set up
properly to lock the PLL loop and that the REFIN clock be
present. The REFIN clock must come from a stable source
external to the AD9520.
VCO calibration can be performed two ways: automatically at
power up and manually. Automatic VCO calibration occurs when
the EEPROM is set to automatically load the preprogrammed
values in the EEPROM, and then automatically calibrate the
VCO. A valid reference must be provided at power-up in order
for the automatic calibration to complete. If this is not the case,
the user must calibrate the VCO manually.
During the first initialization after a power-up or a reset of the
AD9520, a manual VCO calibration sequence is initiated by
setting 0x018 = 1b. This can be done as part of the initial
setup before executing update registers (0x232 = 1b).
Subsequent to the initial setup, a VCO calibration sequence is
initiated by resetting 0x018 = 0b, executing an update registers
operation, setting 0x018 = 1b, and executing another update
registers operation. A readback bit (0x01F) indicates when a
VCO calibration is finished by returning a logic true (that is, 1b).
The sequence of operations for the VCO calibration follows:
• Program the PLL registers to the proper values for the PLL loop.
• For the initial setting of the registers after a power-up or
reset, initiate a VCO calibration by setting 0x018 = 1b.
Subsequently, whenever a calibration is desired, set 0x018 =
0b, update registers, and set 0x018 = 1b, update registers.
• A SYNC operation is initiated internally, causing the outputs
to go to a static state determined by normal SYNC function
• VCO calibrates to the desired setting for the requested VCO
• Internally, the SYNC signal is released, allowing outputs to
• PLL loop is closed.
• PLL locks.
A SYNC is executed during the VCO calibration; therefore,
the outputs of the AD9520 are held static during the calibration,
which prevents unwanted frequencies from being produced.
However, at the end of a VCO calibration, the outputs may
resume clocking before the PLL loop is completely settled.
The VCO calibration clock divider is set as shown in Table 53
The calibration divider divides the PFD frequency (reference
frequency divided by R) down to the calibration clock. The
calibration occurs at the PFD frequency divided by the calibration
divider setting. Lower VCO calibration clock frequencies result in
longer times for a calibration to be completed.
The VCO calibration clock frequency is given by
fCAL_CLOCK = fREFIN/(R × cal_div)
fREFIN is the frequency of the REFIN signal.
R is the value of the R counter.
cal_div is the division set for the VCO calibration divider
The user should choose a calibration divider such that the
calibration frequency is less than 6.25 MHz. Table 30 shows the
appropriate value for the calibration divider.
Table 30. VCO Calibration Divider Values for Different
Phase Detector Frequencies
PFD Rate (MHz) Recommended VCO Cal Divider
12 to 25
4, 8, 16
25 to 50
50 to 100
The VCO calibration takes 4400 calibration clock cycles.
Therefore, the VCO calibration time in PLL reference clock
cycles is given by
Time to Calibrate VCO =
4400 × R × cal_div PLL Reference Clock Cycles
The AD9520 does not automatically recalibrate its VCO when
the PLL settings change. 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 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 may
not be. Additionally, 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.
Initiate a VCO calibration 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.
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