ADF5610 datasheet(23/51 Pages) AD

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Data Sheet

ADF5610

Rev. 0 | Page 23 of 51

For example, it is determined that across all VCO subbands, a

measurement resolution of at least 800 kHz results in the

selection of the correct frequency band. Setting m = 2 and n = 5

provides 781 kHz of resolution and adds 38.4 μs of autocalibration

time to a normal frequency hop. After autocalibration selects

the band (38.4 μs after the frequency change command in the

preceding example), the fractional register is loaded, and the

loop locks with the normal transient predicted by the loop

dynamics. This means that, as shown in this example,

autocalibration adds 38.4 μs to the normal time to achieve

frequency lock. Autocalibration is recommended for all but the

most extreme frequency hopping requirements.

Manual VCO Calibration for Fast Frequency Hopping

When switching frequencies quickly is needed, it is possible to

eliminate the autocalibration time by calibrating the VCO in

advance and storing the step number vs. frequency information

in the host, which is accomplished by initially locking the

ADF5610 on each desired frequency using autocalibration, then

reading and storing the selected VCO band settings. The VCO

band settings are available in Register 0x10, Bits[7:0] after every

autocalibration operation. The host must then program the

VCO band settings directly when changing frequencies.

Manual writes to the VCO bands are executed immediately as are

writes to the integer and fractional registers when autocalibration

is disabled. Therefore, frequency changes with manual control

and autocalibration disabled requires a minimum of two serial

port transfers to the PLL, once to select the VCO band and once

to set the PLL frequency.

When autocalibration is disabled (Register 0x0A, Bit 11 = 1), the

VCO updates its registers immediately with the value written

via Register 0x05. The VCO internal transfer requires 16 VCO

SPI clock (VSCK) cycles after the completion of a write to

are equal to the input reference divided by 0, 4, 16, or 32, as

controlled by Register 0x0A, Bits[14:13].

For settling time requirements faster than 50 µs, contact Analog

Devices, Inc., applications support. Settling times under 50 µs

are possible, but certain limitations on performance may exist.

Registers Required for Frequency Changes in Fractional

Mode

In fractional mode (Register 0x06, Bit 11 = 1), a large change of

frequency may require main serial port writes to one of the

three following registers:

•

The integer register, Register 0x03. This write is required

only if the integer part changes. For integer frequencies,

write this register last because it triggers the frequency

change.

•

The fractional register, Register 0x04. The fractional

Bit 11 = 0, and the fractional frequency change is loaded

into the modulator automatically after the autocalibration

runs. If autocalibration is disabled (Register 0x0A, Bit 11 = 1),

the fractional frequency change is loaded immediately into

the modulator when the register is written with no

adjustment to the VCO. Write this register last for fractional

frequencies because it triggers the frequency change.

•

The VCO SPI register, Register 0x05. This write is required

only for manual control of VCO if Register 0x0A, Bit 11 = 1,

autocalibration is disabled, or to enable or change the VCO

output divider value, VCO bias, or the divider power level

or RFOUT power level. See Figure 19 for more information.

Small steps in frequency in fractional mode, with autocalibration

enabled (Register 0x0A, Bit 11 = 0), usually require only a single

write to the fractional register. In a worst case scenario, three

main serial port transfers to the ADF5610 may be required to

change frequencies in fractional mode. If the frequency step is

small such that the integer part of the frequency does not change,

the integer register is not changed. In all cases, in fractional mode,

it is necessary to write to the fractional register, Register 0x04,

for frequency changes.

Registers Required for Frequency Changes in Integer

Mode

In integer mode (Register 0x06, Bit 11 = 0), a change of frequency

requires main serial port writes to the following registers:

•

VCO SPI register, Register 0x05. This write is required only

for manual control of the VCO when Register 0x0A, Bit 11

= 1 (autocalibration disabled) or when the VCO configura-

tion or the output frequency divider value must change

(VCO Register 0x02).

•

Integer register, Register 0x03. In integer mode, an integer

Bit 11 = 0 and the integer frequency change is loaded into

the prescaler automatically after autocalibration runs. If

autocalibration is disabled (Register 0x0A, Bit 11 = 1), the

integer frequency change is loaded into the prescaler

immediately when written with no adjustment to the VCO.

Normally, changes to the integer register cause large steps

in the VCO frequency. Therefore, the VCO band settings

must be adjusted. Autocalibration enabled is the recom-

mended method for integer mode frequency changes. If

autocalibration is disabled (Register 0x0A, Bit 11 = 1), a

prior knowledge of the correct VCO band setting and the

corresponding adjustment to the VCO is required before

executing the integer frequency change. Failure to select

the proper band may result in loss of lock at temperature

extremes or degraded phase noise performance.

VCO Built In Self Test (BIST) with Autocalibration

The frequency limits of the VCO can be measured using the BIST

features of the autocalibration FSM by setting Register 0x0A,

Bit 10 = 1, which freezes the VCO bands in one position. VCO

bands can then be written manually with the varactor biased at

the nominal midrail voltage used for autocalibration. For example,

to measure the VCO maximum frequency, use Subband 0 (see

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