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ADF5610

Data Sheet

Rev. 0 | Page 20 of 51

When the ADF5610 PLL receives a request to change the

frequency such that a new VCO band must be selected, the

process is as follows. Note that the PLL portion of the

autocalibration circuitry, the FSM, is assumed to already be

enabled (Register 0x0A, Bit 11 = 0).

1. The PLL registers that must be updated to facilitate lock at

the new frequency as well as any changes to the VCO

registers (gain settings, divide ratio) are sent to the

ADF5610 via the SPI. There is no need for users to change

the VCO autocalibration bit when changing frequencies.

The VCO autocalibration bit state is handled automatically

when PLL Register 0x0A, Bit 11 = 0.

2. The FSM takes control of the VCO autocalibration bit

(VCO Register 0x00, Bit 7), changing the bit from 0 to 1,

temporarily opening the loop filter path and switching the

source of the VT pin from the charge pump to the

VPRST pin.

3. With VT set to a value that provides optimal performance,

the PLL FSM begins a binary search for the VCO core and

subband with a frequency that is closest to the desired

frequency. Note that the setting for the VT resolution

(Register 0x0A, Bits[2:0]) can impact band selection as well

as the ability to retain lock over the full operating

temperature range. When in doubt, use the highest setting

for the best resolution.

4. After the proper band has been determined, the VCO

autocalibration bit (VCO Register 0x00, Bit 7) changes

from 1 back to 0, closing the loop filter path and returning

the source of the VT pin to the charge pump.

5. The new VCO frequency is then routed to the input of the

PLL, divided down, and compared to the reference

frequency. The charge pump adjusts the VCO tuning

voltage as necessary to phase lock the divided down VCO

frequency to the reference frequency.

The temperature compensated voltage that is used during

autocalibration is transferred to the VPRST pin (Pin 35) of the

ADF5610. This voltage correlates to the temperature of the

silicon and can be used for a real-time estimate of the junction

temperature of the die. The voltage at VPRST is a function of

the ambient temperature as well as the PCB design and

assembly quality. Take caution if it is desired to continuously

monitor this voltage. The VPRST pin is tied to a low noise

circuit, and although the internal switch has sufficient isolation,

it may still allow external noise to be coupled onto the precision

reference circuit, which results in unwanted spurs, sidebands,

and degraded phase noise.

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

3.0

3.3

6.9

7.6

8.3

9.1

9.8

10.6 11.4 12.1 12.9 13.6 14.4 15.2

CARRIER FREQUENCY (GHz)

–40°C

+85°C

Figure 21. Tuning Voltage vs. Carrier Frequency After Calibration, Maximum

Tuning Resolution, Register 0x0A, Bits[2:0] = 7

Although the digital VCO bands are automatically controlled by

the ADF5610 using the autocalibration feature, the VCO bands

can also be controlled directly via Register 0x05 for testing or

for special purpose operations. Other control bits specific to the

VCO are also sent via Register 0x05. Refer to the VCO

Subsystem Register Map section for further details.

As mentioned in the VCO Subsystem Overview section, during

autocalibration, coarse tuning is provided by the FSM as it

selects the most appropriate band for operation over the full

operating temperature range. Fine tuning is achieved by

varactor diodes after the VCO autocalibration bit (VCO

of the tuning voltage back to the phase detector and charge pump.

When the band is selected, a narrow voltage range on the varactor

is all that is needed for phase lock. Note that the tuning voltage

stays in a narrow range over a wide range of output frequencies.

The calibration is normally run automatically, once for every

change of frequency. Autocalibration ensures optimum

selection of VCO band settings vs. time and temperature. The

user does not need to be concerned about which band setting is

used for a given frequency because this selection is handled by

the autocalibration routine.

The accuracy required in the calibration affects the amount of

time required to tune the VCO. The calibration routine searches

for the step setting where the desired frequency falls within the

frequency range (determined by the FSM resolution setting)

that is centered around the temperature compensated VPRST

value being used during the search. When located, the loop

filter path is closed and the VCO is locked at the programmed

frequency. If the proper resolution is used, the VCO remains

locked and operates away from the charge pump rails over its

full temperature range without additional calibration, regardless

of the temperature at which the VCO was calibrated. Note that

when autocalibration is enabled, preselection of the VCO core

that autocalibration starts in is not possible.

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