ADF4112BCPZ datasheet(23/28 Pages) AD

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Data Sheet

ADF4110/ADF4111/ADF4112/ADF4113

Rev. F | Page 23 of 28

ADF4111

ADF4112

ADF4113

2.7k

Ω

VCO

GND

Ω

100pF

Ω

RFOUT

FREFIN

Ω

100pF

RFINA

RFINB

POWER SUPPLY CONNECTIONS AND DECOUPLING

CAPACITORS ARE OMITTED FOR CLARITY.

RSET

REFIN

LOOP

FILTER

CLK

DATA

SPI COMPATIBLE SERIAL BUS

AD5320

12-BIT

V-OUT DAC

MUXOUT

LOCK

DETECT

INPUT OUTPUT

Figure 34. Driving the RSET Pin with a D/A Converter

USING A D/A CONVERTER TO DRIVE THE RSET PIN

A D/A converter can be used to drive the RSET pin of the

ADF4110 family, thus increasing the level of control over the

charge pump current, ICP. This can be advantageous in wide-

band applications where the sensitivity of the VCO varies over

the tuning range. To compensate for this, the ICP may be varied

to maintain good phase margin and ensure loop stability. See

Figure 34.

SHUTDOWN CIRCUIT

The attached circuit in Figure 35 shows how to shut down both

the ADF4110 family and the accompanying VCO. The ADG701

switch goes closed circuit when a Logic 1 is applied to the IN

input. The low cost switch is available in both SOT-23 and

MSOP packages.

WIDEBAND PLL

Many of the wireless applications for synthesizers and VCOs in

PLLs are narrow band in nature. These applications include the

various wireless standards like GSM, DSC1800, CDMA, and

WCDMA. In each of these cases, the total tuning range for the

local oscillator is less than 100 MHz. However, there are also

wideband applications for which the local oscillator could have

a tuning range as wide as an octave. For example, cable TV

tuners have a total range of about 400 MHz. Figure 36 shows an

application where the ADF4113 is used to control and program

the Micronetics M3500-2235. The loop filter was designed for

an RF output of 2900 MHz, a loop bandwidth of 40 kHz, a PFD

frequency of 1 MHz, ICP of 10 mA (2.5 mA synthesizer ICP

multiplied by the gain factor of 4), VCO KD of 90 MHz/V

(sensitivity of the M3500-2235 at an output of 2900 MHz), and

a phase margin of 45°C.

In narrow-band applications, there is generally a small variation

in output frequency (generally less than 10%) and a small

variation in VCO sensitivity over the range (typically 10% to

15%). However, in wideband applications, both of these

parameters have a much greater variation. In Figure 36, for

example, there is a −25% and +17% variation in the RF output

from the nominal 2.9 GHz. The sensitivity of the VCO can vary

from 120 MHz/V at 2750 MHz to 75 MHz/V at 3400 MHz

(+33%, −17%). Variations in these parameters change the loop

bandwidth. This in turn can affect stability and lock time. By

changing the programmable ICP, it is possible to get compensa-

tion for these varying loop conditions and ensure that the loop

is always operating close to optimal conditions.

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