REV. 0

ADF4212L

–18–

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, or

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 where the local oscillator could have up

to an octave tuning range. For example, cable television tuners

have a total range of about 400 MHz. Figure 8 shows an appli-

cation where the ADF4212L is used to control and program the

Micronetics M3500-1324. The loop filter was designed for an

RF output of 2100 MHz, a loop bandwidth of 40 kHz, a PFD

sitivity of the M3500-1324 at an output of 2100 MHz) and a

phase margin of 45 degrees.

MUXOUT

ADF4212L

SPI COMPATIBLE SERIAL BUS

CLK

DATA

LE

1000pF

1000pF

51

2.7k

LOCK

DETECT

100pF

51

AD820

20k

3.9nF

27nF

470

130pF

1k

20V

3k

V_TUNE

M3500-1324

GND

OUT

100pF 18

18

18

100pF

THE IF SECTION OF THE CIRCUIT HAS ALSO BEEN OMITTED TO SIMPLIFY THE SCHEMATIC.

12V

Figure 8. Wideband PLL Circuit

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

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

variation in VCO sensitivity over the range (typically <10%).

However in wideband applications both of these parameters

have a much greater variation. Variations in these parameters will

change the loop bandwidth. This in turn can affect stability

to get compensation for these varying loop conditions and ensure

that the loop is always operating close to optimal conditions.