ADF4155

Data Sheet

APPLICATIONS INFORMATION

LOCAL OSCILLATOR WITH RF BUFFER

Figure 34 shows the ADF4155 used with a VCO and an RF

buffer to produce a local oscillator (LO) at 5.8 GHz.

The differential reference input signal is applied to the circuit at

divided by 2 to serve as the 61.44 MHz PFD frequency.

drives the loop filter. The ADIsimPLL design tool is used to

calculate the loop filter components. It is designed for a loop

bandwidth of 80 kHz and a phase margin of 45°.

The loop filter output drives the VCO, whose output is fed back

to the RF input of the PLL synthesizer via an RF buffer. It also

the RFIN pins of the ADL5541 RF buffer.

The RF buffer is an optional buffer inserted in the feedback

an improvement in spur performance is required. The pie

attenuator is required to reduce the RF buffer output to within

In a PLL system, it is important to know when the loop is in

lock. The locking is achieved by using the MUXOUT signal

from the synthesizer. The MUXOUT pin can be programmed to

monitor various internal signals in the synthesizer. One of these

is the lock detect signal.

Figure 34. Typical Application Diagram

ADF4155

CE

CLK

DATA

LE

1nF

1nF

100pF

MUXOUT

470pF

10nF

330pF

100pF

470Ω

680Ω

51Ω

100pF

18

Ω

NOTES

THIS IS A SIMPLIFIED SCHEMATIC, DECOUPLING CAPACITORS AND SPI CONNECTION DETAILS HAVE BEEN OMITTED FOR CLARITY.

122.88MHz

LOCK

DETECT

GND

VTUNE RFOUT

VPOS

RFIN

CB

GND

RFOUT

VCO

V940ME03-LF

ADL5541

100Ω

100Ω

68Ω

100pF

100pF

100pF

4.7kΩ

9

18

6

20

7

21

11

10

24

23

12

8

100pF

L1

47nH

C3

1µF

19

4

1

2

3

18

Ω

18

Ω

Rev. 0 | Page 28 of 32