Circuit Note

CN-0174

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced

ADF4156

6 GHz Fractional-N PLL

ADF5001

18 GHz Divide-by-4 Prescaler

OP184

Single-Supply, Rail-to-Rail Input/Output

Operational Amplifier

Low Noise, 12 GHz, Microwave Fractional-N Phase-Locked Loop (PLL)

Using an Active Loop Filter and RF Prescaler

Rev. A

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CIRCUIT FUNCTION AND BENEFITS

This circuit is a complete implementation of a low noise

microwave fractional-N PLL using the ADF4156 as the core

fractional-N PLL device. The ADF5001 external prescaler is

used to extend the frequency range of the PLL up to 18 GHz.

An ultralow noise OP184 op amp with appropriate biasing and

filtering is used to drive a microwave VCO to implement a

complete low noise PLL at 12 GHz with a measured integrated

phase noise of 0.35 ps rms. This function is typically used to

generate the local oscillator frequency (LO) for applications

such as microwave point-to-point systems, test and

measurement equipment, automotive radar, and military

applications.

CIRCUIT DESCRIPTION

A block diagram of the circuit is shown in Figure 1.

A 12 GHz VCO from Synergy Microwave Corporation, the

DXO11751220-5, was chosen for this circuit, although any

VCO operating from 4 GHz to 18 GHz would also work,

provided the loop filter is redesigned appropriately. As with the

majority of microwave VCOs, the Synergy VCO has a wide

input tuning range of 0.5 V to 15 V, which requires an active

PLL loop filter to interface between the lower voltage ADF4156

charge pump (maximum output of 5.5 V) and the VCO input.

The OP184 was chosen as the op amp for the active loop filter

because of its excellent noise performance, as well as its input

ADF5001

PRESCALER

ADF4156

PLL

SYNERGY

VCO

DXO11751220-5

3.3V

3.3V

VDD1

RFIN

RFOUT

RFINA

RFINB

CPOUT

RFOUT

GND

VDD2

3.3V

3.3V

AVDD

5V

VCC

DVDD

5V

VP

GND

GND

DECOUPLING

INTEGRATED

OP184

OP AMP

2.2V

1µF

47k

Ω

100

Ω

8.2k

Ω

1.8nF

15V

47nF

1.8nF

330

Ω

1.8nF

1k

Ω

VTUNE

RFOUT

18

Ω

37

Ω

150

Ω

150

Ω

18

Ω

18

Ω

12GHz OUT

ACTIVE PLL LOOP FILTER, 30kHz CLOSED LOOP BANDWIDTH

15V

6dB PAD

Figure 1. Low Noise Microwave Fractional-N PLL (Simplified Schematic: All Connections and Decoupling Not Shown)