CN-0174 Datasheet, PDF (1/3 Pages) Analog Devices – Low Noise, 12 GHz, Microwave Fractional-N Phase-Locked Loop Using an Active Loop Filter and RF Prescaler

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Circuit Note
CN-0174
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
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
3.3V 3.3V
VDD1 VDD2
RFOUT
ADF5001
PRESCALER
RFIN
RFOUT
GND
3.3V 3.3V
ACTIVE PLL LOOP FILTER, 30kHz CLOSED LOOP BANDWIDTH
1.8nF
5V
DECOUPLING
INTEGRATED
AVDD DVDD VP
RFINA
ADF4156
PLL CPOUT
RFINB
GND
330Ω
47nF
15V
100Ω
1.8nF
15V
2.2V
OP184
OP AMP
47kΩ
1kΩ
1.8nF
8.2kΩ
1µF
5V
6dB PAD
37Ω
18Ω
150Ω
150Ω
VCC
18Ω
18Ω
SYNERGY
RFOUT
VCO
VTUNE
DXO11751220-5
GND
12GHz OUT
Figure 1. Low Noise Microwave Fractional-N PLL (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. A
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