ADL5960ACCZ-R2 datasheet(16/32 Pages) AD

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Data Sheet

ADL5960

THEORY OF OPERATION

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The ADL5960 is designed to enable broadband multiport network

analyzer solutions in a small footprint.

The ADL5960 is built around an integrated broadband bidirectional

resistive bridge and coupled to broadband dual downconversion

mixers. The differential IF outputs of the mixers are passed through

low-pass filters with SPI-programmable bandwidth and IF amplifiers

with individually SPI-programmable gain.

The LO interface to the mixers supports multiple SPI-programmable

configurations, and is designed to simplify the frequency plan in

a vector network analyzer application. The LO interface contains

frequency multipliers and dividers that enable a wider frequency

range of operation than supported by the LO signal source itself.

A second input to the LO interface, the offset interface formed by

the OFP and OFM pins, enables a zero-offset operating mode with

greatly simplified frequency plan. In this mode, only a single swept

high-frequency signal source is needed, driving both the RF and LO

interfaces of the ADL5960. The signal supplied to the OF interface

controls the frequency of the IF output signal as it mixes with the

LO signal supplied to the LOP/LOM interface before driving the

downconversion mixers.

Multiple ADL5960 devices can be phase synchronized and operat-

ed simultaneously, which enables the construction of small footprint

multiport network analyzers using one ADL5960 device per network

analyzer port.

BASIC ONE-PORT VECTOR NETWORK

ANALYZER

A one-port vector network analyzer, or reflectometer, can be used

to measure the magnitude and phase of the reflection coefficient of

an unknown load vs. frequency.

Figure 52 shows the basic configuration of a one-port analyzer

based on the ADL5960. The RFIN interface of the ADL5960 con-

nects to the RF signal source (typically continuous wave), while the

RFOUT interface connects to the unknown load, the device under

test (DUT). Because the ADL5960 is completely symmetric, the RF

source can also be connected to RFOUT and the load to RFIN. The

RF source injects an incident RF signal into the directional bridge

of the ADL5960, traveling from the source to the load. At the load,

part of this incident power wave is reflected and travels back to the

source, while the other part is absorbed by the load. The reflection

coefficient to be measured (both magnitude and phase) equals the

ratio of the reflected power to the incident power at the load. For

proper operation of the bidirectional bridge, it is important that the

RF signal source has a 50 Ω characteristic impedance. The LO

interface can either be driven by a 50 Ω single-ended source or a

100 Ω differential source.

The directional bridge on the ADL5960 supplies a fraction of the in-

cident signal to the input of the forward IF channel, and, likewise, a

fraction of the reflected signal to the input of the reverse IF channel.

Both these IF signals are downconverted, filtered, amplified, and

made available at the differential IF channel output interfaces, IFFx

(with the IFFP and IFFM terminals) and IFRx (with the IFRP and

IFRM terminals), respectively.

After analog-to-digital conversion, the IF output signals representing

the incident and reflected waves are digitally quadrature (complex)

downconverted, filtered, and decimated. Finally, their ratio (reflect-

ed/forward) is calculated to obtain the (complex) reflection coeffi-

cient.

Figure 52. ADL5960 Used as One-Port VNA (Reflectometer)

FREQUENCY PLANNING—LO

CONFIGURATIONS

The LO interface of the ADL5960 supports several different configu-

rations, some of which significantly simplify the VNA configuration

in exchange for slightly degraded accuracy.

The bypass mode, selected by setting Bit 4 in Register 0x20, is

the most basic and highest performance mode of operation of the

ADL5960. In this mode, the LO signal supplied to the LO interface

(that is, the LOP and LOM pins) directly drives the downconversion

mixers, bypassing the frequency multipliers, dividers, and offset

mixer. To maintain a fixed IF output frequency, the LO signal must

maintain a fixed frequency offset relative to the RF signal across

the entire frequency sweep.

fLO=fRF±fIF

(1)

The + sign corresponds to high-side injection, and the − sign

corresponds to low-side injection. High-side injection often results

in slightly improved dynamic range, because more of the mixing

products calculate out at higher frequencies than the desired IF

signal and can be suppressed by low-pass filtering. The ADL5960

supports IF frequencies up to 100 MHz. For high dynamic range

analog-to-digital conversion, an IF frequency of a few MHz is often

preferable, but an IF of 2 MHz to 3 MHz creates challenges for the

signal sources, requiring an accurate, small frequency offset of a

few MHz between RF and LO up to 20 GHz signal frequencies.

The frequency divider and frequency multipliers integrated into the

LO interface of the ADL5960 enable measurement sweeps beyond

the frequency range supported by the LO source itself, typically

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