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Data Sheet

ADL5961

THEORY OF OPERATION

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Rev. 0 | 18 of 34

The ADL5961 is designed to enable broadband multiport network

analyzer solutions in a small footprint.

The ADL5961 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 (LPFs) 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

VNA 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 a 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 ADL5961.

The signal supplied to the OF interface controls the frequency of

the IF output signal because it mixes with the LO signal supplied

to the LOP and LOM interface before driving the downconversion

mixers.

Multiple ADL5961 devices can be phase synchronized and oper-

ated simultaneously, which enables the construction of small foot-

print, multiport network analyzers using one ADL5961 device per

network analyzer port.

BASIC ONE-PORT VNA

A one-port VNA, 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 VNA based

on the ADL5961. The RFIN interface of the ADL5961 connects to

the RF signal source (typically continuous wave), while the RFOUT

interface connects to the unknown load, the device under test

(DUT). Because the ADL5961 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 ADL5961, 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 ADL5961 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. Their ratio (reflected and

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

cient.

Figure 52. ADL5961 Used as One-Port VNA (Reflectometer)

FREQUENCY PLANNING—LO

CONFIGURATIONS

The LO interface of the ADL5961 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

ADL5961. 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 plus sign corresponds to high-side injection, and the minus

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 de-

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

ADL5961 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; however, an IF of 2 MHz to 3 MHz creates

challenges for the signal sources, requiring an accurate, small

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