LINEAR AN97-1 Data Handbook

Application Note 97

January 2006

Accurate Measurement of LT5514 Third Order Intermodulation Products Dorin Seremeta

INTRODUCTION for the maximum resolution achievable with the available test equipment.

Accurate measurement of the third order intercept point

for low distortion IC products such as the LT5514 requires The test equipment required for LT5514 demo board certain precautions to be observed in the test setup and evaluation consists of two signal generators, a 2-way testing procedure. The LT5514 linearity performance is power combiner, a spectrum analyzer, and fixed attenua- high enough to push the test equipment and test set-up to tor pads, as shown in Figures 1 and 2.

their limits. A method for accurate measurement of third

Two steps are required for the evaluation of LT5514

order intermodulation products, IM3, with standard test

intermodulation products.

equipment is outlined below.

It is also important to correctly interpret the LT5514 1. Evaluate and Optimize Test-Setup Performance

specification with respect to ROUT, and the impact of

Begin by combining the signal outputs of two signal

demo-board transmission-line termination loss when

generators (more information is given in Note 2), and

evaluating the linearity performance, as explained in the

connect the resulting two-tone output directly to a spec-

LT5514 Datasheet and in Note 1 of this document.

trum analyzer as depicted in Figure 1. An example initial setting for the spectrum analyzer is: ATTEN = 20d B, SPAN EVALUATION OF LT5514 LINEARITY PERFORMANCE. = 100Hz, Resolution Bandwidth = 3Hz to 10Hz, REFER- ENCE LEVEL = –20d Bm, ATTENUATION AUTO (OR

The difficulty in providing an adequate low distortion, two-

MANUAL, 10d B). Each frequency of interest should be

tone test signal (Note 2) and failure to adjust the spectrum

inspected, one at a time, with the spectrum analyzer at

analyzer for maximum resolution (Note 3) are the most

minimum span. (Spectrum analyzer settings for maxi-

common issues when evaluating the LT5514. Below is a

mum resolution are discussed in Note 3). The two-tone

description of a test set-up and a method for evaluating

test signal should be set to a power level corresponding to

high linearity components using standard test equipment.

the desired input level for the LT5514 (–23d Bm for opti-

This method makes it easy to identify which section of the

mum maximum output). The signal displayed on the

set-up limits the measurement, and to optimize the set-up

, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

10d B PAD

SIGNAL SPECTRUM GENERATOR #1 ANALYZER

2-WAY COMBINER

AN97 F01

10d B PAD

SIGNAL GENERATOR #2

Figure 1. Basic Set-Up for Two-Tone Signal Source IM3 Performance Verification Measurements

an97f

AN97-1