NPR Measurement: Advantages and Disadvantages

This page details the Noise Power Ratio (NPR) measurement method, which is used for amplifier linearity testing. It covers the benefits (advantages) and drawbacks (disadvantages) of using the NPR measurement method.

Introduction

NPR is the ratio of the root mean square (RMS) noise power of the signal inside a notch to the residual noise within that notch when a notch filter is applied. It’s expressed in decibels (dB). The NPR measurement technique is employed to evaluate the performance of high-power Radio Frequency (RF) amplifiers when handling multiple carriers.

In this method, white noise is generated to simulate numerous carriers with random amplitudes and phases. This white noise is then passed through a Band-Pass Filter (BPF) to create a square spectral pedestal of noise. The bandwidth of this pedestal matches the signal bandwidth intended for measurement. Subsequently, this signal goes through a narrow Band-Stop Filter (BSF) to introduce a deep notch (typically >50 dB) at the center of the noise pedestal.

NPR serves as a measure of the multi-carrier intermodulation ratio (C/I).

What is NPR Measurement?

The figure below illustrates a typical test setup for NPR measurement. In this method, an Arbitrary Waveform Generator (AWGN) generates white noise. This noise is band-limited by a filter before being fed into the Device Under Test (DUT), which is the amplifier being evaluated. The filter employed is a notch filter, characterized by a notch depth exceeding 50 dB below the passband amplitude, and a width of approximately 1% or less than the filtered noise bandwidth.

Image Courtesy: Microwave Journal

The amplitude of the applied noise signal is increased until the amplifier begins to exhibit nonlinearity. As we know, Intermodulation Distortion (IMD) products arise from the mixing of multiple tones when the amplifier operates in its nonlinear region. Because a noise signal comprises many tones, it becomes challenging to measure individual IMD products. Therefore, the aggregate power of the IMD products is measured at the notch frequency using a spectrum analyzer or Vector Network Analyzer (VNA). As the magnitude of these products increases, the depth of the notch decreases.

The method is expensive as it requires high quality filter with sufficient notch depth to observe desired IMD products.

Benefits (Advantages) of NPR Measurement

The following are the advantages of the NPR measurement method for assessing amplifier linearity:

• An AWGN generator provides a high crest factor, which more accurately represents a wideband signal compared to a two-tone IMD stimulus.
• The method utilizes white noise to simulate a multi-tone carrier signal. Consequently, only one noise source is needed for the test setup, unlike IMD measurement methods that require two signal generators. This makes the NPR method less expensive.

Drawbacks (Disadvantages) of NPR Measurement

The following are the disadvantages of the NPR measurement method:

• Since the noise signal is equivalent to numerous tones, measuring individual IMD products directly is difficult. Instead, the aggregate power of the IMD products is measured.
• Similar to IMD testing, NPR measurement also requires an expensive spectrum analyzer or VNA. Furthermore, it necessitates a high-quality filter with sufficient notch depth to accurately observe the desired IMD products.