New Approaches to Satellite NPR Testing
2020-01-31 | 6 min read
Satellite innovations promise to transform future communications. From delivery of communications like fifth-generation (5G) cellular to security and streaming applications, next-generation satellite networks will enable the transformation and creation of many applications. The success of all communications systems depends on performance, however. One important indicator of performance is noise power ratio (NPR) – particularly in satellite systems, which need to limit their power consumption. The distortion testing of radio-frequency (RF) amplifiers offers one method to gain insight into NPR.
NPR is actually one of the first distortion tests. It dates back to the 1930s for the testing of frequency multiplexed telephone networks, where many narrowband signals were combined in a trunk line. The engineers at Bell Labs wanted to find out what effect amplifier distortion would have on each signal. So they put signals on all of the channels except one, creating a “notch” in the overall signal. As a result, they could see how much distortion was caused by all the other channels and estimate how much interference could be expected.
Defining Distortion Testing
In distortion testing, a number of tests are designed to measure the amount of distortion added by a given component. Ideally, an active device like an amplifier would amplify the incoming signal without any distortion. In the real world, however, an amplifier distorts the signal via different methods and mechanisms. Distortion falls into two categories: linear distortion (modeled as frequency response or group delay) or nonlinear distortion (in the form of signal compression and mixing effects).
Testing this distortion is very important in systems with wideband signals such as communication signals. In a new webinar, Application Engineer Donald Vanderweit explains that a good design depends on understanding the specifics of distortion – especially as channel bandwidths widen and amplifier performance is pushed to new limits. Many amplifiers operate in a linear power range, where there is little or no nonlinear distortion. As we push the amplifier further, nonlinear distortion appears and begins to dominate the signal.
The NPR test became so popular that we still use it today to measure nonlinear distortion in amplifiers. Where an intermodulation distortion (IMD) test will only show the interference from two tones, the NPR test fills the whole channel bandwidth with signal. Traditionally, an NPR test setup comprises a wideband noise generator and then a bandpass filter to limit the noise to the right bandwidth. To get the notch, you used a notch filter. Although this method is still used, Vanderweit notes that it lacks flexibility. The filters used are typically at a set bandwidth. Changing frequencies means having to get more filters.
A newer method for generating this NPR signal is to use an arbitrary waveform generator (AWG) to create a field of harmonically related tones with the appropriate bandwidth and notch. Because the tones are harmonically related, all distortion products fall on the same frequency grid. As a result, nothing is lost. With this method, the AWG is flexible and programmable. Changing the bandwidth or the notch is simple. Corrections can be applied to individual tones to improve the notch depth and pedestal flatness.
Despite the improvements gained with this new approach, challenges still need to be overcome. Because the generated signal varies in time, Vanderweit explains, the receiver might get different results with each trace. How do we interpret this? Do we record the “worst case” with the highest tone levels in the notch? Do we average them out? This leads to more questions about the measurement. How many tones are needed to give an accurate answer? How many tones need to appear in the notch? These questions are usually addressed with statistics in mind: how many tones are needed to reduce the uncertainty to an acceptable level?
One more hard-to-answer question arises: How wide should the notch be? In the Bell Telephone days, the answer was easy: one voice channel. For a wideband single carrier signal, the answer is not so obvious. Check out our webinar, “Better Noise Power Ratio Measurements for Satellite Components,” to see different examples of setups and approaches that will help you answer these questions.
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