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Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik

RF & Wireless Figure 2:

RF & Wireless Figure 2: Plot of NPR vs. loading condition to determine the optimum operating point for maximum SNR. curves. Gaussian distribution, also known as normal distribution, is the most common way events take place. Most white noise sources provide Gaussian distribution of their spectral power. Why is Gaussian distribution of such importance? The randomness of noise sources requires a mathematical model that allows calculating the probability density of events. Gaussian distribution right and left of the mean probability is of equal size. Crest factor, the peak-to-average ra- tio, is also an important quantity for noise sources. It allows calculating the likelihood of un- wanted events relative to the mean probability. Such renegade events may cause an overload of an amplifier input, distortion of a signal, or a bit error. Obviously, the fewer of those natural but unwanted events happen, the better. High quality white Gaussian noise sources offer crest factors of up to 18 dB, placing the probability of unwanted events far beyond reasonable measurement capabilities. Application 1: Noise as a Broadband Reference We discussed two key characteristics of white noise: It offers all frequencies in a specified frequency band simultaneously, and that with a constant power level. These features make noise sources desirable elements whenever analyzing frequency bands of filters, circuits, instruments, or systems, as, for example, filter analysis or spectrum analyzer calibration. A simple way to calibrate spectrum analyzers is to use a broadband white noise source to monitor the analyzer‘s frequency response. Noise sources are available with spectral flatness that far exceeds typical spectrum analyzer accuracy; hence, this flat frequency distribution makes white noise sources ideal for calibration purposes. A spectrum analyzer‘s flatness is influenced by the effects of its internal RF attenuator, preselector, and mixing mode gain variations. Injecting noise into a spectrum analyzer as a reference source can therefore provide an accurate measurement of the amplitude variations versus frequency. Another way to normalize the errors associated with spectrum analyzer measurement is to employ a signal generator that sweeps the required frequency while the analyzer stores the response. A noise source has several advantages over this approach: Cost: The cost of a noise source is insignificant compared to the cost of a broadband signal generator with similar output-versusfrequency characteristics. Size: A noise source can be made much smaller than a signal generator. Speed: When performing broadband measurements, the oscillator must be set, responses stored, next frequency set, next responses stored, and so on. Even with fast signal generators, this can take quite a long time, depending on the number of samples taken. White noise sources generate all the frequencies simultaneously, so the limiting factor is only the measurement speed of the spectrum analyzer. The procedure requires less time, which dramatically reduces cost. Accuracy: Noise sources can be accurately measured to within tenths of a dB by NIST-traceable methods. Application 2: Noise Load to Determine System Behaviour - Noise-Power-Ratio (NPR) Measurements Serious dynamic range problems can occur in systems loaded with multi-channel signals. Any nonlinearity in the system (e.g. an amplifier) causes intermodulation products among the input frequencies. This produces new frequencies that may fall within the bandwidth of other channels, therefore creating distortions. The amount of distortion increases with the number of channels and the degree of amplifier saturation. For example, two uncorrelated tones may, at times, add in-phase to create peak voltages that are about 6 dB higher than the RMS voltage. The peak-to-RMS voltage ratio increases by 10 log(2n) dB, where n is equal to the number of uncorrelated signals of the same amplitude. Many uncorrelated load signals can be simulated by white Gaussian noise for noise power ratio (NPR) testing. The NPR method is an accurate means of reproducing multi-carrier intermodulation effects and determining the amplifier‘s performance under worst-case loading conditions. Figure 3: Output noise spectrum during NPR Meas- urement. Noise is injected over the whole operating bandwidth (a). Now, a notch filter is applied to sup- press a known amount of power in a specific band- width (b). The excess of measured power is generated by the DUT 60 hf-praxis 2/2017

RF & Wireless Intermodulation testing of amplifiers with only two tones has many shortcomings if the life system is actually loaded with numerous signals. First, the distortion product amplitude may be dependent on the frequency spacing of the input signals. Second, the amplifier will perform differently when loaded with many signals. The peak-toaverage ratio of multiple modulated signals is much greater than two or several tones. These large peaks stress the amplifier or device under test (DUT) to a greater degree. The NPR method emulates many signals by loading the amplifier with white Gaussian noise. Intermodulation distortion manifests itself in two primary ways. In a receiver LNA, adjacent channel signals cause inband distortion products. For a transmitter power amplifier, distortion of the primary signal will cause interference in adjacent channels. Both cases are closely related and can be accounted for with the noise power ratio (NPR) test. NPR is a measure of distortion produced in a particular band by a device that is loaded with just white noise. Measurement of NPR versus output power, or loading, is used to define the optimum operating point for maximum signal-tonoise-ratio (SNR). The NPR of a DUT is degraded primarily by two factors. The first is the distortion products that are produced under high loading conditions. The second is the noise floor of the amplifier that will become dominant under very low loading conditions. By making numerous measurements with different loading levels, a curve will be generated, as shown in Figure 2. NPR is poor at low loading levels because the amplifier is being operated near its own noise floor. The NPR is also poor at very high loading levels. But the slope on this side of the curve is steeper since the distortion products are dominant in this case. If distortions are caused by nth-order harmonics, intermodulation products increase by (n-1) dB for every 1 dB increase of the loading level. For example, for thirdorder distortions, intermodulation products increase 2 dB for every 1 dB increase of the loading level. Systems are often operated at signals a few dB below the point of maximum NPR. To perform an NPR test, an accurate level of white Gaussian noise is applied to the amplifier. A bandstop (notch) filter is then inserted to create a „quiet“ channel. The NPR is the ratio between the noise power measured with and without the notch filter (Figure 3). Conclusion Noise can indeed help to solve test and measurement challenges for engineers. The applications discussed above - noise as a broadband reference and noise to measure system load behavior - are just two of many. A wide variety of noise sources available for specific applications. They are affordable, and offer a flat response over a wide frequency band. ◄ Antennas Ultra-Wideband Gooseneck Antennas New Patch Antenna Solutions Designed for use on Ground Robots, Unmanned Vehicles, Manpacks, Broadcast Cameras and Vehicles, the wideband nature of these antennas allows users to future proof their systems and give increased flexibility when using software defined radios. These antennas can be used to replace several narrow-band antennas reducing a system’s ‘antenna real estate’ which is especially important on manned and un- manned vehicles. Since one antenna can fulfil all needs across multiple bands, Radio suppliers and users can also benefit by reducing inventory. The Gooseneck itself has been specified to be stiff enough to remain where it has been bent even under vibration, which is essential when being used on systems where shock and vibration may be present. OA2-2.1- 5.9-GN-ST/9625 and OA2-2.1- 5.9-GN-SRT/9626 each cover the frequency range 2...6 GHz and offer gain increasing with frequency from 1 dBi to almost 5 dBi gain in the highest frequencies. The options have either a spinning TNC (9625) or spinning RPTNC (9626) connector. Like their counter-parts in the ‘Universal’ spring mount range, the OA2-2.1-5.9-GN-ST/9625 and OA2-2.1-5.9-GN-SRT/9626 are been designed to withstand harsh environments. A rugged glass fibre radome and IP68 rated to 20 m under water, means they are able to withstand heavy impacts as well as extreme weather conditions. These ultra-wideband antennas will also be available in a range of options including spring mount and direct connector mount so please contact us to discuss options. ■ Cobham www.european-antennas.co.uk RFMW Ltd. announced design and sales support for API Technologies’ ceramic patch antennas. Model PA25-1621- 025SALF offers 4 dBic of gain at a center frequency of 1621 MHz +/-4 MHz, and 10 dB bandwidth is 25 MHz. API Technologies offers a wide range of patch antennas for applications such as Globalstar, GPS, Inmarsat, Iridium, RFID and ISM radios and can customize performance for specific requirements. Ground planes have substantial effects on antenna performance and API offers design support to identify the ideal antenna for your application and aid in the design. The PA25-1621-025SALF measures 25 x 25 mm and mounts to a backplane via adhesive tape and a through pin solder connection. ■ RFMW Ltd. www.rfmw.com hf-praxis 2/2017 61

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