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    6 Technologies and Techniques to Know for Measuring Noise Figure

    6 Technologies and Techniques to Know for Measuring Noise Figure


    Posted April 23, 2015


    Innovations in noise-figure and noise-parameter measurements could reduce testing time and increase accuracy to millimeter-wave frequencies.

    Jean-Jacques DeLisle | Microwaves and RF 

    In smartphone and portable-device use, Internet of Things (IoT) wireless sensor networks, and military networks, wireless radio technologies are breaking barriers in terms of throughput and low-power transmission and reception. Yet the effects of noise degradation scale with such performance extremes (Fig. 1). Historically, testing and characterizing the noise figure and noise parameters of the devices driving these applications has been both time-consuming and error-prone.

    6 Technologies and Techniques to Know for Measuring Noise Figure
    1. In addition to the calibrated noise source used in a noise-figure measurement, many other noise generators add uncertainty to a noise measurement.

    Such challenges are now being overcome by performance enhancements in recent test and measurement instruments; optional packages designed specifically for noise testing; and automation techniques. These developments are ushering in more efficient solutions for both noise-figure and noise-parameter testing.  Knowledge of these methods and their implementation can aid in greater measurement accuracy and drastically reduced testing time.

    1. Y-factor with noise source and spectrum/signal analyzer. The Y-factor method is one of the most common solutions for providing noise measurements. Essentially, this method uses a noise source connected to the device under test (DUT). The DUT, in turn, is connected at the input of an RF/microwave power-sensing device. With the noise source “on” and “off,” measurements of the signal power are taken over by a set bandwidth. The ratios of noise power in the “on” and “off” states are taken to generate the Y-factor.

    With the excess noise ratio (ENR) specification provided by the noise-source manufacturer, the noise factor of the test system can be calculated next. Using knowledge of the DUT’s gain and the noise figure of the test-system components, it is then possible to extract and calculate the noise figure of the DUT.

    To shorten noise-figure measurement times while increasing accuracy and repeatability, optional packages have been made available for several recent spectrum/signal analyzers. Companies such as Keysight, Tektronix, Anritsu, and Rohde & Schwarz offer options that include a software enhancement that guides an engineer through the measurements. Doing so ensures repeatability while automatically calculating the noise figure from the noise factor measurements.

    6 Technologies and Techniques to Know for Measuring Noise Figure
    2. The Y-Factor solution requires a calibrated noise source external to the analyzer and DUT, whereas the cold-source method generally demands an electronically calibrated tuner.

    Keep in mind that these measurements require calibrated noise sources with known ENR. Such sources are provided by companies like Noisewave, Keysight, Noisecom, Pasternack, and Mercury Systems.

    When using spectrum/signal analyzers with very-low-power noise signals, a low-noise amplifier (LNA) will need to be added to the analyzer’s signal chain. Many of the companies that provide noise-figure options include low-noise preamplifiers for their spectrum/signal analyzers. Those preamplifiers are even built into the analyzer. With the internal preamplifier, measurement accuracy is increased while test-set component count and calibration complexity are reduced.

    Many of the latest spectrum/signal analyzers reach 26.5 GHz and 43 GHz with a native analyzer from Rohde & Schwarz recently hitting 67 GHz. These analyzers allow noise measurements to be performed into the upper microwave and millimeter-wave bands without the need for external mixers. With external mixers, noise measurements to 110 GHz can be obtained with a spectrum/signal analyzer and the Y-factor method (Fig. 2).

    2. The cold-source or gain solution. To characterize and measure noise figure, the cold-source solution employs a noise source held at a reference cold temperature, a network analyzer, and several step-calibration and gain measurements. A calibrated noise source is utilized to measure the noise figure of the instrument’s receiver while the network analyzer measures the DUT’s gain.

    Once the prior measurements have been performed, the cold source can drive the DUT connected to the network analyzer. With the latest network analyzers with noise-figure packages, an additional pre-amplifier is often included inside the network analyzer. It enhances low-noise measurement capability (Fig. 3).

    6 Technologies and Techniques to Know for Measuring Noise Figure
    3. Compared to the Y-Factor method, high-performance VNAs with automated test equipment—using the cold-source method—can significantly reduce the DUT’s reflection and noise interaction during a noise test.

    Similar to spectrum/signal analyzers with noise-figure packages, network-analyzer noise-figure packages often include software guides that aid in increasing test repeatability. With this method, the noise figure and S-parameters of a DUT can be measured in the same test setup. This aspect enhances the automation potential of the approach. An automated-test-equipment (ATE)-based setup could provide very rapid chip-and larger-device-level noise-figure testing. Obviously, automating the cold-source method could prove more repeatable and reliable than using multiple instruments and test setups.

    Conveniently, software programs like MATLAB and LabView also include instrument-control and analysis toolboxes. These toolboxes enable the control and analysis of test instruments that can create complex and user-defined ATE systems. When coupled with ATE systems, software control and analysis tools enable the rapid and repeatable use of additional testing devices, such as impedance tuners, for optimizing the low-noise performance of non-50-O devices.

    An example is finding the noise figure of unmatched transistors and low-noise amplifiers (LNAs). Here, the minimum noise-matching impedance can be found with an impedance tuner in conjunction with a cold-source, network-analyzer noise-figure measurement setup. Companies like Maury Microwave and Focus Microwaves offer impedance tuners, some of which are manual or automated. They can range from 2 to 26 GHz and 8 to 50 GHz for Maury Microwave tuner systems and from 100 MHz to 67 GHz for Focus Microwaves’ coaxial impedance tuners.