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RF Components: Noise Source Selection Guide

Noise Source Selection Guide

Noise Sources are divided into three (3) major categories, Packaged Noise Sources, Amplified Noise Modules and Simple Noise Diodes. These devices are listed in the various product families below. In each are a brief description and a link to the product line data sheet.

Coaxial Noise Sources: These devices are more often in microwave frequency, but lower frequency and millimeter wave frequencies are not uncommon. They contain all the biasing circuitry so just a simple positive voltage is needed. Noise sources typically have no amplification, so just the power output from the raw noise diode is output. This has a maximum of about 35 dB over ambient thermal noise in a resistor at room temperature, known as Excess Noise Ratio (ENR) expressed as 35 dB ENR. These noise sources have two major usages, in the lab as calibration standards for noise figure measurement, and built into systems. Noise sources used in the lab are broadband (i.e. 10 MHz - 26.5 GHz), and are calibrated to an absolute level traceable to a NIST calibrated standard and have very good VSWR, when the noise is on and off. Very low power is needed, from 5 to 15 dB ENR for noise figure measurement. This is convenient because a pad is inserted in the device, which serves the dual purpose of lowering the amplitude from the raw noise circuit, which increases the return loss, by twice this amount, thus yielding a very low VSWR.

The Mercury Systems 346 Series Noise Source is designed specifically for use with noise figure meter. Noise sources when built into systems often require higher power as there may be substantial insertion loss in the RF path caused by some combination of, 10 or 20 dB coupler, splitter, filter, length of coaxial line, etc. When used in a live system such as to calibrate a radar receiver's noise temperature, good VSWR and protection from potential high incident RF power is needed. For these applications, an isolator will solve both these problems. Fortunately most systems are either narrow band or octave band, in which standard isolator designs are available. Some systems require the noise sources built-in to be calibrated. Others use an external noise source to calibrate the internal noise source and RF path to the measurement plane. Mercury Systems' noise sources for systems can supply noise sources calibrated to a standard if required.

Waveguide Noise Sources: These are similar to coaxial noise sources. Noise sources up to 40 GHz are built with modern hybrid coaxial construction with a coax to waveguide adaptor or adaptor/isolator in the end. These noise sources are calibrated with the waveguide in place so there is no need to de-embed the insertion loss of the adaptor/isolator. Above 40 GHz, the noise sources are full waveguide construction. These millimeter waveguide are fairly expensive. Substantial cost savings can be realized if the source only needs to operate over a narrow band within the waveguide size frequency rang. Noise sources can be tuned anywhere in the overall frequency range, but full-band tuning can be quite time consuming.

Amplified Noise Modules: These modules not only have the noise diode circuitry built-in, but also one or several gain stages to boost the output power. Lower frequencies (under 2 GHz) are more common, but usages up to 6 GHz are fairly common. Mercury Systems produces powerful amplified noise modules up to 10 GHz. Applications vary for these modules, from setting up a signal to noise ration to frequency response measurement to random number generation. Noise modules are available in coaxial, surface mount and pin type construction.

Surface Mount Noise Sources/Modules: A full suite of surface mount noise packaged and amplified modules for today's modern circuits. These devices are easy to integrate into circuit boards without having to spend the high cost of engineering required when using raw noise diodes.

Noise Diodes: Noise diodes fall into four (4) primary categories, surface mount, axial leaded, pill package and raw die form. The last three types represent traditional noise diodes. For modern, higher volume circuits, Mercury Systems has a large offering of surface mount noise diodes to 3 GHz. These devices have very good unit-to-unit consistency without requiring screening, making them very inexpensive in quantity. They are available in tape-and-reel for automated pick and place machines.

When choosing between a noise diode and packaged noise sources, consider the engineering hours required to design the noise source biasing circuit and amplification vs. the annual usage of the overall circuit. Calculating the return on investment (ROI) of reinventing the wheel vs. the cost savings of using a diode over packaged surface mount noise modules is an important analysis to make prior to endeavoring to use a noise diode.
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