It is often essential to test the performance of RF, microwave, and millimeter-wave communication, sensing, and signal generation systems to the effects of noise. However, to gain the most insight from testing systems with artificial noise sources, it is important to have noise sources that behave in a measurably predictable manner. This is the purpose of the various types of RF noise source generators. These devices are typically connected inline with a device-under-test (DUT) or system-under-test (SUT) with some type of measurement system, often a vector network analyzer, on the other end of the device or system. The goal here is to measure the differences in performance of the device while being exposed to calibrated levels of noise or to determine the limits of the device or system functionality under excessive noise conditions.
Key RF Noise Source Generator Electrical Specification Performance Parameters & Features
- Frequency range [Hz]
- Impedance [Ohms]
- Output ENR [dB]
- Flatness [dB]
- VSWR [ratio]
- Output power [dB]
- Crest factor [ratio]
- Output power spectral density (noise power) [dBm/Hz]
- Output variation vs. input voltage (supply voltage sensitivity) [dB/%V]
- Output variation vs. temperature (temperature sensitivity) [dB/deg C]
- Bias voltages [V]
- Input currents [A]
- Calibration points [Hz] *applicable to calibrated noise sources
- Connection [coaxial connectorized, surface mount technology etc]
Many modern noise sources use diodes specifically designed for their noise generation capability. An example of this is Zener diodes being driven in reversed-biased mode to the point of breakdown where these diodes generate avalanche noise, which results in a relatively flat frequency spectrum noise source, i.e., white noise. At this stage there is additional manipulation to the white noise, additional circuitry, and other design features that can be incorporated to result in a noise source that exhibits specific characteristics used for a wide range of applications. The difference with an RF noise source and typical noise sources is that RF noise sources are specifically designed to reach higher frequencies relevant to testing RF devices and systems.
There are several different variations of noise generators, namely:
- Amplified Noise Source Generators
- Calibrated Noise Source Generators
- Calibrated, Integral Isolator Noise Source Generators
- Calibrated, Output Isolator Noise Source Generators
- SMT Noise Source Generators
In some use cases it is desirable to have a noise source with a higher output noise power than is generally used, which requires precise broadband amplification. Hence, amplified noise sources are used to provide this higher noise power level while minimizing distortion and nonlinearities over a wide bandwidth that would otherwise degrade the noise quality. Calibrated noise sources are designed to provide a precise noise level at type over a specific range of frequencies. A calibrated noise source is ideal for precision measurement and testing. Some noise sources include isolators at the output that provide enhanced port matching at the output. Output isolation on noise sources results in more accurate measurements, as well as protection of the noise source from incident RF power at the output port of the noise source. This is especially useful when a noise source is connected to reflective components and devices, such as some filters, mixers etc.