Noise and Noise Figure
Noise occurs naturally in any active device or circuit, and limits the minimum levels of useful signals. With a cell phone, for example, it can interfere with a weak signal, and interrupt a call. Therefore, it is important to design circuits to minimize the effects of noise. To do this, the noise must be quantified and measured. The most common measure of added circuit noise is a figure-of-merit called noise figure, which quantifies the signal-to-noise degradation caused by an amplifier. While noise figure is most commonly measured in a 50-ohm environment, a device’s noise figure varies with the source impedance presented to the device. This variation can be expressed in terms of noise parameters, which are essential to know when designing low-noise amplifiers using highly mismatched devices.
Noise Parameters are comprised of four distinct characteristics, Fmin the minimum noise figure of the transistor, Γopt the optimum impedance at which Fmin occurs (in real and imaginary), and Rn the equivalent series resistance of the transistor. In theory, any four controlled impedances Γs can be presented to the transistor and the corresponding noise figures F measured in order to satisfy equation and solve four simultaneous equations for four unknowns. Practically, the impedances selected for Γs should be in the range of Γopt.
Swept-Frequency Noise Parameters
A new ultra-fast noise parameter measurement method is able to improve overall calibration and measurement time by a factor of 100X-400X, bringing measurements that could once take tens or hundreds of hours to tens of minutes. The new method has two key features that contribute to the breakthrough speed improvement: 1) The tuner is characterized with one set of states (physical tuner positions) that are selected to give a reasonable impedance spread over the frequency band of interest; and 2) the noise power measurement is swept over the frequency range at each state, so that the tuner only moves to each position once. This takes advantage of the fast sweep capability of modern instruments, as well as saving time by minimizing tuner movement.
The new noise parameter measurement method provides two orders of magnitude speed improvement. It also produces data that is smoother and has less scatter than the traditional method. The fast measurement speed eliminates temperature drift, and using a VNA with an internal noise receiver simplifies the setup and makes it much more stable and consistent. The much higher speed makes it practical to always do a full in-situ calibration to minimize errors, and to measure more frequencies to get a better view of scatter and cyclical errors, and to be able to use smoothing with more confidence. The higher frequency density also enhances accuracy by reducing shifts due to aliasing.