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 in the form of noise parameters, comprised of Fmin, Gamma,opt (mag and phase), and Rn.
An impedance tuner is often used in conjunction with a noise figure analyzer (or alternate noise measurement instrument) to measure the noise figure, or noise power, as a function of source impedance presented to the DUT, from which the Noise Parameters are extracted.
Note, the term Noise Figure is a commonly referenced parameter when discussing LNAs, and most often refers to the 50ohm noise contribution of a device. The 50ohm Noise Figure of a device can be directly measured using the Noise Parameter system, or extrapolated from the Noise Figure contours. Direct measurement is achieved by using an impedance tuner to present exactly 50ohm to the DUT and measuring the associated noise figure (note, the tuner can correct for the non-50ohm system impedance normally presented without a tuner). Noise Figure extrapolation is a standard function within a noise parameter measurement system and uses mathematically determined contours to calculate the expected Noise Figure contribution at 50ohm.
Ultra-Fast 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.
The Keysight N524x-series PNA-X is the only instrument of its kind to include an embedded electronic tuner capable of presenting impedances up to Γ=0.3 (VSWR~2:1). When measuring a near-50Ω transistor, the electronic tuner may be sufficient in presenting impedances to solve the unknown equations and determine noise parameters. For non-50Ω transistors, the embedded electronic tuner is bypassed and an external automated impedance tuner is used to present impedances closer to Γopt. Tuner model MT982BL01 is recommended for measurements between 0.8-18 GHz; tuner model MT983BL01 for measurements between 2-26 GHz; tuner model MT984AL01 for measurements between 8-50 GHz.
The PNA-X is also the only instrument to combine a vector receiver with a dedicated and optimized noise receiver. The sensitivity of the noise receiver is critical in determining the noise figure F by accurately measuring the noise power contribution of the transistor. The accuracy of the noise measurement is directly related to the second-stage noise figure of the noise receiver, the lower the better. While it is possible to directly use the noise receiver of the PNA-X, the second-stage noise figure can be reduced by 5-6 dB with the addition of an external noise receiver module (NRM) MT7553B03.