What Are Dynamic Range, SNR, P1dB, and Other Common RF/Microwave Characteristics

There are several key methods of describing RF signal quality and device performance in respect to noise and linearity. As noise and linearity are often key considerations of both transmit and receive chains in communications and sensing systems, it is valuable for a technical professional to understand these parameters. The following is a brief description of a few key noise and linearity parameters commonly used to describe RF/microwave signals and devices.

Signal To Noise Ratio (SNR)

The signal to noise ratio (SNR) is merely a comparison of the maximum (sometimes the average) signal power to the maximum (sometimes the average) background noise of a system. The SNR of a system is often described in decibels, which allows for easy SNR calculation, as the SNR in dB is just the maximum signal strength minus the maximum noise strength. 

Dynamic Range

Similar to SNR, dynamic range is also a measure of the difference between a signal degrading factor and maximum good signal The difference with SNR is that the dynamic range is based on the difference between the maximum non-distorted signal and the minimum discernable signal. In many cases, the minimum discernable signal is the noise floor of the system, however, there are other signal degrading factors that may also be used to determine the minimum discernable signal, such as harmonics or spurs. The choice of what limits the dynamic range depends on application considerations.

Harmonic Distortion and Third-order Intermodulation Distortion

Due to nonlinearities of devices in an RF signal chain, harmonics are generated that are multiples of the input frequency. It is typical that with harmonic distortion, the signal energy drops off for the higher order harmonics and they are further from the fundamental frequency of the system, so often only the first few harmonics are considered or the harmonics within the bandwidth of the system.

When two or more tones are present in a nonlinear system they may also mix creating additional distortion that is a product of the sum and differences of those mixed tones and the sums and differences of their distortion products. A common way of describing the impact of this distortion is based on the third-order terms, or the third-order intermodulation distortion (IMD3) terms. The IMD3 products are the closest products to the fundamental signals and are typically the strongest distortion terms in the system bandwidth.

Spurs

Spurs are unwanted signals created by nonlinearities in mixers, frequency converters, power amplifiers, and other nonlinear components that appear in the bandwidth of interest (sometimes limited to the intermediate frequency band). Spurs include image signals from mixers and other distortion products created by the mixing of two or more signals. As spurs are typically predictable mathematically given the input frequencies, good design usually accounts for filtering or otherwise mitigating the spurious content within the bandwidth of interest. For test instruments and broadband systems eliminating all of the spurs is usually not feasible, and spurs act as limiting factors to the dynamic range of a system (i.e. spurious-free dynamic range).

OIP3/IIP3

The output third-order intercept point (OIP3) and input third-order intercept point (IIP3) are methods of measuring the linearity of a RF device. The third-order intercept (TOI) point is where the linear amplified signal at one input tone curve intersects with the third order nonlinear product when plotted on a graph of input power versus output power. As the third order nonlinear product (IM3) increases 3dB for every 1dB of power raised, the 1:1 linearly amplified signal eventually intersects with the IM3 curve on the graph. The input power at the TOI point is the IIP3 and the output power at the TOI is the OIP3. These are important figures for system design, as using these figures provides linear boundaries on system behavior based on nonlinear component response.