Phase noise and the Y-factor noise figure

-December 19, 2013

Noise figure is a critical performance parameter that defines radio receiver component performance. Accurate measurements of noise figure let you select components and optimize receive chain tradeoffs. The phase noise in a signal source used for measuring noise figure will influence the final measurement.

You can measure noise figure using a technique called the Y-factor method, which requires the use of a signal source whose phase noise is a factor in the measurement. The signal source can be a signal generator or a frequency synthesizer.

Figure 1 shows a typical receive chain. Noise figure measurements can help you system decisions on which components to use in your system. Decisions include selecting ADC (analog-to-digital converter) resolution, gain distribution, and antenna selection, all of which influence the detectable signal strength of a receive chain.

Figure 1: Receive chain block diagram.

Noise figure defined
Noise figure quantifies the amount of noise added to a signal chain by an individual component or a cascaded set of components. Specifically, noise figure is the SNR (signal-to-noise ratio) at the output of a component or block compared to the SNR at the input of the component or block.

Unfortunately, noise figure can be a difficult measurement to accurately make on modern components. Contributed noise levels often are so low that they approach the thermal noise. Plus, the measurement equipment adds as much noise as the device itself, and must be compensated out in a two-step process. Measurements of noise figure can lead to very poorly-conditioned calculations and significant instrumentation challenges.

Noise figure of a receive mixer device presents a special problem. Noise measurements at the mixer's output includes contributions from the mixer device itself, the LO (local oscillator), and the RF input. Figure 2 illustrates the LO mixing with the noise of the mixer device. At the IF frequency where the measurement is made, a filter removes the main peak from the LO, but LO phase noise at the carrier frequency adds to the device noise that we are trying to characterize.

Figure 2: Noise components mixing to measurement frequency.

To isolate the mixer noise itself, it is common to generate the LO using a high-quality lab signal generator or a tone passed through a narrowband filter to reduce phase noise spreading. At the same time, radio circuitry continues to evolve towards higher levels of integration and thus, the industry is starting to introduce wireless infrastructure class mixers with integrated synthesizers.

Phase noise from a synthesizer integrated with the mixer is high compared to well-controlled external oscillators. Since the synthesized signal is completely contained inside the device, it cannot be filtered to clean up imperfections. This higher phase noise from an integrated, unfiltered synthesizer perturbs noise figure measurements.

Now, let's evaluate noise figure measurements of a receive mixer, comparing the results found using a filtered signal generator for an LO source against results found using a high-performance synthesizer LO source. The synthesizer used to generate the LO is an LMX2581, a high-performance synthesizer with integrated VCO. The synthesizer's in-band phase noise is higher than the signal generator, but its phase noise floor is 6dB lower. Although the synthesizer is not integrated with the mixer, it represents the level of performance that could be expected in an integrated configuration.

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