Originally posted Jul 22, 2016
An overview to complement your equations
After some recent conversations about noise figure measurements, I’ve been working to refresh my knowledge of what they mean and how they’re made. My goal was to get the essential concepts intuitively clear in my mind, in a way that would persist and therefore guide me as I looked at measurement issues that I’ll be writing about soon.
Maybe my summary will be helpful to you, too. As always, feel free to comment with any suggestions or corrections.
- Noise figure is a two-port measurement, defined as an input/output ratio of signal-to-noise measurements—so it’s a ratio of ratios. The input ratio may be explicitly measured or may be implied, such as assuming that it’s simply relative to the thermal noise of a perfect passive 50Ω.
- The input/output ratio is called noise factor, and when expressed in dB it’s called noise figure. Noise figure is easier to understand in the context of typical RF measurements, and therefore more common.
- It’s a measure of the extra noise contributed by a circuit, such as an amplifier, beyond that of an ideal element that would provide gain with no added noise. For example, an ideal amplifier with 10 dB of gain would have 10 dB more noise power at its output than its input, but would still have a perfect noise figure of 0 dB.
It’s important to understand that noise figure measurements must accurately account for circuit gain because it directly affects measured output noise and therefore noise figure. Gain errors translate directly to noise figure errors.
The Y factor method is the most common way to make these measurements. A switchable, calibrated noise source is connected to the DUT input and a noise figure analyzer or signal analyzer is connected to the output. An external preamp may be added to optimize analyzer signal/noise and improve the measurement.
The central element of the noise source is a diode, driven to an avalanche condition to produce a known quantity of noise power. The diode is not a very good 50Ω impedance, so it is often followed by an attenuator to improve impedance match with the presumed 50Ω DUT.
The noise figure meter or signal analyzer switches the noise source on and off and compares the results, deriving both DUT gain and noise figure versus frequency. It’s a convenient way to make the measurements needed for noise figure, and specifications are readily available for both the noise source and the analyzer.
However, the impedance match between the noise source and the DUT affects the power that is actually delivered to the DUT and therefore the gain calculated by measuring its output. The impedance match is generally very good at low frequencies and with an attenuator in the noise source output. This enables accurate estimates of measurement uncertainty.
Unfortunately, as you approach millimeter frequencies, impedances are less ideal, gains are lower, and noise source output declines. Noise figure measurements are more challenging, and uncertainty is harder to estimate. In at least one upcoming post, I’ll discuss these problems and some practical solutions and measurement choices.
Why go to all the trouble? Whether or not it has mass, noise is a critical factor in many applications. By making individual or incremental noise figure measurements, you can identify and quantify noise contributors in your designs. This is the knowledge that will help you minimize noise, and optimize the cost and performance tradeoffs that are an important part of the value you add as an RF engineer.