Noise parameter extraction using source pull

Updated August 27, 2005

New for September 2005!

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Here is an index to the page you are on now:

Definitions of load pull versus source pull

Types of microwave tuners (separate page)

Why source-pull a low noise device?

Noise parameters: Gamma Opt, Fmin and RN

Source pull bench block diagram (coming soon)

Definitions of load pull versus source pull

Microwaves101 convention: When we're talking about the process of load pull or source pull as nouns (or adjectives like "load pull bench") we won't hyphenate. When we're telling you to load-pull or source-pull something (verbs), we'll hyphenate. Just so you know.

Load pull: the process of varying the impedance seen by the output of an active device to other than 50 ohms in order to measure performance parameters, in the simplest case, gain. In the case of a power device, a load pull power bench is used to evaluate large signal parameters such as compression characteristics, saturated power, efficiency and linearity as the output load is varied across the Smith chart.

Harmonic load pull: the process of varying the impedance at the output of a device, with separate control of the impedances at F0, 2F0, 3F0, etc. A very tricky measurement!

Source pull: the process of varying the impedance seen by the input of an active device to other than 50 ohms in order to measure performance parameters. In the case of a low noise device, source pull is used in a noise parameter extraction setup to evaluate how signal-to-noise ratio (noise figure) varies with source impedance.

In practice, power benches employ both load and source pull, and noise parameter setups employ both as well, but the emphasis is on one or the other. In power measurements, the input is source-pulled to a single location that provides good power gain, while the output is swept all over the Smith chart. In noise parameter extraction, the output is load-pulled to an impedance that provides good gain, then the input is swept all over the Smith chart.

Microwave impedance tuners are the "engines" that let you drive all over the Smith Chart.

Why source-pull a low noise device?

The performance of a low noise device is a function of many things:

• Frequency
• Bias point
• Temperature
• Source impedance
• Device geometry (number gates, gate width, etc.)

When you measure noise figure on a noise figure meter or noise parameter analyzer, you typically evaluate signal-to-noise ratio (noise figure) while restricted to a fifty-ohm system. At the device level, your low noise transistor is not going to give you the best response at fifty ohms. Measuring noise parameters using source pull is what is used to empirically gather all of the data you need to design a low noise amplifier, starting from the device level.

Noise parameters

It turns out that signal-to-noise ratio of a device is one of those "magic parameters" that are exactly described by circles of constant value on the Smith chart (much like small-signal gain). Contrast this with power amplifier responses such as contours of constant maximum efficiency, which are more generally described as "potatoes".

Once you have the noise parameters in hand using linear CAD software you can mathematically calculate the signal-to-noise response if the device sees impedances other than fifty ohms. Noise parameters are can be very accurate, if they are measured carefully.

There are three things that you are after when you measure noise parameters (four if you consider that gamma opt is a vector and has a real and imaginary part). Remember, the three noise parameters are functions of bias point, frequency and temperature, and of course, device geometry.

Gamma Opt
Gamma opt is the optimum impedance that must be presented to a low noise device to achieve its lowest possible noise figure.

Fmin
Fmin is the minimum noise figure of a device, and it is only achieved at gamma opt.

RN (equivalent noise resistance)
RN gives an indication of how fast the noise figure of a device degrades when it's input is matched to other impedances beside gamma opt. Lower RN is always desirable, it is a measure of the slope of noise figure versus impedance.

More to come!