Noise-figure meter

Key takeaways

Noise-figure meter belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Noise-figure meter to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Noise-figure meter from memory before moving on to harder problems.

Reference excerpt

A noise-figure meter is an instrument for measuring the noise figure of an amplifier, mixer, or similar device. An example instrument is the 1983-era Agilent 8970A. The 8970A Noise Figure Meter is a Keysight product numbers that were formerly part of Agilent.

Measurement methods One way to perform the measurement is described on the Y-factor page. A noise-figure meter could automate that procedure as follows: A gated broadband noise source (such as an avalanche diode) drives the device under test. A measurement is made with the noise source on; another measurement with the noise source off. From those measurements and the characteristics of the noise source, the noise figure can be calculated.

Noise source Some noise figure meters need a calibrated broadband noise source—a noise generator. Several methods are used to generate broadband noise. Some methods require two sources: a "hot" and "cold" source. For high frequency measurements, the noise source will be embedded in a transmission line.

Thermal noise Noise (electronics)#Thermal noise Thermal noise in a resistor. Resistor in liquid nitrogen. Resistor in boiling water.

Shot noise Noise (electronics)#Shot noise Electrons crossing a gap make discrete arrivals. Impulse. White noise. Compare to thermal electrons. Motchenbacher & Fitchen (1973, p. 292) describe using a forward biased diode as a calibrated noise source. They also describe a generator made from a low-noise amplifier with a shorted input. Its noise voltage is determined by the shot noise of the amplifier's input transistor.

Vacuum tube

Random noise generators can be made from temperature-limited vacuum tube diodes. (Motchenbacher & Fitchen 1973, pp. 289–291) The vacuum tube's anode (plate) is high enough to collect all the electrons emitted from the hot cathode. The operating conditions are set to avoid a space charge around the filament/cathode that would affect the electron emission. The anode current exhibits shot noise. The noise current is set by the filament temperature. The current is an exponential function of filament temperature. At low frequencies, there is 1/f noise. At high frequencies, the transit time of the electron becomes an issue. Ott (1976, pp. 218–219) describes using a noise diode to measure noise factor.

Zener and avalanche diodes Voltage breakdown diodes are often used as noise generators. (Motchenbacher & Fitchen 1973, pp. 180–182) There are two breakdown mechanisms: Zener and avalanche. Diodes with the corresponding effects are known as Zener diodes and avalanche diodes. The two mechanisms have different noise behaviors. The Zener effect (or internal field emission effect) dominates below 7 volts. The junction is thin, and the electric field is large enough that electrons jump the energy gap. The primary noise is shot noise. There is little other noise (excess noise). Avalanche breakdown is noisier. A carrier traversing the semiconductor junction is accelerated by the reverse-bias field, and it can generate new electron-hole pairs in a collision. Those new carriers can also generate more carriers in a subsequent collisions. The carriers don't arrive singly but rather in bunches. The result is avalanche multiplication of what would have been just shot noise. The spectrum, like shot noise, is white. Avalanche breakdown can also exhibit multi-state noise. The generated output noise appears to switch between two or more distinct levels. This noise has a 1/f characteristic. The effect can be minimized. Motchenbacher & Fitchen (1973, pp. 291–292) describe a noise source using a Zener diode (also suitable for an avalanche diode). Some commercial microwave noise generators use avalanche diodes to create a large excess noise figure that can be turned off and on. The impedance of the diode is different during the two states, so an output attenuator is used. The attenuator reduces the noise source output, but it minimizes mismatch loss. (Swain & Cox 1983, p. 26).

See also Noise (electronics) Noise figure Radio noise source

Notes

References Keysight (June 2009), Keysight 346A/B/C Noise Source (PDF) Hewlett-Packard (May 1985), HP 8970A Noise Figure Meter: Operating & Service Manual (PDF) (29MB download) Motchenbacher, C. D.; Fitchen, F. C. (1973), Low-Noise Electronic Design, New York: John Wiley & Sons, ISBN 0-471-61950-7 Ott, Henry W. (1976), Noise Reduction Techniques in Electronic Systems, New York: John Wiley & Sons, ISBN 0-471-65726-3 Swain, Howard L.; Cox, Rick M. (April 1983), "Noise Figure Meter Sets Records for Accuracy, Repeatability, and Convenience" (PDF), HP Journal, 34 (4): 23–34 Also

HP Application notes Newer generation manuals Ailtech fixed IF

Worked examples

Example 1 — a first encounter with Noise-figure meter

Start with the simplest possible case. Write down what Noise-figure meter claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Noise-figure meter before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Noise-figure meter ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Noise-figure meter

In research: Noise-figure meter appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Noise-figure meter in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Noise-figure meter is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic test equipment, so understanding it makes those chapters shorter.
In everyday life: Look for Noise-figure meter outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Noise-figure meter in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Noise-figure meter means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Noise-figure meter in simple terms?

A noise-figure meter is an instrument for measuring the noise figure of an amplifier, mixer, or similar device. An example instrument is the 1983-era Agilent 8970A.

Why does Noise-figure meter matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Noise-figure meter?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Noise-figure meter.