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Two-tone testing

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A spectrum analyser – typically used as the measuring instrument in two-tone testing

Two-tone testing is a means of testing electronic components and systems, particularly radio systems, for intermodulation distortion. It consists of simultaneously injecting two sinusoidal signals of different frequencies (tones) into the component or system.

Rationale

An electronic device can be tested by applying a single frequency to its input and measuring the response at its output. If there is any non-linearity in the device, this will cause harmonic distortion at the output. This kind of distortion consists of whole-number multiples of the applied signal frequency, as well as the original frequency being present at the device output. Intermodulation distortion can produce outputs at other frequencies. The new frequencies created by intermodulation are the sum and difference of the injected frequencies and the harmonics of these. Intermodulation effects cannot be detected with single-tone testing, but they may be just as, or more undesirable than harmonic distortion depending on their frequency and level.

Two-tone testing can also be used to determine the discrimination of a radio receiver. That is, the ability of the receiver to distinguish between transmissions close in frequency.[1]

Testing

Component testing

Receiver testing

Two-tone receiver testing using direct injection
Two-tone receiver testing using off-air method

A test setup suitable for testing receivers at microwave frequencies is shown in the figure. The two signal generators, F1 and F2, are combined using a directional coupler in reverse. That is, the two generators are connected to what would normally be the coupled and transmitted output ports respectively. The combined signal appears at what would normally be the input port. The advantage of using a directional coupler rather than a simple summing circuit is that the directional coupler provides isolation between the two generators. Another signal being injected into the output of a signal generator can cause intermodulation distortion within the generator, leading to a false measurement of distortion in the device under test. Practical directional couplers do not provide perfect isolation. For this reason, isolators are used at the output of the signal generators to provide additional isolation.[2]

The combined test signal can be injected directly in to the receiver if the antenna is removable. A second directional coupler, connected in the conventional configuration, can be used to provide a feed of the input to a spectrum analyser. This allows confirmation that the input signal is free of intermodulation products. If the test signal cannot be directly injected, for instance, because the receiver uses an active antenna, then the test signal is transmitted through its own transmitting antenna. A feed for a spectrum analyser can be provided by connecting a receiving antenna to its input. Tests done by the latter method are normally performed in an anechoic chamber to avoid broadcasting the test signal to the world at large.[3]

The consequences of intermodulation distortion depend on the nature and purpose of the receiver. For a set receiving audio, it can manifest itself as an interfering signal making the wanted station unintelligible. In a radar receiver, it can manifest as a false detection of a target.[4]

Transmitter testing

For transmitters that are designed for the transmission of speech or music, two frequencies within the audio band can be injected into the normal input of the transmitter. The output of the transmitter can be examined with a spectrum analyzer to look for intermodulation products. This kind of end-to-end testing tests all parts of the transmitter for non-linearity: from the audio stage, through the mixing and IF amplifier, to the final RF power amplifier. Likewise, a transmitter used for passing data can be injected with two frequencies within the baseband of the data stream. In some cases, there is no accessible input to a transmitter. Radar transmitters, for instance, do not take an input; the circuitry generating the radar signal is internal to the transmitter. In such cases the tones must be injected at some internal point of the device, or else the amplifiers and other stages must be tested as separate components.[5] A dummy load may be connected to the output of the transmitter to prevent it actually broadcasting, and a directional coupler, possibly together with an attenuator, used to provide a feed to the spectrum analyser.[6]

References

  1. ^ Avionics Dept., p. 5-7.7
  2. ^ Avionics Dept., p. 5-7.1
  3. ^ Avionics Dept., pp. 5-7.1–5-7.2
  4. ^ Avionics Dept., p. 5-7.9
  5. ^ Ghannouchi et al., pp. 156–157
  6. ^ Carr, pp. 224–226

Bibliography

  • Avionics Department, Electronic Warfare and Radar Systems Engineering Handbook 4th edition, Naval Air Warefare Center Weapons Division, October 2013, NACWD Tech. Pub. 8347.
  • Carr, Joseph J., Practical Radio Frequency Test and Measurement, Newnes, 1999 ISBN 0750671610.
  • Rudersdorfer, Ralf, Behavioral Modeling and Predistortion of Wideband Wireless Transmitters, John Wiley & Sons, 2015 ISBN 1118406273.

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