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
.svg?lang=en)
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
References
Bibliography
Avionics Department, Electronic Warfare and Radar Systems Engineering Handbook 4th edition, Naval Air Warefare Center Weapons Division, October 2013, NACWD Tech. Pub. 8347.