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. Intermodulation distortion usually occurs in active components like amplifiers, but can also occur in some circumstances in passive items such as cable connectors, especially at high power. Measurement in two-tone testing is most commonly done by examining the output of the device under test (DUT) with a spectrum analyser with which intermodulation products can be directly observed. Sometimes this is not possible with complete systems and instead the consequences of intermodulation are observed. For instance, in a radar system the result of intermodulation might be the generation of false targets.

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.

Testing

Component testing Circuit components such as amplifiers can be tested using the two-tone method with a test setup like that shown in the figure. Two signal generators, set to two different frequencies F1 and F2, are fed into a power combiner through circulators. The combiner needs to have good isolation to prevent the signal from one generator being sent to the output of the other. If this happens, intermodulation can occur in the non-linear parts of the generator internal circuit. The resulting intermodulation products will give a false result to the test. The circulators are there to provide even more isolation between the generators and isolation between any signal that might get reflected back from the device under test (DUT) and the generator. The circulators have one port connected to a resistive load so that they act as isolators. Low-pass filters may also be provided at the generator outputs to remove any harmonic distortion. These harmonics could cause unexpected intermodulation products in the DUT, again giving misleading results. The output of the DUT is fed to a spectrum analyser where the results are observed, possibly via an attenuator to reduce the signal to a level the instrument can cope with.

Passive components Passive components such as cables, connectors and antennas, are generally expected to be linear and therefore not liable to generate any intermodulation. However, especially at high power, a number of effects can lead to non-linearity through formation of a metal–semiconductor junction at what is supposed to be a metal-metal junction. These effects include corrosion, surface oxidisation, dirtiness, and simple failure to fully make mechanical contact. Some passive materials are intrinsically non-linear. These include ferrites, ferrous metals, and carbon-fibre composites. Intermodulation distortion is a particularly difficult problem at the cellular base stations of mobile phone cellular networks. These have to deal with multiple transmissions at closely spaced frequencies and it is necessary to ensure that these do not interact with each other. A typical specification is that intermodulation products should not exceed −125 dBm in the presence of 40 dbm transmissions. This equates to a requirement for a signal to intermodulation ratio of 165 dB, an exceedingly stringent specification. To achieve this, materials and components must be chosen with great care and installation and maintenance done to a high standard. Likewise, two-tone testing of these components needs to be done with great care and precision since intermodulation products at these low levels can easily be generated within the test setup accidentally. There is an international standard, IEC 62037 "Passive RF and microwave devices, intermodulation level measurement", for measuring intermodulation distortion of passive components. Testing to the standard ensures that specifications from different manufacturers are done under the same conditions and can be compared with each other. Militaries will typically use their own standards for testing. For instance US procurement contracts may specify MIL-STD-461.

Receiver testing

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