Two-tone testing
It consists of simultaneously injecting two sinusoidal signals of different frequencies (tones) into the component or system.
An electronic device can be tested by applying a single frequency to its input and measuring the response at its output.
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 signal generators, set to two different frequencies F1 and F2, are fed into a power combiner through circulators.
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.
[3] Passive components such as cables, connectors and antennas, are generally expected to be linear and therefore not liable to generate any intermodulation.
These effects include corrosion, surface oxidisation, dirtiness, and simple failure to fully make mechanical contact.
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.
A second directional coupler, connected in the conventional configuration, can be used to provide a feed of the input to a spectrum analyser.
For a set receiving audio, it can manifest itself as an interfering signal making the wanted station unintelligible.
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.
[13] 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.
However, the greater frequency difference between the wanted and unwanted signal makes out-of-band intermodulation products relatively easy to remove with filters.
The idea is to spread the tones over the bandwidth of the real signal with a similar frequency power density.
