In the Test & Measurement field a frequency extender is defined as an electronic device that allows for extending the frequency coverage of the equipment that it is intended to work with. A frequency extender performs frequency conversion, either by multiplication or by heterodyne mixing, or by a combination of both.
Frequency multiplication is realised by a device in which its output frequency is the harmonic (multiple) of the input frequency i.e. FOUT = N x FIN. A frequency multiplier typically uses nonlinear semiconductor circuits to generate harmonics of the input signal. Typically, we see such devices with multiplier factors of 2, 3, 4 and 5, however 6 and beyond are also possible in a multistage or cascaded circuit configurations.
Heterodyne mixing is achieved in a device called a mixer, which operates on a principle of mixing one of the two signals, either radio frequency (RF) or intermediate frequency (IF), with a local oscillator (LO) frequency to produce new frequencies. In its most common application, the mixer produces a sum and a difference of the two signals applied to its inputs i.e. IF = RF ± LO for a mixer operating as a down-converter, or RF = LO ± IF in case of an up-converter. Other more complex scenarios, in which mixing of signals’ harmonics is considered, are also possible.
In principle, a relatively simple device such as a mixer or a multiplier, can be considered a frequency extender. In practice however, extending the range of a modern electronic equipment requires more complex solutions. In such cases, frequency extension is achieved by a combination of both frequency multiplication and heterodyne mixing, and aims to extend the frequency coverage of the equipment without deteriorating its other key parameters – for example output power, stability and/or dynamic range.
There are various types of test & measurement equipment for which frequency extenders are designed and manufactured, with vector network analysers, spectrum & signal analysers, noise figure analysers, signal generators to name just a few. Such frequency extension systems are used in applications that require high frequency operation such as radar testing, antenna measurements, on-wafer measurements, wireless communication and material characterisation.
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