What is a Frequency Extender?

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 realized by a device in which its output frequency is the harmonic (multiple) of the input frequency i.e. F_OUT = N x F_IN. 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.

Figure 1. A conceptual diagram of a multiplier

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. 

Figure 2. A conceptual diagram of a mixer

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 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 analyzers, spectrum & signal analyzers, noise figure analyzers, 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 characterization.

Figure 3. Antenna range measurement setup

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