By Tomasz Waliwander, Michael Crowley, Courage Mudzingwa (Farran Technology)
There has been a vast research effort and academic development in the past three decades in millimeter wave (mm-wave) technology. Such an effort has been corresponding steadily in the growth in customer demand for mm-wave components and systems which has in turn created a need for a cost-effective test and measurement solutions for high frequency applications. There is a large number of test instrumentation already available in the field as well as new developments coming on-stream to the engineers ranging from signal generators, spectrum analyzers to network and noise figure analyzers to choose from to fulfill test and evaluation duties. The choice of instrumentation as well as ways of extending their measurement capabilities will be discussed in this article.
Introduction
Millimeter waves are electromagnetic signals with frequencies ranging from 30 to 300 GHz that correspond to wavelengths of 10 to 1 mm in the free space. Such signals in natural atmosphere environment are susceptible to attenuation at different rates for different wavelengths (frequencies) which makes them very useful in specific applications. The atmospheric attenuation of mm-waves is caused by gases and constituents that naturally occur in the environment. The atmospheric attenuation characteristic from 10 GHz to 1 THz under various levels of humidity [1] (large H2O droplets) and fog (small H2O droplets) is shown in Figure 1 :
Figure 1. Atmospheric attenuation characteristic from 10 GHz to 1 THz.
Frequencies where the average absorption of mm-waves is the lowest are called low attenuation windows and occur mainly around 35, 77 and 94, 140, 220, 340, 410, 650 and 850 GHz. The regions with highest averaged absorption levels are called attenuation lines and can be seen around 22, 60, 118, 183, 320, 380, 450, 560, 750 GHz. It is the oxygen molecules that are responsible for high attenuation at 60, 118 and 560 and 750 GHz. The rest of the attenuation lines are caused mainly by water droplets of various diameter sizes as well as other chemical species (CO2, N2O, NO, SO2 and SH2) at submillimeter wavelengths.
Due to different properties of mm-waves at different frequencies and environmental conditions the applications of mm-waves vary largely from communications, imaging and security applications, radar, radiometry and atmospheric sensing. All the applications mentioned, at some stage of their development, need to employ a mm-wave measurement system for component and system level testing and evaluation.
1.1 Mm-wave Applications
The mm-wave applications correlate closely with how such signals propagate in the atmosphere. The frequencies for which atmospheric attenuation is low (44, 86, 94, 140 GHz) are particularly useful in communication system operating at long ranges such as: satellite communications, backhaul mm-wave radios and point to multi-point radio links. For short range communications the 60 GHz band provides enough range where only a local area, short distance transmission is required. Other application benefiting from low atmospheric attenuation would include automotive radars at 24, 77 and 94 GHz where a long-range transmission and reception is possible [2].
Imaging and security utilize a mixture of high and low attenuations bands for passive and active systems. These use 77, 94 and 183 GHz frequencies as these signals present good properties to penetrate many materials (i.e. clothing) and see through the fog and rain. For those materials that can not be penetrated the atmospheric environment provides a good thermal contrast from which images can be synthesized at post processing level (see Figure 2).
Figure 2. Typical human body image obtained with mm-wave imaging system.
Other applications include scientific research as well as radiometry and ground-based astronomy. These applications would mainly concentrate at 183 and 220 GHz as well as higher frequencies.
Rectangular waveguide due to its inherently low loss properties is the medium that is most frequently used in mm-wave applications. Under normal conditions the electromagnetic field propagates through the waveguide in transverse electrical dominant mode TE10 and has a cut off point below which it does not propagate in any form or mode.
Millimeter wave waveguide bands are shown in Table 1 and contain band designation, internal waveguide dimensions as well as cut off frequency.
1.2 Mm-wave Frequency Extensions
The mm-wave frequencies can be generated in general using two methods: up conversion by means of solid-state devices such as Schottky diodes, or down conversion using optical and quasi optical methods. It is the former method that is used predominantly in millimeter wave range and thus will be discussed here.
The mm-wave signals are created with either multiplying or mixing lower frequency signals (<30 GHz). This is achieved by using active (requiring DC biasing) MMIC based multipliers or mixers at the lower end of mm-wave band (<80 GHz typically) and devices built with Schottky diode devices for higher end of mm-wave spectrum. To reach frequencies beyond 110 GHz harmonic mixers (mixers utilizing a nth harmonic of the LO signal) or chain of multipliers (lower frequency modules driving the input of higher frequency ones – i.e. a doubler driving a tripler) are most commonly used. Due to conversion efficiency constraints in general those devices would be limited to doublers and triplers only.
Most commonly purchased and used test and measurements instruments operate below 20 GHz. Such operating range is adequate to fulfill the evaluation and test purposes in most cases. However, with the advent of mm-wave applications, more so than ever before, there is a need for measurement systems operating at frequencies above 20 GHz and very often beyond 110 GHz. Such systems in principle are thought to extend the range of standard instruments beyond their range by means of frequency multiplication or mixing. In general, there are 4 groups of test and measurement equipment commonly used in component and system evaluation by engineers. These include: signal generators, spectrum and signal analyzers, vector network analyzers and noise figure analyzers.
1.2.1 Signal Generators Frequency Extension Sources – FES
In most cases the mm-wave users already own a microwave signal generator that are capable of supplying frequencies up to 20 GHz. For scalar measurements and modulation, the frequency extension can be achieved by a frequency multiplication. The generator is used as a driver for a amplifier-multiplier chain where the input signal is firstly multiplied and then amplified by an active MMIC module and used internally to drive a stand alone passive multiplier – usually a doubler or a tripler. To reach the higher end of mm-wave spectrum a chain of passive multipliers might have to be used to achieve best output power and frequency coverage. Farran Technology offers a full range of Frequency Extension Sources (FES) for extending the coverage of microwave signal sources. These modules operate on principle of frequency multiplication and amplification to offer best performance on the market. In the Figure 3 a typical block diagram of such modules is shown:
Figure 3. Block diagrams of Frequency Extension Sources for signal generators.
Modern test laboratories require high stability signal sources that can be applied as local oscillators in mixer applications or as an RF source in antenna and receiver applications. Also, mm-wave on wafer device testing requires a repeatable, as well as compact, extension source that can be easily mounted on wafer probing station positioners close to a DUT reducing therefore unnecessary losses associated with long cables at high frequencies. Farran Technology’s (FTL) signal generator frequency extenders (FESs) provide unparalleled in industry performance that will fulfill the requirements needed at high frequency applications in terms of frequency coverage, output power, stability etc. The full specification of FES range is show in Table 2:
The FES series are small size high performance source extenders that provide mm-wave coverage for any signal generator that has the ability of supplying microwave signal from 9 to 20 GHz with at least +5 dBm of drive level. The range covers 40 to 325 GHz and provides high output power in full waveguide bands. These units are designed to maintain the stability and repeatability of the signal provided by the signal generator with as little distortions and harmonic content as possible. Their small sizes make them particularly useful in complex bench-top setups as well as wafer probe and antenna range applications.
The modules are designed to be used specifically with test equipment and cover full waveguide bands without a need for tuning or re-configuration. The signal can be either continuous wave or swept frequency and the modules are capable of reproducing pulse, phase and frequency modulation. For phase and frequency modulation the modulation index will be increased by multiplication factor of the modules, therefore to achieve the desired index modulation adjustments are normally made at the signal generator. The frequency band coverage with the relevant output power is shown in Figure 4:
Figure 4. Frequency coverage and relevant output power for FES modules.
An example of Farran Technology’s frequency extenders for signal generators is shown in Figure 5:
Figure 5. Frequency Extension Source.
The connectivity diagram of FES module and typical signal generator is shown in Figure 6:
Figure 6. Block diagram connectivity setup for signal generator with FES extension module.
The signal generator in most cases can be setup to take into account the multiplying factor of the source extenders. In such condition the actual mm-wave frequency will be then readily displayed on the screen of the instrument.
1.5 Conclusion
The rapid growth in millimeter wave technologies across communications, sensing, security, and scientific domains has created a parallel demand for accurate, flexible, and cost-effective test and measurement solutions. As standard instrumentation is often limited to frequencies below 20 GHz, the ability to extend their capabilities into the mm-wave domain is essential for engineers and system developers working at the forefront of innovation.
Frequency Extension Sources (FES), such as those developed by Farran Technology, provide a compelling solution for extending the frequency range of widely available microwave signal generators. Designed with high stability, repeatability, and compactness in mind, these modules enable efficient coverage across full waveguide bands from 40 to 500 GHz, supporting a wide range of scalar and modulated signal requirements. Their robust integration into bench-top, antenna range, and on-wafer measurement setups illustrates the critical role of modular, high-performance extensions in bridging the gap between traditional RF instruments and next-generation mm-wave applications.
Alongside other extension modules for analyzers and network analyzers, FES units exemplify how tailored frequency extenders can transform existing laboratory infrastructure into advanced mm-wave testing environments—empowering researchers, developers, and integrators to meet the challenges of tomorrow’s high-frequency systems with confidence.