Why do we use Frequency Extenders?

Frequency extenders are used when one wants to perform measurements that go beyond the standard range of the test & measurement equipment that is available. This is often the case with pre-existing as well as newly purchased equipment. There are three fundamental reasons for which frequency extenders are considered the best solution for expanding the measurement capability of any test equipment.

1. Technological constraints

Due to technological constraints, covering the required frequency range cannot be often achieved using an all-integrated test equipment that is fully equipped with all the hardware required to perform such measurements. This is particularly the case for testing in high millimeter wave and sub-THz frequencies of 70 GHz and beyond. Implementing such system in an integrated form is mainly constrained by the electrical and mechanical limitations of the available interconnects and the transmission media. Coaxial interface has been the transmission line of choice for decades when it comes to the test & measurement equipment with various types of connectors and cables employed to achieve broadband coverage. Although the highest frequency for which coaxial connectors can be currently made is 220 GHz (0.6 mm connector), the machining limitations and mechanical sturdiness that is required to form a reliable and repeatable connection make such an interface rather impractical [1].

For that reason, oftentimes rectangular waveguide is a preferred choice of a transmission line, especially for frequencies of 50 GHz and beyond, and becomes a dominating interface for high millimeter, sub-THz and THz bands in 100 GHz to 1 THz range. The usable coverage of a rectangular waveguide transmission line is defined between the fundamental and the higher order mode cut-off frequencies to ensure that only the desired TE₁₀ mode propagates. Therefore, covering a very wide band requires using a number of different waveguide bands to achieve the desired frequency coverage. For example, while a single 0.6 mm coaxial interface can cover a frequency range from DC to 220 GHz, covering 40 to 220 GHz with a rectangular waveguide requires using at the very least four waveguide bands: WR-19, WR-12, WR-08 and WR-05. The standardized table of waveguide band designations can be found here.

With mechanical (poor robustness) and electrical (high losses) limitations of high frequency coaxial connectors the rectangular waveguide becomes the preferred interface for high frequency test & measurement systems. However, due to a lack of a suitable broadband alternative, integrating waveguide electronics and interconnects that are bandwidth restricted into the test equipment does not make practical sense. Instead, it is much more practical to add this functionality externally by means of dedicated ‘banded’ frequency extenders that cover the frequency range of interest. 

It must also be noted that especially when it comes to performing measurements at a very high frequency, there is no other alternative but to use frequency extension systems.

Technological constraints impact significantly on cost of upgrade and flexibility of test equipment, which are shortly discussed below.

2. Cost of changing equipment.

The cost of upgrade may be difficult to justify, as the high frequency test & measurement equipment becomes prohibitively expensive. For example, replacing a 26.5 GHz analyser with a 67 GHz unit can be several times more expensive than extending its range with frequency extension systems. Also, for cases when only a specific range of frequencies is of interest, a dedicated frequency extender covering the required range is a much more cost-effective solution compared to a broadband analyser. One may wish to only extend the range to a band of interest as required at the time, and extended into other frequency bands in the future when such need arises.

3. Flexibility

Flexibility is another reason why using frequency extenders may be a preferred option for expanding the capabilities of one’s test & measurement equipment. Frequency extenders can be easily moved and shared between different test equipment so that the assets are used the most effectively. 

Additionally, frequency extenders offer placement flexibility as they can be positioned very close to the test subjects - that being integrated circuits in an on-wafer scenario, or antennas tested in anechoic chambers with long distances away from the test equipment. In such cases, extenders allow for a close placement at the plane of a measurement and therefore maximizing of the performance of such a test system. 

Conclusion

In conclusion, we use frequency extenders because we have to perform measurements at frequencies that are not catered for by standard test & measurement equipment, or the cost of such equipment, if available, is prohibitively high.

References:

[1] T. Roberts, J. Martens, “The Flanged Coaxial Connector System: Enabling DC – 220 GHz Connections”, Anritsu Technical Bulletin, No. 95, Mar. 2020.