Network Analysers Moving to Lasers for Increased Effectiveness

Saurabh Durgapal is working as technology journalist at EFY

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Benchtop network analysers have seen huge design changes in the last few years, mostly to ensure an integrated approach to measurements on active devices such as amplifiers and frequency converters. With the growing popularity of dual-source network analysers, measurements can be done with a single instrument. These are also much faster as the sources and receivers are on the network analysers. But more on that later; let us first look at the changes in network analysers over the past few years.

Requirement influencing design

Vector network analysers (VNAs) have proven themselves to be accurate, owing to their sophisticated test set design and the kind of applications these are used in. Changes in these systems are concentrated on developing innovative calibration algorithms, measurement applications and digital signal processing techniques. These techniques, for example, are applied once the signal is down-converted and digitised by an analogue-to-digital converter.

System design has changed to accommodate advanced measurements like built-in pulse generators and modulators that help characterise radio frequency or microwatt components in pulsed mode. There is a lot of development in new concepts in millimetre range for a range of applications, especially related to RADAR.

With two-source network analysers having a built-in combiner, these measurements can be done very easily. Built-in second source and combiner networks help with accurately measuring and calibrating inter-modulation distortion (IMD) for amplifiers and frequency converters.

Special low-noise receivers are being used for source-corrected accurate noise figure measurements for low-noise amplifiers and receivers. True-mode stimulus allows for fast and fully error-corrected mixed-mode S-parameter measurements.

Pulsed measurements.

Pulsed measurements for devices such as transmitter and receiver modules, and pulse amplifiers, and noise figure measurements on converters and amplifiers, are some of the recent features being focused on for the design of network analysers. Harmonic specifications of sources improve with the use of filters in more advanced network analysers to enable better mixed measurements and IMD measurements. Third-order IMD in amplifiers requires two sources and a spectrum analyser with traditional approach.

But it is not just the feature sets that are changing. Analysers are going through physical changes, too.

Aiming for a better handheld device.

Connectivity with the network analyser is simpler, and measurements over a wide frequency range can be made much faster. Vendors now aim for smaller, lightweight devices. “Smaller the size, easier the usage and lower the power losses,” is how one vendor succinctly puts it.

This has led to an increase in field testing equipment as well. Handheld VNAs cover up to 50GHz range, and integrate full two-port VNAs, spectrum analysers, vector voltmeters, power meters and channel scanners, among other measurement devices. Field testing equipment are being used by engineers, but these have a frequency limitation. Designing handheld equipment means reduction in size, which causes removal of fans, resulting in reduced frequency range.

Cabling with focus on quality.

Field equipment needs to be rugged and should not give in to wear and tear easily. Not only are the test equipment being ruggedised, but also test cables are being improved for better operation. Test cables for improved measurements are a necessity but are often forgotten. Of late, cables are being ruggedised and being made safe for operation at high frequencies up to 40GHz.

VNA test cables from Pasternack can withstand production testing for 50-ohm communication systems. Torsion-resistant connector heads are directly attached to the steel conduit cycle to ruggedise the design for up to 75,000 flexure cycles.

A word of caution for optoelectronics.

With the optoelectronics time-domain measurement method, scattering parameter measurements on planar waveguides up to 500GHz with 500MHz frequency spacing are easily achievable. In addition to high-frequency measurements, this could also be used to characterise high-frequency coaxial devices and to help realise a precise voltage pulse standard. You need to ensure that you remove the effects of the optical to electrical converter of the photo diode by de-embedding these. This is done after the traditional calibration at the network analyser reference plane to remove the effects of RF cables and connectors.

USB testing made easy.

Windows based operating systems have enabled development of applications for automated gain-compression measurements, mixer measurements, IMD measurements and measurements on balanced device architectures. Also, with the advent of USB based power sensors, network analysers can be used with USB sensors as power meters as well.
More recently, femtosecond lasers are making a major difference. Network analysers today offer a higher dynamic range with tuned measurements and excellent receiver linearity. This makes femtosecond lasers a viable option for use in network analysers.

Femtosecond lasers are here

For accurate characterisation of a high-frequency device, a directional coupler is used to separate forward and backward propagating signals. After research showed that a frequency-resolved scattering parameter could also be realised using laser based measurement techniques, the femtosecond laser began to be utilised. But, how?

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