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Eye on Vector Network Analyzers

Mon, 12/14/2009 - 11:47am
By Harry Momjian, Anritsu Company

Eye on Vector Network Analyzers

Figure 1: The Anritsu VectorStar™ is an example of VNAs that combine wide frequency range, fast measurement speed, and high overall measurement performance.
Vector Network Analyzers (VNAs) are very powerful and flexible measuring instruments. Their basic capability is to measure the S-parameters of an RF or microwave device and display the result in the frequency domain. Today's VNAs perform many more measurements, such as those typically associated with spectrum analyzers, power meters, and even high-speed oscilloscopes. This provides valuable data for design engineers to develop a design and/or substantiate the performance of the device or system.

Engineers need to consider a few factors when selecting the proper VNA (figure 1) for their application. Common product-related questions that need to be answered include frequency range, measurement performance, measurement speed, and measurement medium. Other non-product attributes, such as support, reliability, and cost-of-ownership, are also extremely important.
Frequency Range
Determining the proper frequency range is contingent upon the device and the application. Historically, VNAs were grouped into two main categories – RF VNAs, which typically went up to 6 GHz, and microwave/millimeter wave VNAs that covered up to 100+ GHz. Today, even RF customers are interested in a 20 GHz microwave VNA, since they need a minimum of 13.5 GHz to cover the 5th harmonic of the cellular frequency bands.

Another reason microwave VNAs have become more prominent is because the price of the instruments has come down. The result is a strong price/performance scenario in which it no longer makes sense to purchase a VNA with a frequency range under 20 GHz, unless cost is the primary factor or the application is more for the field and maintenance.

Minimum frequency coverage of a VNA is also important. Lower frequency coverage allows for a wider span, which is necessary when making time domain measurements, which will be discussed later. Also, some VNA architectures provide 10 MHz coverage but deliver such poor performance at the low end that measurements taken below 1 GHz are not reliable.

Related to frequency coverage is the device under test (DUT) connectors, such as K (2.92 mm) up to 40 GHz, and V (1.85 mm) up to 70 GHz. The VNA must have total solutions, including calibration and verification accessories, to support the connectors over the covered frequencies as well.
Measurement Performance
Eye on Vector Network Analyzers
click to enlarge

Figure 2: The graph above shows two Time Domain Measurements of a 25-ohm stepped impedance line. One with a traditional microwave VNA, with couplers rolling-off below 1 GHz, showing 23 ohms mean and 0.5 ohms variation due to DC extrapolation error, and another with VectorStar's bridge-based VNA below 2.5 GHz, showing a more accurate impedance of 25 ohms mean and 0.17 ohms varaiation.
When considering the performance of a VNA, the most important specification is dynamic range. Synonymous to signal-to-noise ratio, dynamic range affects the sensitivity and accuracy of the measurement. High dynamic range is particularly important when making low-level measurements because the lower noise floor gives engineers space to make these measurements with higher accuracy.

The necessary dynamic range is contingent upon the DUT. If passive devices such as cables are being measured, a dynamic range of 60 dB is sufficient in most cases. Handheld 20 GHz VNAs provide such performance for field applications. However, filters with high reject bands or high gain amplifiers need the widest dynamic range available in the premium benchtop VNAs, such as in excess of 100 dB at 70 GHz.

Active device measurements also require non-linear measurements. A wide ALC range with high maximum output power allows for characterization of the compression characteristics of most amplifier classes. A minimum of 35 dB at 70 GHz (wider at the lower frequencies) can easily be obtained to cover most applications. Higher power can be achieved by adding external amplifiers in front panel loops.

Understanding VNA receiver compression is also vital. As the high output power of an amplifier is being sent into the VNA, its test receiver should not be compressing or it will adversely affect measurement accuracy. It is also critical to understand the relationship between dynamic range and receiver compression. The dynamic range of the VNA is calculated as a ratio of the maximum output power to the noise floor. Engineers need to ensure that the maximum power used for dynamic range specifications does not exceed the 0.1 dB compression level of the receiver, otherwise the full dynamic range specified is not usable. Most manufacturers use 0.1 dB compression. Increasing that figure signifies less performance.
Measurement Speed
Measurement speed is essential for manufacturing throughput or for tuning purposes. The former may not need the highest performance, so economy models that emphasize speed over performance are suitable. Tuning needs speed for immediate feedback and wide dynamic range for applications such as tuning a filter. Simply lowering the IF bandwidth to realize wider dynamic range does not work. By providing the lowest noise floor at standard IF bandwidths, and by designing the fastest locking, acquisition, processing, and displaying architectures, today's premium VNAs can sweep at 20 usecs per point while synthesized, leveled, and displaying data for tuning purposes.
Time Domain as Typically Found in Oscilloscopes
Eye on Vector Network Analyzers
click to enlarge

Figure 2b
A VNA that operates in the frequency domain can supply time domain information using Inverse Fast Fourier Transform (FFT) to convert frequency domain data to the time domain. A key benefit of time domain is characterizing a device's impedance over distance or time.

The most important VNA characteristic for time domain measurements is the overall frequency span, because it dictates measurement resolution. Resolution is inversely proportional to the frequency span of the VNA, so a 70 GHz VNA can achieve a 2-mm resolution, while a 20 GHz VNA can achieve an 8-mm resolution. Notice that this is independent of the frequency band of the DUT, assuming its response allows some information to pass through over the entire VNA bandwidth.

The other mathematical limitation of time domain is that FFT is a circular function and repeats. The period called alias free range is equal to the inverse of the step size. The longer the device to be measured, the wider the required alias free range and the smaller the necessary step size. Since the widest span for resolution and the smallest step size for alias free range are necessary, today's VNAs offer up to 100,000 points.

The most powerful time domain processing capability is the low pass method, which requires data starting at the lowest possible frequency. Since it needs to integrate the impulse response over time, the more low-frequency data, the better the accuracy. It also uses the DC data as phase reference, to provide capacitive or inductive impedance information. Though traditional 10 MHz start frequency VNAs provide enough low-end data, it is useless because the VNA performance below about 1 GHz is very noisy and unstable. Today's microwave VNAs can provide a 70 kHz start frequency, meaning 13 more octaves of usable data for higher time domain performance than the old 10 MHz products that use broadband couplers. 70 GHz couplers cannot physically be stretched to operate below 1 GHz, thus roll-off heavily down to 10 MHz, degrading dynamic range and increasing noise and uncertainty. An example of the benefit of time domain can be seen in Figure 2.
Conclusion
Analyzing current and future applications will dictate which VNA should be used. Since these instruments are significant capital equipment expenses that must pass multiple layers of approval, it is imperative to understand the application's technical requirements and the appropriate VNA's differentiating capabilities that enable measurements. Using these criteria, you will be able to select the best VNA for your application.

Harry Momjian is project manager for Anritsu Company, www.us.anritsu.com; 408-778-2000.

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