VNA Reflections

Ensuring Large Vehicle Comms System Performance Using VNAs with NLTL Technology

February 14, 2022

Large vehicles, such as aircrafts, ships, and trains, rely on advanced wireless technology for communications, systems control, and sensor interconnects. Successful operation of these systems on large commercial and military vehicles requires interference mitigation. Often, vector network analyzers (VNAs) are used to ensure signal transmission. In today’s post, we explain how VNAs can ensure systems on large vehicles operate according to specification.

Interference can be caused by numerous devices. Laptops, electronic games, and mobile devices are common examples. There are also nefarious acts designed to intentionally create interference. Autopilot disconnects, erratic indicators, and vehicles veering off course are potential consequences that may be caused by interference.

Testing Cabling on Large Vehicles

RF/µW cabling is used throughout modern vehicles. Checking cable phase and magnitude performance over frequency becomes more important – and challenging. This is especially true as systems used in larger vehicles are increasingly sophisticated and complex, and connections between different parts of the vehicle are often well over several meters in length.

Once installed, fully characterizing long RF/µW cables with vector S-parameter measurements creates a connection challenge between a traditional VNA and the widely spaced cable ends. The interconnect cables required to attach the VNA ports to the cable under test adds significant insertion loss and phase instability to the measurements over the distances involved.  Cable replacement due to wear-and-tear adds considerable expense to the test setup, as well.

Figure 1 shows how to conduct cable measurements on ships and aircraft using the Anritsu ShockLine™ ME7868A VNA. As depicted, the portable port modules connect directly to the cable under test.

Figure 1: Cable measurements on airplane and ship using the ShockLine ME7868A VNA.

Four advantages are realized using the ME7868A VNA-based solution:

1. Eliminates coax cable insertion loss and phase instability
2. Complete vector S-parameter characterization
3. Full wide band frequency sweep coverage
4. Easier and faster setups for more efficient testing

The configuration can be used to conduct the following tests on cabling used in large vehicles:

• RF/µW vector S-parameter characterization
• Test phase matched cabling
• Group delay cable testing
• Insertion loss measurements
• Cable fault detection

Eliminating EMI

With so much cabling and wireless equipment now aboard large vehicles, electromagnetic interference (EMI) is a critical factor that must be considered. It can cause equipment performance to degrade or even malfunction. EMI can affect radios and radar signals, interfering with communication between the aircraft or ship and the control tower and ports, respectively.

As a result, EMI effects must be considered in all aspects of large vehicle design and certification. Standards such as RTCA DO-160 for environmental conditions and test procedures for airborne equipment or MIL-STD-461, developed by the Department of Defense (DoD) to control EMI issues, are often used as the basis of evaluation.

One major method to combat EMI is to provide shielding of various line replaceable units (LRUs) and harnesses. Shielding a device or system not only reduces EMI emissions, it improves susceptibility performance. With advances in wireless technology and increased device signal sensitivity, shielding becomes even more important to maintain the functionality and safety of equipment.

OTA Solution Brings Benefits

Engineers often use VNAs to test RF/µW shielding and propagation on large vehicles. Typically, the configuration requires very long test port cables to reach around the vehicles. At the distances required for these tests, coax cables introduce insertion loss and phase instability so significant that different transmission media (such as optical cabling) must be used to enable S-parameter measurements. The additional hardware and setup complexity significantly raise the cost and time required to make the propagation and shielding measurements.

To alleviate additional expense and improve test times, the ME7868A VNA (figure 2) can be used to conduct OTA measurements on large vehicles, including aircraft and ships.

Figure 2: The Anritsu ShockLine ME7868A 2-port VNA.

The ME7868A is configured with portable port modules that eliminate the need for long test port cables. It supports typical large vehicle tests, including:

• RF/µW shielding characterization
• RF/µW propagation studies
• Vehicle antenna pattern measurement

Figure 3 shows the difference between a traditional complex configuration and the more efficient, streamlined ME7868A OTA set-up to conduct measurements on aircraft.

Figure 3: Comparison of testing aircraft using traditional VNA vs. ME7868A setups.

The ME7868A solution provides four key benefits in these OTA environments:

1. Eliminates coax cable insertion loss and phase instability
2. No different transmission media (E/O, O/E) required; additional costly hardware to overcome coax cable issues unnecessary
3. Full wide band sweep coverage
4. Easier and faster setups for more efficient testing

Unique VNA Architecture

The Anritsu ShockLine ME7868A has a unique design that makes it well suited for conducting measurements on systems used on large vehicles. As outlined in the examples described, it is a chassis-less 2-port VNA with portable port modules that supports a frequency range from 1 MHz to 43.5 GHz.

It can conduct tests with port modules spaced up to 100 meters apart. The ability of the VNA ports to be connected directly to each end of long cables in large vehicles optimizes dynamic range and improves phase stability in vector S-parameter testing. VNA ports can be physically placed close to large DUTs or at widely placed test antennas without the need for additional complex test hardware.

Many of the advantages are realized due to the patented nonlinear transmission line (NLTL) technology used in all ShockLine VNAs. It delivers wider bandwidth and higher dynamic range than competing technologies, enabling better measurement accuracy and repeatability with longer intervals between calibrations. NLTL is also employed on Anritsu’s VNA-on-chip design, which has inherent temperature and time stability advantages. NLTL technology also enables cost-effective, high-frequency measurements.

To learn more about large vehicle testing using a VNA, download leaflets on large vehicle OTA tests and cable tests.

Ensuring High-accuracy VNA Measurements with Type-B Two Tier Calibration with Flex Standards

November 2, 2021

Calibration is critical for engineers to conduct precise S-parameter measurements using a vector network analyzer (VNA). While a VNA is a highly-linear receiver and has sufficient spectral purity in its sources to make accurate measurements, there are imperfections if calibrations are not performed.  As designs continue to extend into higher frequencies, such as 5G Frequency Range 2 (FR2) millimeter wave (mmWave), calibration takes on added importance.

Standard network extraction and de-embedding capabilities are common methods of calibration to characterize and remove test fixture measurement contributions. Such techniques are used for all VNAs, including the ShockLine™ MS46122B series of PC-controlled, compact VNAs that support 1 MHz up to 43.5 GHz.

All three models in the ShockLine MS46122B series are full-reversing, 2-port VNAs aimed at RF and microwave applications within manufacturing, engineering, and education organizations. To accurately conduct necessary S-parameter measurements on RF passive components, some of which are listed in table 1, proper calibration must be conducted.

Table 1: 5G RF passive components tested with the ShockLine MS46122B Compact USB VNA.

Type-B Two Tier Calibration

One calibration method engineers can employ to ensure measurement accuracy when using the ShockLine Compact USB VNA MS46122B is type-B, two tier calibration with flex standard network extraction. The method can de-embed a fixture when only partial standards are available. By doing so, the insertion loss will be close to 0 dB.

Type-B, two tier calibration with flex standards is particularly useful when the other end of the fixture is not accessible to standard calibration kits. Using this method, engineers can de-embed the fixture with one (either open or short), two (open and short), or three (open, short, and load) standards. The more standards applied, the more accurate the measurement.

The main purpose of type-B, two tier calibration with flex standards is to extract the .s2p file of the fixture with different or incomplete calibration standards. Algorithmically, it is similar to the Bauer-Penfield technique (full calibration standard: short, open, and load) but an incomplete calibration is performed at the fixture output plane. A general setup for Type-B with a fixture attached to a VNA is depicted in figure 1.

Figure 1: Type-B with a fixture attached to a VNA.

The fixture can be represented by:

• 2 x 2 scattering parameters array denoted as S₁₁, fixture (fixture outer plane reflection on port 1)
• S₂₁, fixture (transmission loss on the fixture from port 1 to port 2)
• S₁₂, fixture (transmission loss on the fixture from port 2 to port 1)
• S₂₂, fixture (fixture inner plane reflection after connecting to reflection standards).

Non-Touching Loop Rule

Engineers can also implement the non-touching loop rule when calibrating. It is a technique that can be applied on any signal flow model. Figure 2 is a network signal flow where the fixture is placed between the VNA and a reflection standard. The system has only one independent variable (a1).

Figure 2: Fixture network signal flow model.

The flow diagram contains “paths” and “loops.” A “path” (P) is defined as a series of lines follow in sequence and in the same direction, so no node is touched more than once. The value of the path is the product of all coefficients encountered in the path. There are also loops in the flow diagram.

Example Measurement

The impact of type-B, two tier calibration with flex standards can be seen in a sample measurement conducted using the ShockLine MS46122B. For the measurement, the Compact USB VNA had a starting frequency of 100 MHz and a stop frequency of 10 GHz. One hundred points were acquired. A full 2-port SOLT calibration using TOSLKF50A calibration standard was performed.

In the sample calibration, the short and open are TOSLKF50A calibration standards and its offset is 5.01 mm. The estimated delay for the fixture was set 0 ps and the software automatically measured the fixture delay. The ShockLine VNA GUI will extract a .s2p file using a network extraction tool.   

A sample of the fixture .s2p file is shown in figure 3. When using open and short standards, the inner plan, which is S₂₂, is assumed perfect match and also S₁₂ = S₂₁ (assume reciprocal). After de-embedding the fixture, S₁₂ and S₂₁ traces become approximately 0 dB (as depicted in figure 4). S₁₁ is a bit worse before de-embedding because the S₁₁ fixture is not a perfect match.

Figure 3: Fixture .s2p file plots.

Figure 4: S12 and S21 plots after the fixture has been de-embedded.

Using the non-touch loop method with the signal flow diagram, the scattering parameters of the fixture can be mathematically modeled. With Type-B, two tier calibration with open and short standard, the inner plane is assumed to be perfect.

Learn More

The application for the Type-B two tier calibration with flex standard is a useful tool when the fixture has incomplete calibration standard and the user is only interested in de-embedding the loss of the fixture and measure the device under test (DUT) for the insertion loss.

To learn more, download the application note on type-B two tier calibration with flex standard network extraction.

Effective VNA Calibration Methods for Next-generation R&D Designs

September 15, 2021

Research and development (R&D) teams are tasked with finding more efficient ways to move data as they work on the next great technologies, such as 6G and machine learning. One commonality with all these efforts is to move operating frequencies into the millimeter-wave (mmWave) bands.

Using higher frequencies to transfer increasingly large amounts of data can be complicated, especially when you consider bandwidth as a percentage of the carrier frequency. The higher a carrier frequency, the larger the bandwidth. Data rate is a function of bandwidth. In simplest terms, higher frequency leads to greater bandwidth and that leads to larger data rates.

Another reason for the move to mmWave is because there are more swaths of contiguous bandwidth available. Wider bandwidth modulation is supported in a single carrier, compared to overcrowded lower frequencies unable to accommodate the massive data throughput associated with emerging designs.

As device developments are engineered in the 100+ GHz space, challenges arise. Hardware is certainly one. Another is measurement setup, including calibration of vector network analyzers (VNAs), such as the VectorStar™ family.

What Engineers Need to Know About Calibration

VNAs are the default test equipment for active device testing. These instruments typically have generic accuracy for various S-parameter and power measurements, but also have the ability to calibrate the source and receiver based on the test requirements.

A VNA provides accurate results for most measurements without the engineer conducting calibration. There are many setups, however, that require higher accuracy of interpolation in frequency and power axes. Another reason for user calibration is to move the power measurement away from the test port and establish a power reference plane in a desired location.

Engineers use VNAs to measure active devices using familiar power measurements, such as those in figure 1. While this is common, especially for low frequency calibrations, challenges can present themselves when designing in the mmWave frequencies. Complete frequency coverage of the instrument, automatic level control trade-offs, calibration time, and voltage standing wave ratio (VSWR)/ accuracy across frequency can all be present when performing a broadband calibration up to 110 GHz.

Figure 1: Power measurement types

Frequency Coverage

For many users, nothing is more frustrating than test equipment that covers the frequency range of interest but is limited by components and/or equipment for calibrations and operation. Often, engineers must resort to separate low- and high-frequency power sensors to conduct a broadband calibration. Obviously, this is not ideal because it adds cost and time.  

Level Control Coverage

Another challenge is the lack of available calibrated automatic level control (ALC) to address power requirements. While an instrument may address the power requirements, the power sensor used for calibration sometimes becomes non-responsive during the calibration. This is generally due to the sensor’s inability to resolve power levels for extremely low signals or for signal bandwidth that may be too large to cover all the desired power levels.

Voltage VSWR and Accuracy

A common challenge for many RF systems and components is good VSWR over a very broadband frequency set, especially up to 110 GHz. The main issue is that VSWR deteriorates as the frequency climbs above 90 GHz in most well-designed RF systems.

Poor VSWR can have a direct impact on measurement accuracy. Fortunately, there are ways to improve accuracy for test systems, including power sensors, such as Anritsu’s Power Master, available.

Calibration Time

Time-to-market is critical in any industry but it is particularly true as 5G and 6G progress. Because power measurements are a requirement for amplifiers, especially for the receiving and transmitting sides of a transceiver system, measurements should be fast and accurate. Assuming engineers can get the full frequency and ALC levels corrected, calibration time can come into play. Considerable hours can be spent on calibration throughout a year, especially for custom calibrations with a multitude of points across a broadband frequency set.

Cost

As we all know, cost can increase steeply as designs extend in frequency for R&D and/or testing. Engineers are continuously challenged with finding an instrument that meets their performance needs and is in their budget. Many times, a product’s price tag is the sole contributor for a customer to consider a product.

Calibration Technologies

When conducting calibrations with power sensors, there are options. For example, thermal sensors and receiver-based Non-Linear Transmission Line (NLTL) sensors are available up to 110 GHz. There are some stark differences and trade-offs, however, between both technologies. 

There are a few approaches for thermal sensing. A Wheatstone bridge with a thermistor can be used to sense RF power. The RF energy is applied to the thermistor, a change in temperature is detected, and an alteration in resistance can quantify the power level.

Another approach is to utilize a Seebeck effect in which dissimilar materials are joined at one end and a potential difference is produced when the junction changes temperature. Like the former example, changes in the temperature can allow the characterization of power through the device. Figure 2 shows the Wheatstone and Seebeck technologies.

Figure 2: Wheatstone bridge with thermistor (Left) and Seebeck effect thermal circuit.

This technology is widely used and features the best accuracy for power sensors but there are trade-offs. One is slow settling times, which translate to an overall increase in calibration time. This may not be an issue for small frequency coverage but over large frequency bandwidths, such as 9 kHz to 110 GHz, time can be important. Long calibration time can also affect overall throughput in manufacturing.

Another limiting feature of this technology is level control coverage. Thermal sensors are limited to approximately -30 dBm, so they may not adequately address manufacturing or design concerns.

One important distinction is that thermal sensors measure integrated power. This approach is useful in some applications but falls short on designs that require an emphasis on fundamental power.

NLTL Architecture

NLTL is a pseudo-transmission line where voltage-dependent varactor diodes are in place where a shunt capacitor would exist on a standard transmission line. When voltage is applied across the NLTL, a shockwave is produced. Based on the applied voltage, the incident wave compresses the fall time on the shockwave, creating a train of very narrow gating pulses that feeds into a sampler and can be used to characterize power (figure 3). These pulses contain harmonics with nearly the same power as the fundamental frequency and can be multiplied to provide input power levels across a broad frequency range of 9 kHz to 110 GHz.

Figure 3: Shockwave creation from CW signal.

NLTL implementation also allows for level control coverage down to -90 dBm and up to +23 dBm. Thermal sensors are limited to -30 dBm to +10 dBm, typically.

Calibration time can be alleviated through NLTL because it doesn’t rely on physical changes. Physical changes can take several seconds per minute to settle and report in the sensor to create a good power characterization.

To learn more about the need of calibrations in mmWave designs and their importance, download an Anritsu application note.