Testing automotive RF systems, part 2

 

By Xiao Li

Sophisticated RF electronic components in automobiles have resulted in much safer, efficient and connected vehicles. They have also created new challenges for RF testing and verification. In this two-part series, we will discuss the ins and outs of testing common RF automotive applications: Tire Pressure Monitoring System (TPMS), Remote Keyless Entry (RKE), Passive Keyless Entry (PKE) and Remote Start (RS). In the first part we reviewed the block diagrams and took a look at the RF physical layers of these systems. In this second part we examine test procedures for transmitters, receivers and EMI pre-compliance.

RF applications for new vehicles need a thorough checkup via testing of their transmitter and receiver systems as well as their EMI signatures. What’s at stake is more than owners’ satisfaction with their vehicles’ RF applications. Automotive manufacturers have strict regulatory and sometimes internal standards to meet. Here is an overview of the testing procedures for transmitter, receiver and EMI fields.

The transmitter tests

Channel power/transmit power gives an indication of the total average (and other measures) of RF power in a given channel. It is an important factor to deciding on the transmit range of communication systems. However, the maximum regulatory transmit power level limits are regulated on a country-by-country basis. The actual maximum transmit power levels need to be measured to make sure they are less than the published regulatory limits.

Frequency offset is the difference between receive and transmit frequencies of a radio channel. Mismatch in carrier frequency can result in inter-carrier interference (ICI).

Occupied bandwidth is a measurement of the frequency band bandwidth that contains a specified percentage of the total power of the signal. For example, for non-specific short-range applications in Europe, the occupied bandwidth has to be less than 25 kHz from 433.05 to 434.79 MHz.

Modulation quality measures the degree of closeness to which the modulation follows the ideal modulation determined by the rms difference between the actual deviation and the expected deviation for the transmitted symbols. In FSK, frequency deviation is the most often used for that purpose.

Out of band emission is any radio frequency not deliberately created or transmitted, primarily in a device that normally creates other frequencies. A harmonic or other signal outside a transmitter's assigned channel would be considered a spurious emission. The local regulatory standards, such as FCC in the U.S., delineates the top permissible value of spurious emission power of a given unwanted emission domain.

SignalVu-PC is the RF and vector signal analysis software used with Tektronix Real-Time USB Spectrum Analyzers, RSA300/500/600 series. The general purpose digital modulation analysis provides modulation analysis of 23 modulation types.

The screen capture below shows SignalVu-PC software with a RSA306B spectrum analyzer to do transmitter testing on a wireless key fob. The DUT was sending out an FSK-modulated signal in the 433 MHz band, and SignalVu-PC provided six different sets of test results including occupied bandwidth, signal quality summary, frequency deviation, constellation diagram, channel power and ACPR, eye diagram, demodulated symbol table and spectrum analysis. More tests can be done as needed. 

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The receiver tests

Sensitivity is one of the most important measurements of the receiver’s capability in wireless communication systems. The ability of the radio receiver to pick up the required level of radio signals enables it to operate more effectively within its application.

Blocking measures the receiver's ability to receive a wanted signal at its assigned channel frequency in the presence of an unwanted interferer on frequencies other than those of the spurious response or the adjacent channels.

The TSG4100A Series RF Vector Signal Generator offers mid-range performance at an entry-level RF signal generator price. Soft key upgrade is available for vector/digital modulation at very low cost. The most common modulation used in TPMS and RKE/PKE systems, ASK and FSK, are supported by TSG4100A.

The image below shows the TSG4100A as used with a RSA600 as a complete solution for the transceiver tests of TPMS and RKE/PKE systems.

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EMI pre-compliance test

Electromagnetic interference (EMI) regulations are in place throughout the world to provide improved reliability and safety for users of electrical and electronic equipment. The automotive industry and individual vehicle manufacturers also need to meet a variety of EMI requirements.

CISPR 25 is an international standard that applies to any electronic/electrical component intended for use in vehicles, trailers and devices. In addition to CISPR 25, most manufacturers have their own set of standards to augment the CISPR 25 guidelines. For example, Ford Motor Company has FMC1278 as their own EMI specifications. The emission requirement of RKE and TPMS systems are specified RE 310 Level 1 and 2 requirements.

Much time and effort go into the design of products to minimize their EMI signatures. Most engineers employ good design practices to minimize the potential for EMI problems. It is common today to perform pre-compliance measurements during the design and prototyping stages to find and fix EMI issues before the product is sent out for compliance testing. This reduces the risk that the product will fail the final full compliance testing and helps minimize time and expense at the compliance test house.

Performing pre-compliance testing can help you catch out-of-specification conditions before formal testing. If you have already been to a test house and your product failed the emissions test, testing in your own lab gives you time to isolate and fix problems.

Tektronix’ RSA300/500/600 series USB-based Real Time Spectrum Analyzer and similar low-cost products make pre-compliance testing easier or more cost effective. Test setups using the RSA306 are used to perform both radiated and conducted emission measurements that can help you keep your testing costs down and stay on schedule for getting your products EMI certified.

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The DPX technology in Tektronix RSA300/500/600 helps you to discover the most difficult-to-find problems with the powerful real-time architecture. You can see RF characteristics that are nearly invisible to a conventional spectrum analyzer. Figure 10 shows a near-field probe is used to locate emission sources in the near-field. In a comparison of the DPX spectrum and the regular spectrum displays, with Real-Time DPX the spurs with lower power can be seen even though they are hidden under the wideband noise.

The spurious measurement in SignalVu-PC is set up to change limits vs. frequency, and antenna correction factors can be applied. Violations above the user-set limits are shown in red on the table. The screen capture below shows how the SignalVu-PC and the RSA perform the CISPR 25 EMI pre-compliance test.

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Tire Pressure Monitoring System (TPMS), Remote Keyless Entry (RKE), Passive Keyless Entry (PKE) and Remote Start (RS) are more and more often used in today’s automobiles. Testing for these automotive applications assures drivers are kept happy with their cars and regulations aren’t breached. The Tektronix Real-Time USB Spectrum Analyzers RSA300/500/600 with SignalVu-PC software is a cost-effective system for measuring the transmitters of these automotive applications before more costly compliance testing is done.

To learn more about how computing power is replacing horsepower in modern networked cars and the full range of Tektronix solutions for the automobile industry, go to: https://www.tek.com/automotive/trends

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