By Dieter Mahnken and Ute Philipp, Rohde & Schwarz Munich
Nearly all new vehicles today offer Bluetooth® handsfree equipment. The Bluetooth connection between smartphone and infotainment system can also be used for numerous other applications. The individual components must undergo RF and audio tests to ensure that the headset, speakers, infotainment system and smartphone all work seamlessly with each other.
During the development of new radio components, complex and often costly tests are required. These tests should be reproducible and fast to execute. Certification by officially approved test houses is also necessary to be able to use and sell these components on the market. These certifications are cost-intensive. With suitable T&M equipment, you can perform precertification tests in your own lab to determine whether components will be certified by the test houses. The R&S CMW wideband radio communication testers from Rohde & Schwarz were designed for such precertification testing and are approved by the Bluetooth Special Interest Group (SIG). The testers offer solutions for RF and audio testing during development, production and service. Users can simulate realistic conditions for networks such as Bluetooth, WLAN, LTE-A, WCDMA and GSM.
What began with the Bluetooth headset has led to numerous additional functions that can be transferred from a smartphone via Bluetooth to the infotainment system and its speakers. Bluetooth is a radio transmission medium that is primarily used for the transmission of audio data and for control purposes. Users can make phone calls via the Bluetooth connection and upload their contact lists to the infotainment system. They can also play music or podcasts from a smartphone and listen to it over the vehicle’s speakers. Some systems can read out text messages as audio. And in some vehicles, drivers can operate their smartphone apps over the vehicle’s infotainment system when their smartphones connect to the system. Such apps include a navigation system or an info service for traffic reports, weather forecasts or hotels, gas stations and restaurants in the vicinity.
Audio transmission via Bluetooth
The audio Bluetooth transmission is based on Bluetooth Classic specifications from the Bluetooth Special Interest Group (SIG). Due to the effective data throughput of 0.7 to approx. 2.1 Mbps and the adaptive frequency hopping (AFH) transmission method, Bluetooth Classic technology is a short-range radio technology for distances up to 10 m that is robust even in noisy environments. Today’s widespread Bluetooth Low Energy technology, also called Bluetooth Smart, has been around since Bluetooth specification 4.0, which came out in 2010. However, it is currently not used for audio transmissions. The two Bluetooth technologies have been continually developed in parallel.
Bluetooth Classic operates with a synchronous link for voice transmission (synchronous connection-oriented: SCO) and an asynchronous link for data transmission (asynchronous connectionless: ACL). Audio signals can be transmitted with different Bluetooth profiles for the synchronous link.
The handsfree profile (HFP) for handsfree units, for example, transmits audio signals from the microphone near the driver via the infotainment system to the smartphone and back. For voice transmissions in HFP, the continuously variable slope delta modulation (CVSD) voice codec with a maximum transmission rate of 64 kBit/s is used.
The advanced audio digital profile (A2DP) is used for stereo playback of music stored on a device with a Bluetooth interface.
According to the Bluetooth standard, A2DP sources must support the low complexity subband coding (SBC) audio codec, which does not require a license. During SBC-based transmission of music from the smartphone, the device first decompresses the music, which is usually saved in compressed form, and then compresses it using the SBC algorithm for Bluetooth transmission. SBC coding can have a bit rate of up to 345 kBit/s. This is high enough that any receiver loss is no longer in the audible range and good audio quality can be ensured. The smartphone sends this Bluetooth sound stream via ACL to the infotainment system, which then transfers it via cable to the speakers.
Criteria for audio measurements
For Bluetooth transmission of sound information, it is crucial that the acoustic signal be transmitted with as high a quality as possible, without noise and dropouts. For this reason, Bluetooth headsets as well as microphones and speakers and the infotainment system’s associated radio components undergo radio frequency (RF) tests to ensure error-free Bluetooth transmission. The sound also needs to be played back in good audio quality. This is checked using audio analyses that test the frequency response, total harmonic distortion and noise and also offer a detailed spectrogram of the signal. Due to the low frequencies in the audio range, the settling times of filters and other components can play a role. Therefore, the Bluetooth RF tester should be able to adjust to the frequency of the test signal to perform measurements as quickly as possible. This applies to level measurements and to complex analyses such as total harmonic distortion plus noise (THD+N).
A Bluetooth tester must be able to establish a complete Bluetooth connection to the device under test (DUT) via the SCO link or ACL link. The tester should also support the relevant codecs and profiles for all audio transmissions. The following codecs and profiles are currently required for testing purposes: the narrowband CVSD codec, the broadband mSBC codec in conjunction with the handsfree profile and the broadband SBC codec with the A2DP Profile. For precise audio analysis, the tester should also be able to send commands to set the level of the microphone and speakers. These level settings are specified in the Bluetooth SIG Audio Video Remote Control Profile (AVRCP). All this is assured if the Bluetooth tester being used is approved by Bluetooth SIG.
Relevant audio tests for developers
The R&S CMW wideband radio communication tester uses an integrated two-channel audio generator to check the Bluetooth audio quality. If offers various measurement procedures. Via the multitone mode, a developer can define up to 20 sounds for each audio channel (level and frequency) and measure the associated frequency responses.
In single-tone mode, the following parameters can be specified for a sine signal: audio level, frequency, signal-to-noise and distortion ratio (SINAD), total harmonic distortion (THD) and THD plus noise (THD+N). There are also several filters that can be selected for the audio measurement.
Stereo transmission of music
Stereo transmission of music is carried out via the A2DP profile with SBC coding and uses the broadband, asynchronous link (ACL) for data transmission. Bluetooth devices that support A2DP must correctly process signals from SBC codecs. Tests with SBC-coded signals based on ACL also show whether or not the DUT correctly transmits longer packets. It is also practical if the Bluetooth tester can analyze and play all SBC codec modes such as dual, mono, stereo and joint stereo (dual mode for high-quality transmissions). The R&S CMW platform with its different models supports all this.
Example: headset testing
Testing the microphone of a Bluetooth headset also involves testing the headset’s audio input amplifier and A/D converter. The Bluetooth tester’s audio generator produces the audio signal that is transmitted to the microphone under test via a reference speaker. The microphone then transmits this audio signal to the tester’s audio analyzer via a Bluetooth link. The tester measures this signal and compares it to the original outgoing signal. (Fig. 1)
Testing a headset speaker includes testing the headset’s D/A converter and output amplifier. The Bluetooth tester’s audio generator produces an audio signal and transmits it to the headset via a Bluetooth link. There it is amplified and converted to an acoustic signal via a sound converter. This signal is picked up by a reference microphone and sent via a reference amplifier to the tester’s audio analyzer where it is displayed and evaluated. Audio tests on the Bluetooth module of an infotainment system are performed in much the same way (Fig. 2).
Relevant RF tests for developers
When developing a device with a Bluetooth interface, functional tests, interoperability tests and the range are relevant. The receiver sensitivity and transmit characteristics of the components are crucial for the range.
In order to measure the transmit characteristics of a Bluetooth component, the developer must be able to reproducibly determine characteristic values such as power, spectrum, frequency accuracy, frequency drift, frequency deviation and the modulation index calculated from these values. Receiver sensitivity is another important parameter for a Bluetooth device. This parameter is measured using an artificially impaired signal generated by the Bluetooth tester (dirty transmitter).
Bluetooth module receiver tests are performed by sending highly accurate data from a precise generator to the receiver. The data is analyzed either by having the receiver return the bit sequence or by using an external control PC.
Numerous, additional RF signalling tests as well as spectrum measurements on newly developed Bluetooth components will be required until they receive Bluetooth certification from Bluetooth SIG. In addition to the described audio measurements, the R&S CMW communication testers support all Bluetooth RF tests cases for prequalification purposes for Bluetooth Classic and Bluetooth Low Energy up to (the latest) specification 5.
For the time-consuming spectrum measurements that are also part of Bluetooth qualification tests, the R&S CMW provides first test results in less than one second, which no other Bluetooth tester on the market can do. The parametric test concept allows users to set all parameters themselves. They now have a compact solution for performing automated prequalification tests for Bluetooth Basic Rate, Enhanced Data Rate and Bluetooth Low Energy in line with Bluetooth core specifications 2.0, 2.1+EDR, 3.0+HS, 4.0, 4.1, 4.2. The easy-to-use R&S CMWrun sequencer software also helps.
Especially for production use, the R&S CMW platform offers numerous hardware and software options so that the tester can be customized exactly to the measurement requirements on site. The R&S CMWrun sequencer software can be used to automate production tests. The software also makes it possible to integrate the tester into a comprehensive test system. The platform offers all essential measurements at an excellent price/performance ratio.
Bluetooth plus other radio technologies
The user can expand testing to include other radio technologies. Many devices support more than just Bluetooth. They also support WLAN, GNSS and/or various other mobile technologies such as LTE, LTE-A, WCDMA and GSM. When WLAN and Bluetooth components use the same antenna, the developer can test both radio technologies with one test configuration. If the R&S CMW is equipped with the appropriate hardware and software options, it is possible to test all integrated radio modules of a component up to precertification. This allows the developer to check whether and to what extent the individual radio modules affect each other and to define any necessary measures. This test is called the coexistence test and it is also possible with the R&S CMW tester family. The R&S CMW500 wideband radio communication tester is the only test platform on the market to offer a one-instrument solution for testing all cellular and non-cellular standards such as WLAN and Bluetooth.
Dieter Mahnken is a product manager at Rohde & Schwarz in Munich. He is responsible for Bluetooth functions and applications within the complex R&S CMW platform. Ute Philipp is also a product manager for the R&S CMW platform. She is responsible for RF testing for non-cellular technologies such as Bluetooth and Zigbee as well as for IoT applications.
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