White Paper. Solving Extreme Dynamic Range Issues for Car Radios Capturing Adjacent Field Signals to Improve FM Radio Design and Testing.

Transcription

1 Solving Record and and Playback Extreme Playback System Dynamic for System GNSS: Range What for You GNSS: Issues Need to What Know for Car You for Successful Need Radios to Know Testin for Successful Testing White Paper Solving Extreme Dynamic Range Issues for Car Radios Capturing Adjacent Field Signals to Improve FM Radio Design and Testing Executive Summary Radio quality is a highly important part of a car s user experience. Consequently, designers strive to constantly improve the performance, feature set, and basic flexibility of their radios. While digital signal processing (DSP) has led to many quality improvements and new features such as satellite radio and hybrid digital (HD) radio, testing these radios is not a trivial task. Since harsh environments are nearly impossible to create strictly in a lab environment, drive-testing is typically used to verify radio design. However, drive-testing is expensive and not reproducible. To address this testing challenge, Averna has developed solutions that reliably capture field conditions and allow repeatable real-world testing in the lab, greatly reducing drivetesting costs and accelerating lab work. Introduction A difficult field issue to reproduce is that of a strong adjacent channel interferer in an area where fading may also be present such as is found in portions of Ann Arbor, Michigan. There, local station WQKL, transmitting 3 kw at a frequency of MHz, acts as an adjacent interferer for a signal transmitted from Detroit radio station WDTW, transmitting at MHz. In portions of Ann Arbor during a typical drive test, the WQKL signal is received at 65 to 95 dbµv. For someone listening to WDTW, which is received at 25 to 50 dbµv, the WQKL signal represents a very strong interference source. That makes this drive test an excellent choice for testing a radio s ability to mitigate this interference, allowing the listener to have a positive user experience while listening to such stations. However, making the trip to Ann Arbor to do this testing can be cost prohibitive. A more cost-effective solution is to bring the Ann Arbor environment back to the lab. Of course, this same technique can be used for any channel combination in any market. Figure 1 Averna RP-5120 RF Recorder November 2012 / / Averna p_1

2 Capturing the RF Environment The Difficulties of Extreme Dynamic Range To bring the RF environment back to the lab, first it needs to be captured. Using the Averna RP-5120 RF Recorder (Figure 1), the environment can be fully recorded. However, an examination of the power versus frequency spectrum of the station frequencies shows that the power difference is on the order of 60 db (Figure 2). And the effect of fading during a drive test can lead to differences of an additional db. While the 80 db operational dynamic range of the Averna RP-5120 may seem large, it is actually less than the dynamic range of a high-quality radio. Furthermore, the optimal use range is about 60 db. So the power spectrum of the two signals in this environment makes it difficult to get a recording of both signals at the same time. Figure 2 Power Patterns of a Strong Signal (WQKL 107.1) and a Weak Signal (WDTW 106.7) However, by using clever synchronization of the two Averna 5120 channels and external hardware, the RF environment can be successfully captured for use in the lab. Solving the Extreme Dynamic Range Recording Issue Because the Averna RP-5120 has two recording channels, it can easily be configured to capture both the strong WQKL signal and the weak WDTW signal as high-quality recordings. In addition to recording the RF signal, it is often desirable to record how a receiver processes the signals this recording can then be used later in the lab to verify the fidelity of the recording. The block diagram in Figure 3 shows an example of the setup needed to make the recordings. November 2012 / / Averna p_2

3 Figure 3 Recording Block Diagram In Figure 3, the cavity bandpass filter and the Averna RP are the main elements needed in the recording. The DV Camcorder can be used to record video of the environment during the drive. This can be reviewed later to provide information about the environment at the time of any noticed impairments during testing. The block diagram also shows several different receivers. The purpose of these receivers is twofold. First, during the preparation and actual drive test, these receivers can be monitored. This ensures that there are usable signals present at the start of the drive and for the duration of the drive. By using various recording devices, such as those shown at the right side of the block diagram, a baseline can be determined. This baseline can then be compared with results of using the recording, thus verifying that the recording is accurate and usable. The baseline is also useful for making objective decisions in determining if modifications to DSP algorithms are indeed beneficial to the radio performance. When using this setup, channel 1 of the recording, which records the signal presented to RF1 of the Averna RP-5120, records the entire FM band this provides the full FM interference, including the strong signal in question. November 2012 / / Averna p_3

4 URT SETTINGS Bias-T Voltage: FM radio does not require an active antenna; therefore this should be left at 0 for both channels. External Gain Loss: Due to the two splitters shown in Figure 3, there is a 9 db loss between the antenna and the RF inputs. Therefore the checkbox should be checked, and the value below should be set to -9 db to account for these losses in both channels. If the cables contribute significant losses, that should be accounted for here as well. Low Noise Amplifier: Because the signal is expected to be 95 dbµv or less, this amplifier can be turned on for each of the channels. Comments: Freeform recording comments can be stored here. Comments and markers can also be used to document any abnormalities noted at the test receivers during the drive test. Record: The Record button starts the recording. Click the Record button on the slave (typically channel 2) recording first the system then waits for a signal from the master indicating that the recording should begin. When all settings are configured, click the Record button on the master (typically channel 1) recording to start the actual recording process for both channels. Destination Folder: It is recommended that two removable hard drives be inserted into the Averna 5120 one for each recording. Set one of the recordings to store one of the hard drives, and the other recording on the other hard drive. File Name: Enter a valid file name for the recording. If no name is provided, one will be generated based on the current date and time. By using a 200-kHz bandpass filter with a center frequency of the station of interest, in this case MHz, channel 2 records only the channel of interest. Setting Up the Recording Parameters The other part of the recording scenario is to set up the recording software parameters to drive the recording process and save the data. Figure 4 shows the default configuration of the Universal Receiver Tester (URT) software. Two URT windows are required one for each channel recorded. Figure 4 URT Software A passenger should operate the Averna RF Recorder equipment, while the driver focuses on safe driving. November 2012 / / Averna p_4

5 URT SETTINGS Center Frequency: For the recording of channel 1, set the center frequency to the center frequency of the FM band to allow capturing of the entire FM band. For this application, use 98 MHz (enter 98M). For the recording of channel 2, set the center frequency near the center frequency of the weaker station, WDTW MHz; in this case, set the value to MHz (enter 106.4M), which when used with a 1-MHz recording bandwidth will record to MHz. This records the desired station while excluding the strong interference from WQKL transmitting at MHz. Recording Bandwidth: The recording bandwidth specifies the RF bandwidth to record. For channel 1, set this value to 20.0 MHz to record the entire RM band. For channel 2, set this to 1 MHz to record a single station. Record Mode: Because this is a drive test, manual stop should be used. When the drive has completed, stop the recording on channel 1, and this will cause both channels to stop recording. Reproducing the RF Environment Once a recording has been made, it can be used in the lab. Use the Averna URT (Figure 5) to play the recording, thus reproducing the RF environment. Two URT-5000s are needed, and one of them needs to have the Dynamic Range Extender (DREX) option installed. Figure 5 Averna URT-5000 The first step is to remove the hard drives from the Averna RP-5120 and install them in the URT-5000s this is the easiest way to transfer the files. The drive that holds the channel 2 data for the lower power channel should be installed in a URT-5000 that has the DREX option installed. Because the channel 1 recording is a full FM band recording (including station WDTW MHz FM), and the channel 2 recording is a WDTW MHz FM-only recording, before the signals are passed to the receiver, station WDTW needs to be filtered out of the interfering signal using a 200 khz bandwidth bandstop filter centered at MHz to remove the signal, as shown in Figure 6. November 2012 / / Averna p_5

6 Figure 6 Playback Block Diagram Figure 6 shows the connection between the master and slave URT-5000s. This is the trigger signal that is used for synchronization purposes. It is very important to set this trigger correctly, otherwise the two RF signals will not play synchronously as desired meaning that the system would no longer be a recording of the actual environment. To set the trigger correctly, you need to use the Sync option. Ensure that the Sync output of the master URT-5000 is connected to the Trig In port of both the master and the slave URT-5000s. Playing back the signal requires that you load it with the URT software. Configuring the playback is straightforward. When you select the file, the playback parameters are read back from the file. The URT Playback software is shown in Figure 7. Figure 7 Playback Software November 2012 / / Averna p_6

7 RF PLAYER SETTINGS File to Play: Select the file to play this is the file recorded earlier. Playing Mode: For continuous testing, leave this as Loop. For single-pass testing, change this to Only Once. Play Position: If only a portion of the recording is of interest, set the Play position to jump to that portion of the recording. Play: Click the Play button of the slave channel first, then click the Play button of the master channel to start the playback. Playing Back the Recording in the Lab By playing the recorded signal, designers can now test situations that are normally only available during an actual drive test. Using the separate recordings and playing them back on separate players allows for accurate testing of extreme dynamic range conditions. And because the recordings are so separated, the problem can be adjusted. Unlike in a drive test, with these recordings the designer can easily change the power offset between the two stations, which can be useful during analysis because the offset can be reduced to determine when performance issues start to occur. As well, the power offset can be made even more severe for testing other e0vironments. Another advantage of using the Averna RP-5120 to record and the URT to play back the RF environment is that the URT software can be used to override the output frequency of the playback. For signals such as FM radio where the channel is determined entirely by the operating frequency, this means that the designer now has the opportunity to test the design not only at the recorded frequencies, but across the entire band of operation. Additional Testing Capabilities Since the entire FM band is captured in the channel 1 recording, and because the region has a relatively dense RF environment, this recording can be used in a standalone mode to do other testing, such as selectivity tests or more typical interference testing. November 2012 / / Averna p_7

8 Conclusion With the record-and-playback technique explained in this document, the designer can test the extreme dynamic range of environments that a radio might encounter without having to perform multiple driving tests. Furthermore, because the signal is recorded, it is a reproducible environment, allowing the designer to test various changes to the DSP software and be assured that the same impairments are used at every iteration of testing. Averna is a premier manufacturing solution provider for communications and electronics device makers worldwide, helping them deliver a better end-user customer experience. Key Averna clients in the communications, aerospace, defense, automotive, consumer electronics, and medical device industries use Proligent, RF Test Instruments, and other test solutions to accelerate product development, reduce manufacturing costs, achieve superior quality throughout the lifecycle, and solve critical supply chain issues. Averna has multiple centers of expertise worldwide, supported by channel sales partners across North America, Europe and Asia. Incorporated in 1999, Averna is a 2010 Best in Test award winner and an Ernst & Young Entrepreneur of the Year 2009 winner, and since 2007 has been honored as one of the Deloitte Fast 500 fastest-growing technology companies in North America. Averna is a registered trademark. All other brand names, product names, or trademarks belong to their respective holders. Copyright 2012 Averna. All rights reserved. November 2012 Canada United States Mexico Japan Hungary Telephone: Toll free in NA: Fax: