• Document: FM stereo multiplex (MPX) generation, including RDS data
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dScope Series III Application Note FM stereo multiplex (MPX) generation, including RDS data Introduction This application note describes how dScope’s versatile signal generator can be programmed to produce an MPX stimulus (stereo multiplex) for testing FM radio receivers or transmitters. The generated MPX has optional 50us or 75us pre-emphasis, and may also contain an RDS sub-carrier transmitting basic RDS data if required. In the supplied example, a multi-tone stimulus is generated, with distinct left and right channel content, and analysis scripts are included to simultaneously measure channel balance, distortion and noise, high and low-frequency roll- off and cross-talk at the output of the equipment-under-test. Since the dScope is programmed to fulfil this function using Visual Basic Scripting (VBScript) alone, it is easy for users to modify the supplied scripts to meet their particular requirements. Quick start The attached files should be copied to the following existing folder locations within the dScope installation directory (normally \Program Files\Prism Sound\dScope Series III\): Copy contents of supplied folder… … into installation directory folder Configurations Configurations (File: mpxtest.dsc) User Wavetables User Wavetables (File: mpxtest.dss) (File: MPX.wfm) Scripts\FFT Detector Calculations Scripts\FFT Detector Calculations (Folder: MPX, containing:) (File: Balance.dss) (File: Cross-talk.dss) (File: LF Rolloff.dss) (File: HF Rolloff.dss) (File: TD+N.dss) The dScope should be connected to the receiver-under-test via an FM exciter as shown: Run the dScope application, and load the configuration file: mpxtest.dsc Ensure that the receiver is tuned to the same channel as the exciter. The exciter’s input sensitivity should be adjusted to accommodate the dScope’s MPX output level. If this is not possible, the dScope’s output level can be modified by adjusting the output amplitude setting in the Signal Generator dialogue box. The receiver should indicate that it is locked to a stereo transmission and, if RDS-enabled, should display the eight-character station identifier string, “PRISMRDS”. If the output of the receiver has a gain control, set it to the desired nominal position. The dScope’s Signal Analyzer should now simultaneously display the various measured parameters for each channel, as well as the recovered spectra, as shown below: NOTE: Depending on the nature of the exciter, it may be necessary to place an attenuating pad between the exciter’s output and the receiver’s RF input. If so, this attenuator should match the output impedance of the exciter and the input impedance of the receiver (usually 75R). Some receivers incorporate an adjustable RF attenuator. Troubleshooting If the receiver does not recognise a stereo transmission, check the following: • Is the dScope’s A-channel analogue output connected to the exciter’s MPX input? • Is the exciter’s RF output connected to the receiver’s RF (antenna) input? • Is the receiver set to the same RF channel as the exciter? • Is the exciter switched on? • Is the dScope application running, with the mpxtest.dsc configuration loaded? • Is the input sensitivity of the exciter matched to the dScope output level? • Is the RF output level of the exciter appropriate for direct injection into the receiver? You may need to use an in-line attenuator or adjust the RF input sensitivity of the receiver. • Is the dScope generating the MPX signal correctly? You can check this by connecting the dScope’s A-channel analogue output directly to one of the analogue inputs and examining the spectrum using the FFT Trace in the Trace Window. The spectrum of the MPX should look similar to this: MPX spectrum at dScope output dBu -20.00 -40.00 -60.00 -80.00 -100.00 -120.00 -140.00 0.00 Hz 10000.00 20000.00 30000.00 40000.00 50000.00 60000.00 • Is your dScope capable of analogue sampling at 192kHz? Some models aren’t – check whether 192kHz sampling is selected in the Analogue Inputs dialogue box. If your dScope is not capable of analogue sampling at 192kHz, you can have it upgraded – contact your local distributor, or Prism Sound directly (sales@prismsound.com). If the receiver recognises the stereo transmission, but the dScope Signal Analyzer does not receive a decoded multi-tone (as shown in the ‘Quick Start’ section above), check the following: • Are the analogue outputs of the receiver connected to the analogue inputs of the dScope? • If the outp

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