ISO 21806-13 pdf download Road vehicles — Media Oriented Systems Transport (MOST) — Part 13: 50-Mbit/s balanced media physical layer conformance test plan
ln order to get to a representative average value, it requires a long-term observation. Dependingon the chosen SP2 and applied channel losses,intersymbol interference impact affects the signal tobe measured. It may lead to locally distributed RMS minima and maxima when choosing only shortsnippets of the signal. The acquired waveform shall have a minimum length of 125 us ( 125 us equals sixframes with a frame rate of 48 kHz].
DC offset in the measurements shall be minimized as it may indirectly compromise RMS signal voltageamplitude results, see 10.2 and 10.3.
8.3PSD of SP2 output signal
PSD as specified in ISo 21806-12:2021,9.2 is used as a link quality criterion at SP2.The main purpose isto limit pulse shape variations and inherently limit the transmitted signal bandwidth.
Several measurement options are available to perform spectral signal analysis. A method using time-domain data acquisition followed by FFT post-processing is given for reference. Other measurementmethods are permitted.In the case of discrepancies, the reference method shall be used.
PSD shall be measured with an RMS detector and using an effective RBW of 500 kHz.Besides directlymeasuring PSD with 500 kHz resolution bandwidth, this can be achieved by using lower RBW settingand averaging spectral results in the amount n of overlapping groups of the lower RBW bands to producethe effect of 500 kHzRBW sliding window (linear scale), (i.e.measurement with RBW 10 kHz, averagedin overlapping groups of fifty bands, therefore n = 50).To achieve statistical representation, the spectraldensity results of multiple trace segments are averaged to form the final result. The number of tracesegments contributing to the averaged spectrum equals the sweep time.
The following is an example procedure for measuring PSD.
– The lUT transmitter sends multiple MOST50 bPHY PSD patterns.
Differential signal at terminated SP2 is measured with a differential probe. Other methods tomeasure a single ended representation of the differentialsignal are acceptable(e.g.use BALUN or usetest fixture with matched length 50 .Q coaxial cables, measured with 2 channels and mathematicalcombination).
An oscilloscope acquires the SP2 signal. To reduce noise in the measurement channel,it isrecommended to use an averaging technique for time domain data acquisition. Selecting oscilloscopesampling rate and acquisition length leads to the inherent RBw for the acquisition, which is thereciprocal of the acquired duration time. The appropriate duration can be achieved by adjustinghorizontal oscilloscope settings accordingly or by acquiring longer traces and slice the trace intoappropriate trace segments for the further processing.
For further post-processing FFT algorithm can be applied on the oscilloscope or via processing perexternal script on a PC.In frequency domain, PSD is then formed as a moving average (linear scale)of n consecutive samples of the inherent RBW bands.
To achieve statistical representation, the spectral density results of multiple trace segments areaveraged to form the final result. The number of trace ‘segments contributing to the averagedspectrum equals the sweep time.
Described procedure provides spectral density for consecutive 500 kHz bands in the relevantfrequency range and can be directly compared with the limit lines.