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A7-MX With Real Time Close in Phase Noise PlotQuartzlock A7-MX Frequency Analyser Introduction A quasi real time plot of the Phase Spectral Density (PSD) has been added to the A7-MX virtual front panel. This shows phase noise as a graph of L(f) in units of dBc against offset frequency on a log scale. Various window functions and averaging modes are provided. The routines are identical to those used in the Industry standard software "Stable32". The user can select the basic length of the FFT, and also the degree of overlap. As data is accumulated, new FFTs are performed on a mix of old and new data depending on the overlap parameter. For example, with an FFT length of 256 points, and an overlap of 50%, with a tau (sample rate) of 10ms, a new FFT will be available every 1.28 seconds. Each FFT result can either replace the last graph, be added to a block average, or be used in a continuous or exponential average. All FFTs are correctly normalised for bin noise bandwidth, window coherent gain, and nominal frequency. Frequency data always has a fixed offset removed before being used for the FFT calculation. Phase data has a fixed slope ramp removed by linear regression. This avoids a large component in the lower frequency bins which will distort the result, even when windowing is used. Applications Phase noise measurement at very small frequency offsets Identification of spurious components in the data which can distort an Allen variance plot Specifications Maximum offset frequency 500Hz (at 1ks/s) |
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