
5.4.2 FFT Power Spectrum
5.4.3 Single FFT Statistics
(1)
(2)
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User Interface
FFT calculation from presenting a smeared power spectrum due to the fact that the FFT presumes thesample of the input is part of a continuous input signal. If the input and sampling frequency is notcoherent, and the sampled data is appended end to end to form a continuous input signal, then there is anapparent phase discontinuity at the beginning and the end of the sampled data. Making the sampling andinput frequencies coherent avoids this apparent discontinuity. If the input frequency cannot be madecoherent, then the windowing functions other than Rectangular can be used to process out this effect tosome degree.
The FFT record length can be set in the FFT Record Length (NS) input text box. The TSW1200EVMsupports FFT record lengths of as much as 65536 samples, or as little as 4096 samples.
The FFT power spectrum of the captured data is displayed in the major center portion of the window. TheTSW1200 software automatically scales the horizontal axis from DC through the Nyquist frequency,although the scale of the horizontal axis can be changed simply by highlighting the text and typing in anew value. For example, the display in Figure 11 can be used to zoom in on the input frequency byhighlighting the 0MHz and typing 25M, and then highlighting the 62.5M and typing in 35M. This causes theportion of the power spectrum from 25 MHz through 35 MHz to fill the power spectrum display.
The vertical scale of the power spectrum is automatically scaled to display the noise floor of the FFT resultup through 0 dBFS. The vertical scale can also be manually adjusted by highlighting the limits of thevertical scale and typing in new limits.
By default, the first few harmonics of the input frequency are marked in the display, as well as anadditional marker that can be placed by dragging the marker to any place in the power spectrum, such asa noise spur that is not already marked as a harmonic. By default this additional marker initially goes tothe highest spur that is not identified as a harmonic.
Display properties can be edited by using the mouse to right-click in the power spectrum display. Visibleproperties such as the graph palette or plot legend can be edited, and auto-scale of the vertical andhorizontal axises can be enabled or not.
For the Single FFT test, a number of calculated statistics and AC performance measurements aredisplayed to the right of the power spectrum display, grouped into several categories.
AC
Signal-to-Noise Ratio – SNR is the ratio of the power of the fundamental (P
S
) or input frequency to thenoise floor power (P
N
), excluding the power at DC and the first five harmonics. SNR is either given in unitsof dBc (dB to carrier) when the absolute power of the fundamental is used as the reference, or dBFS (dBto full scale) when the power of the fundamental is extrapolated to the converter’s full-scale range.
Signal-to-Noise and Distortion (SINAD) – SINAD is the ratio of the power of the fundamental (P
S
) to thepower of all the other spectral components including noise (P
N
) and distortion (P
D
), but excluding DC.
Spurious-Free Dynamic Range (SFDR) – SFDR is ratio of the power of the fundamental to the highestother spectral component (either spur or harmonic). SFDR is typically given in units of dBc (dB to carrier).
SFDR w/o 2,3 – Spurious-Free Dynamic Range without the second or third harmonic. Commonly thelargest spectral components after the fundamental are the second and third harmonics of the inputfrequency, and commonly the input frequency can contain significant power in the second and thirdharmonics. SFDR w/o 2,3 reports the SFDR with these two harmonics ignored.
Total Harmonic Distortion (THD) – THD is the ratio of the power of the fundamental (P
S
) to the power inthe first five harmonics (P
D
). THD is typically given in data sheets in units of dBc (dB to carrier).
SLAU212A – April 2007 – Revised August 2008 TSW1200EVM: High-Speed LVDS Deserializer and Analysis System 19Submit Documentation Feedback