Real-Time FFT Analyser - Functional Specification

Transcription

1 Real-Time FFT Analyser - Functional Specification Input: Number of input channels 2 Input voltage ranges ±10 mv to ±10 V in a sequence Autorange Pre-acquisition automatic selection of full-scale input voltage range based upon input signal (data sampled at 12.8 khz (5 khz bandwidth) with anti-alias filters switched off) Input coupling AC, DC, ICP ('ACCEL') Overload detection In-band signals only (after anti-alias filtering) at full-scale input voltage range - indicated on-screen Under-range detection In-band signals only (after anti-alias filtering) when signal <25 % of full-scale input voltage range - indicated on-screen Alias Protection (Anti-alias filtering) Can be switched 'on' or 'off'...default is 'on', this is the recommended condition Enveloping Single channel (ch 1) enveloping facility for signal envelope demodulation and analysis Trigger: Data acquisition modes Source External trigger modes Analogue trigger settings Independent channel delay Normal (repeat after average count satisfied) or '1-SHOT' (stop after average count satisfied) Free-running, channel trigger, external trigger, delta time, or delta RPM (requires external analogue tacho signal connected to the external trigger connector) TTL, analogue or analogue plus phase-locked-loop (EXT+PLL) slope - positive or negative, level - variable in steps of 1 % of range, up to ±10 V Pre-trigger 0 to time points Post-trigger delay 0 to time points Sampling: Internal Controlled by internal sampling clock. Data sampling rate determined by specified analysis bandwidth or by frequency of 'external trigger' signal (EXT+PLL trigger mode only). Sampling multiplier 2.56 External Sampling rate controlled by external clock signal (using 'external clock' BNC connector on rear of analyser) Maximum data sampling rate khz per channel Analysis: Types Single-/dual-channel FFT spectrum analysis, auto- and cross-correlation, full & third octave synthesis Baseband frequency ranges From DC (0 Hz) to 25, 50, 100, 200, 500,1k, 2k, 5k, 10k, 20k, or 40k Hz Baseband resolution (max) 25 Hz 1600 lines = 15.6 mhz Zoom analysis (real-time) Up to x 80 expansion in 1, 2, 5 sequence Zoom resolution (max) 5 Hz 1600 lines = mhz Time data block sizes 256 to 4096 data samples Number of spectrum lines Real-time bandwidth 100 to 1600 lines up to 20 khz - single channel up to 10 khz - dual channel

2 Order analysis Order filtering Octave synthesis A-weighting...assumes: 'turbo mode', 'fast averaging', acquisition mode 'normal', time averages '1', spectral (process) averages '100', number of lines '200', single or dual spectrum display respectively (EXT+PLL mode) A special case of internally clocked data acquisition where the analysis frequency range is determined by the period of a tachometer signal connected to the external trigger connector and measured immediately prior to the acquisition of each data block. This feature allows the x- axis of the spectrum to be calibrated in 'orders' (multiples of the tachometer frequency) and allowing any components that are directly related to the machine's operating speed to remain at a fixed location on that axis. Note: To avoid the effects of 'spectral smearing' when using this form of analysis it is required that the fundamental (tachometer) frequency does not change significantly within the acquisition of each data frame. This is sometimes referred to as 'pseudo-steady-state' operation. Removes all data components other than a single specified integer order component (requires EXT+PLL mode) Computation of full and third octave from narrowband spectral data to ANSI standard S (R 1975) type E Class II specifications. Available over 5 khz, 10 khz & 20 khz analysis ranges using a dual-pass measurement. One 2048 point / 800 line measurement is made at the specified bandwidth and another at 1/10 th of that frequency range. The required octave bands are then computed from those spectra. Applied to full and third octave functions only. The filter shape complies with definitions given in ANSI standard S and IEC

3 Functions: Time domain functions Correlation functions Single-channel spectra Dual-channel spectra and derived spectral functions Octave functions - Instantaneous or averaged time data blocks - Differentiated time (single time domain differentiation) - Time domain difference between channel 1 and channel 2 - Windowed time data (for Force/Exponential window combination only) - Time data 'preview' before being passed to the averaging stages ('manual preview' mode only) - Auto- and cross-correlation (for auto-correlation 'T' signal into both channels) - Instantaneous spectrum (complex)...no time or process averaging - Synchronous spectrum (complex)...time averaging only - RMS Auto-spectrum expressed in linear units (square rooted)...process averaging enabled - Power Spectral Density (PSD). RMS spectrum normalised to compensate for the effect of spectral resolution, expressed in linear units...process averaging optional - Cross-spectrum - Frequency Response Function (H1 FRF) - Coherence - Full and Third Octave, synthesised from narrowband spectral data to ANSI standard S (R 1975) type E Class II specifications. 2 khz, 10 khz & 20 khz analysis ranges using a dual-pass measurement, one 2048 point / 800 line measurement at the specified bandwidth and another at 1/10 th of that frequency range. Processing: Windowing Averaging Reprocessing Fast averaging Turbo mode Block rejection Rectangular, Hanning, Flat-top & Force/Exponential Time and Spectral ('Process') averaging performed in sequence (setting either average count to '1' disables the corresponding form of averaging):...from 1 to 4096 time domain averages...from 1 to 4096 'process' averages. Process average modes include; linear (RMS), RMS exponential and peakhold In normal operation (turbo mode 'off') all intermediate functions and spectral cross-products are retained in memory. This allows data acquisition to be paused and any other function to be computed and displayed without loss of Note: Only limited reprocessing options are available for octave functions. Option to suppress display of intermediate average results (time or time and process) Optimises real-time processing speed by suppressing computation of any unnecessary intermediate or crossspectral functions not required for display. Note: No 'reprocessing' is available when 'turbo mode' is switched on. Optional automatic rejection of overloaded data blocks (before inclusion in average) or manual preview of time domain data with option to accept/reject

4 Octave analysis Post-processing Display: Formats Cursors X-axis options Y-axis options Scaling Offsets Synthesis of full or third octave spectra from narrowband Notes: i. When octave analysis is required octave functions must be specified for both data traces, ii. Octave functions can not be 'reprocessed' from normal Computation of various functions from data recalled from function memory. Single trace - 2-D spectrum - 3-D map (waterfall) with low level component suppression, skew, hidden-line removal and 'repeat', 'scroll' and 'one-shot ' modes - 'table display'...graphical display and tabular listing of peaks, harmonics or octave band data values Dual trace - top/bottom - overlaid - cross-plotted, Note: Combination of above options and available data types allows selection of popular display formats including Bode (magnitude and phase vs. frequency), Nyquist, Orbit etc. Single (X-Y readout), harmonic (max 20), sideband, peak, power in specified band Specific cursor options for 3-D map display: - active trace selection & extraction of 2-D spectrum - z-axis slice function - overall slice function (shift + slice)...computation of spectral overall for each trace and display against z-axis variable Linear or log Frequency in Hz, CPM or Orders Linear, Logarithmic or db magnitude, phase, real part or imaginary part Default scaling (based upon measurement input range), auto-scaling (based upon maximum value in data) or fixed scaling (user-specified pre-selected display range - Auto EU mode and single-channel displays only) Reduction of displayed dynamic range for db plots Offset phase display range to reduce 'phase wrapping' effects Scaling: Modes Unit systems Volts mode (no scaling), 'manual' EU and 'automatic' EU scaling Metric (g, m/s, m) or 'English' (g, ips, mils) - 'auto EU' mode User-defined scaling and unit labelling - 'manual EU' mode

5 Integration Frequency domain integration; 'manual EU': single ( jω) or double ( -ω 2 ) 'auto EU': acceleration to velocity, acceleration to displacement, velocity to displacement, all with preservation of proper data scaling in accordance with the specified 'unit system' RMS (default), 'pseudo peak' (RMS x 2) or 'pseudo peak-to- Spectral function scaling peak' (RMS x 2 2)