NVGate time shift resampling
SUBJECT: Time Shift Resampling in OR3x
Hardware Version : OR3x
Software version : any
/ Introduction
This document intends to highlight internal signal processing of OR3x with regards to phase precision of Time trigger blocks in NVGate. When trigger event occurs in between two samples of acquisition error may be introduced in absolute phase measurement of the acquired Time block (see Picture 1). Enhanced signal processing of NVGate enables to minimize this error by finding start of trigger even with a much higher precision and performing time shift resampling.
/ Principles of OR3x enhanced acquisition
a/ oversampled External Sync input
OR3x Front end features 2 External Sync inputs (2 logic inputs) that have integrated pre-divider enabling oversampling of external trigger channel. For instance when Input Front End Sampling is set to 102.4kS/s (40 kHz bandwidth) External Sync 1 and 2 have detection precision equivalent to 6.5MHz (#102.4*64). Trigger event resolution therefore increases from 9.7 micro seconds (sample rate @ 102.4 kS/s) to 151 nano seconds.
b/ Time Shift Resampling
Enhanced signal processing of OR3x enable phase improvement of trigger block by time resampling of acquired trigger block (at sampling frequency) with a resolution of 1/16th of a sample. This is made according to the External Sync precise start event. (See Picture 2)
/ Benefits of Time shift resampling
Signal processing for time shift resampling is automatically engaged when "Time" averaging is selected in NVGate FFT plug in (see Picture 3).
When "Time" averaging mode is selected, precision of absolute phase of the trigger block signal is therefore increased by a factor of 16.
Picture 3: Time domain averaging in FFT
4/ Precision calculation:
a/ Without time shift resampling
Standard absolute phase error calculation without time shift resampling can be determined as follow (worst case where event occurs right after last sample)
Phaseerror = (360/2.56)*f/fe
Where f is the frequency of interest
fe is the frequency band of analysis
for instance for a total analysis band of 20kHz, maximum error at 10kHz frequency will be Phaseerror = (360/2.56)*10/20 = 70.3 deg
b/ With Time shift resampling
Standard absolute phase error calculation with Time Shift Resampling can be determined as follow (worst case where event occurs right after 1/16th of a sample )
TSR_Phaseerror = (360/[2.56*16])*f/fe
Where f is the frequency of interest
fe is the frequency band of analysis
for instance for a total analysis band of 20kHz, maximum error at 10kHz frequency will be TSR_Phaseerror = (360/[2.56*16])*10/20 = 4.4 deg
NOTE : time shift oversampling does not affect the phase match between channels (10V range, 0 to 20kHz , +/- 0.02deg on OR36-OR38) it only improves absolute phase information of acquired time block.