VTI Instruments EMX-4016B, Руководство пользователя

Страница: 89 / 118

www.vtiinstruments.com
APPENDIX B 89
A
PPENDIX
B
P
HASE
M
EASUREMENT AND
C
ORRECTION
O
VERVIEW
The Fourier transform of the time domain signal acquired by EMX-4250/4350/4380 gives the
amplitude and phase of each frequency components. The amplitude indicates the magnitude of the
phenomena and the phase represents the time shift. When the instrument acquire signal, the analog
signal goes through multiple stages of analog and digital filters before the data reaches to the user’s
application. Each filter adds a certain time delay to the signal.
When you are trying to measure the time difference between two signals of the same frequency, if
both signal goes through the same analog and digital filters, then the amount of time shift can be
considered to be equal, since digital filters can achieve the perfect phase match. In this case, the
phase difference between two signal does not require any special correction.
On the other hand, if one of signal does not go through these filters, the phase of the other signals
must be corrected for the delay added in each filtering stages. This is the case when the user triggers
the measurement with a phase reference signal, such as TDC (Top Dead Center) pulse. The trigger
signal is directly detected at the trigger detection circuit for the trigger inputs, while all the analog
signals go into ADC channels and they are filtered to the desired frequency span. Thus, delay is
introduced. Fortunately, our digital filters are all linear phase FIR filters that adds a constant time
shift independent of the signal frequency, so they can be corrected mathematically. The timestamps
associated to the data samples are already corrected for delays introduced by analog and digital anti-
aliasing filters. However, there are other phase corrections that the users have to consider, depending
on the test setup and the accuracy requirement.
S
UB
-
SAMPLE
T
RIGGER
D
ELAY
When the measurement is triggered by a trigger signal sent directly to the front panel trigger
connector, or backplane trigger line, the trigger event occurs asynchronous to the ADC sampling
clock. This trigger event time is measured by the timestamp clock and recorded, and returned to the
user. This information can be obtained by parsing
AdditionalData string returns at the
Measurement . Read (or MemoryRead for streaming) method. The Read method also returns the
timestamp of the first sample in the data record. The difference between the trigger timestamp and
the data timestamp can be used to correct for more accurate phase measurement.