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Tek Exponential Window
The Tek Exponential window was invented at Tektronix. In the time domain, it is not a
symmetrical bell shape as is the case with the other windows. Instead, it is exponential with a peak
at the 20% position of the time domain gate. The frequency domain shape is triangular. Use this
window for impulse-response testing where the 20% position is the zero phase reference point.
More of the acquired data record length is used to capture the impulse response. Exact details of
how to compute its values were published in the article; Impulse-response testing lets a single test
do the work of thousands by John Pickerd in EDN magazine, April 27, 1995, page 95.
Effects of Trigger Jitter
The instrument acquisition system has a sample clock that is asynchronous with respect to the
input signal. This means that from one acquisition to the next, samples may be in a different
position on the waveform with respect to the trigger. Samples may vary in position by up to one
sample interval.
There are only two samples per cycle of a signal that has a frequency equal to one half of the
sample rate. This is the highest nonaliased signal that can be output from the spectral analyzer.
Thus, at this frequency, one sample of acquisition jitter will show up in the spectrum as 180
degree phase variations. Therefore, when you analyze phase, make sure to oversample the signal
sufficiently such that the one sample interval of jitter that is showing up in the phase is acceptable
to the accuracy you want to achieve.
Effects of Average and High Res acquisition modes
The result of averaging the time domain acquisition using either Average mode or Hi Res
acquisition mode affects the frequency response of the instrument. This is due to the one sample of
jitter in the acquisition system. Both Hi Res and Average acquisition modes have the same affect
on the frequency response. These modes cause the response to roll off from a magnitude value of
one at DC to a magnitude value of 0.63 at Nyquist, which is the frequency equal to one half of the
sample rate. This is true regardless of the real time sample rate setting.
Frequency Domain Averaging
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