t

i

i

i

ijiii i

i

i

 

I

WI STMaT-5 “a

 

 

 

 

STOP

I n.

 

 

 

 

(a)

Short Circuit

 

 

 

(b) Short Circuit at

 

 

 

 

 

 

 

 

the End of a 3 dB Pad

pg5194_c

Figure 6-70. Masking Example

Windowing

The analyzer provides a windowing feature that makes time domain measurements more useful for isolating and identifying individual responses Windowing is needed because of the abrupt transitions in a frequency domain measurement at the start and stop frequencies. The band limiting of a frequency domain response causes overshoot and ringing in the time domain response, and causes a non-windowed impulse stimuh~s to have a sin(kt)/kt shape, where

k = r/frequency span and t = time (see Figure 6-71). This has two effects that limit the usefulness of the time domain measurement:

nFinite impulse width (or rise time). F’inite impulse width limits the ability to resolve between two closely spaced responses. The effects of the Gnite impulse width cannot be improved without increasing the frequency span of the measurement (see ‘IMAe 6-12).

4

, WINDOWING

 

SIDELOBES

pg665d

 

Figure 6-71. Impulse Width, Sidelobes, and Windowing

nSidelobes. The impulse sidelobes limit the dynamic range of the time domain measurement by hiding low-level responses within the sidelobes of higher level responses. The effects of sidelobes can be improved by windowing (see ‘J%ble 6-12).

Windowing improves the dynamic range of a time domain measurement by altering the frequency domain data prior to converting it to the time domain, producing an impulse stimulus that has lower sidelobes. This makes it much easier to see time domain responses that are very different in magnitude. The sidelobe reduction is achieved, however, at the expense of increased impulse width. The effect of windowing on the step stimulus (low pass mode only) is a reduction of overshoot and ringing at the expense of increased rise time.

B-136 Application and Operation Concepts

Page 417
Image 417
HP 8753E manual Masking Example