Time domain low pass

This mode is used to simulate a traditional time domain reflectometry (TDR) measurement. It provides information to determine the type of discontinuity (resistive, capacitive, or inductive) that is present. Low pass provides the best resolution for a given bandwidth in the frequency domain. It may be used to give either the step or impulse response of the test device.

The low pass mode is less general-purpose than the bandpass mode because it places strict limitations on the measurement frequency range. The low pass mode requires that the frequency domain data points are harmonically related from dc to the stop frequency. That is, stop = n x start, where n = number of points For example, with a start frequency of 30 kHz and 101 points, the stop frequency would be 3.03 MHz. Since the analyzer frequency range starts at 30 kHz, the dc frequency response is extrapolated from the lower frequency data. The requirement to pass dc is the same limitation that exists for traditional TDR.

Setting frequency range for time domain low pass

Before a low pass measurement is made, the measurement frequency range must meet the

(stop = n x &,&) reqdement described above. me ~~~~~~~~~~~~~~

softkey perfoms

....i.. .<:.:. .:..:.: /.~.:,.>;;.:: ..........;;.:I . . ..T :. :..,.... . . .

 

this function automatically: the stop frequency is set close to the entered stop frequency, and the start frequency is set equal to stop/n.

If the low end of the measurement frequency range is critical, it is best to calculate

approximate v&es for the St& ad stop frequencies before pressing ,~~~~~~~~~~~,

and calibrating. This avoids distortion of the measurement results. ‘Ib see an example, select the preset values of 201 points and a 300 kHz to 3 GHz frequency range. Now press

~~~~~~~~~~~~ad observe me change h frequency values. me stop frequency changes _/.~~~~.....................>.z.....A.... ...z............... .._........................~.~~.. .....A.ii;;>........; ....Ai.,.,....i:>;::..

to 2.999 GHz, and the start frequency changes to 14.925 MHz. This would cause a distortion of measurement results for frequencies from 300 kHz to 14.925 MHz.

Note If the start and stop frequencies do not conform to the low pass requirement before a low pass mode (step or impulse) is selected and transform is turned on, the analyzer resets the start and stop frequencies. If error correction is on when the frequency range is changed, this turns it off.

‘Ibble 6-11. Minimum Frequency Ranges for Time Domain Low Pass

Number of Points

Minimnm F-4-w m

3

30kHztQ0.OfIMHz

11

3OkHzto0.33MHz

26

30kHzto0.78MHz

51

3OkHzto1.53MHz

101

30kHzto3.03 MHz

201

30 kHzti 6.03 MHz

401

3OkHzto12.03MHz

801

30kHzto24.03 MHz

1601

3OkHzto43.03MHz

 

 

6-130 Application and Operation Concepts