HP 8753E manual General Theory

Time domain low pass impulse mode simulates the time domain response of an impulse input (like the bandpass mode). Both low pass modes yield better time domain resolution for a given frequency span than does the bandpass mode. In addition, when using the low pass modes, you can determine the type of discontinuity. However, these modes have certain limitations that are defined in “Time domain low pass, n later in this section.

The analyzer has one time-to-frequency transform mode:

Forward transform mode transforms CW signals measured over time into the frequency domain, to measure the spectral content of a signal. This mode is known as the CW time mode.

In addition to these transform modes, this section discusses special transform concepts such as masking, windowing, and gating.

General Theory

The relationship between the frequency domain response and the time domain response of the analyzer is defined by the Fourier transform. Because of this transform, it is possible to measure, in the frequency domain, the response of a linear test device and mathematically calculate the inverse Fourier transform of the data to find the time domain response. The analyzer’s internal computer makes this calculation using the chirp-Z Fourier transform technique. ‘Ihe resulting measurement is the fully error-corrected time domain reflection or transmission response of the test device, displayed in near real-time.

Figure 6-62 illustrates the frequency and time domain reflection responses of a test device. The frequency domain reflection measurement is the composite of ail the signals reflected by the discontinuities present in the test device over the measured frequency range.

Note In this section, all points of reflection are referred to as discontinuities.

(a) Frequency Domain (b) Time Domain Bandpass

Figure 6-62. Device Frequency Domain and Time Domain Reflection Responses

The time domain measurement shows the effect of each discontinuity as a function of time (or distance), and shows that the test device response consists of three separate impedance changes. The second discontinuity has a reflection coefllcient magnitude of 0.035 (i.e. 3.5% of the incident signal is reflected). Marker 1 on the time domain trace shows the elapsed time

6-126 Application and Operation Concepts