HP 8753E manual Transforming CW Time Measurements Into the Frequency Domain

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Figure 6-76. Gate Shape

Selecting gate shape. The four gate shapes available are listed in lhble 6-13. Each gate has a different passband flatness, cutoff rate, and sidelobe levels.

‘able 6-13. Gate Characteristics

Gate	Passband	Sidelobe	cntoff	Minimnm
saspe	Ripple	Levels		Gate span
Gate Span Minimum	fO.10 dJ3	-48dB	1.4IFreq Span	Z.S/Freq Span
Nod	• 0.01dB	-68 dE3	Z.S/Freq Span	6.6/l%eq Span
Wide	fO.O1 dF3	-57 dB	4.4fFreq Span	S.S/Freq Span
Maximum	fO.O1 dB	-70 dB	12.7IFreq Span	25.4lFYeq Span

The passband ripple and sidelobe levels are descriptive of the gate shape. The cutoff time is the time between the stop time (-6 dB on the Alter skirt) and the peak of the f&t sidelobe, and

is equal on the left and right side skirts of the Inter. Because the minimum gate span has no passband, it is just twice the cutoff time. Always choose a gate span wider than the minimum. For most applications, do not be concerned about the minimum gate span, simply use the knob to position the gate markers around the desired portion of the time domain trace.

Transforming CW Time Measurements Into the Frequency Domain

The analyzer can display the amplitude and phase of CW signals versus time. For example, use this mode for measurements such as amplifier gain as a function of warmup time (i.e. drift). The analyzer can display the measured parameter (e.g. amplifier gain) for periods of up to

24 hours and then output the data to a digital plotter for hardcopy results.

These “strip chart” plots are actually measurements as a function of time (time is the independent variable), and the horizontal display axis is scaled in time units. Transforms of these measurements result in frequency domain data. Such transforms are called forward transforms because the transform from time to frequency is a forward Fourier transform, and can be used to measure the spectral content of a CW signal. For example, when transformed into the frequency domain, a pure CW signal measured over time appears as a single frequency spike. The transform into the frequency domain yields a display that looks similar to a spectrum analyzer display of signal amplitude versus frequency.

6-142 Application and Operation Concepts