HP 8530A, Руководство по эксплуатации и программированию, Страница: 227 / 439

Time Domain General Theory
Time Domain General Theory
In Frequency Domain measurements, a device's response to RF energy at CW or sweeping
frequencies is measured. A Time Domain measurement determines a devices response to a
specic waveform, as a function of time . An example is when bouncing an RF radar pulse o
an object and measuring the return RCS signal.
With \direct measurement" systems (such as time domain reectometers), Time Domain
measurements are made by sending a known waveform pulse (or impulse) out to the device
under test (DUT), and measuring the waveform returned as a function of time . The HP 8530A
does not measure time domain directly , since it is a frequency measuring device . However ,
any waveform can be mathematically formed by adding many frequencies together . Therefore ,
the receiver can measure DUT performance at various frequencies and then mathematically
calculate its Time Domain response . In most ways , the mathematical model is actually superior
to systems that measure Time Domain directly . Noise performance is usually much better ,
and the time axis is very stable and accurate . The calibration feature of the HP 8530A also
improves measurement performance , which is not possible in direct measurement systems .
The relationship between the Frequency Domain measurement and the Time Domain response
is described by the F ourier Transform:
Frequency Domain ! Time Domain
H(f) 0 ! h(t)
It is therefore possible to measure the response of an antenna or an RCS target in the
Frequency Domain and then mathematically calculate the inverse F ourier Transform of the
data to give the Time Domain response . The receiver does this calculation using Chirp-Z F ast
F ourier Transform (FFT) computation techniques . As explained later in this chapter , the
Chirp-Z FFT has advantages over standard FFT techniques .
13-2 Introduction to Time Domain RCS and
Antenna Measurements