HP 8530A, Руководство по эксплуатации и программированию, Страница: 234 / 439

Time Domain RCS Measurements
Measurement Errors and Calibration
Measurement errors are caused by undesired range reections , dierent electrical
characteristics between the test and reference channels , and other causes . This section
discusses the causes of common measurement errors , and explains how calibration is used to
reduce these errors .
Time Domain Noise Floor Reduction
One advantage of the HP 8530A Time Domain conversion technique is that it reduces receiver
noise in the Time Domain, proportional to the Number of P oints measured in the Frequency
Domain. The Time Domain noise is lower by 10 log (Number of P oints), which greatly increases
the Time Domain dynamic range .
F or example , for a measurement at 801 frequency points , converting to the Time Domain will
reduce the receiver noise by 29 dB! (This is also known as \processing gain.") Because of this ,
most RCS measurements will be limited by residual chamber clutter rather than receiver noise .
Eects of RCS Calibration on Time Domain Responses
RCS calibration reduces the eects of systematic measurement errors (those that are stable and
repeatable), and displays the RCS of the target in dBsm (dB relative to a square meter). RCS
calibration aects both Frequency and Time Domain data. Refer to the two-term error model
shown in Figure 13-5.
Figure 13-5. RCS Isolation and Response Error Model
Isolation Error T erm (Range Clutter). The RCS Isolation error term accounts for RCS \range
clutter ." This is caused by leakage between the transmit and receive antennas and by spurious
reections within the anechoic chamber . These signals arrive in parallel with the target
responses , and they limit the receiver's ability to measure small RCS targets .
Frequency Response Error T erm. The Frequency Response error term accounts for electrical
dierences between the test and reference paths , caused by the test system. The signals in the
test and reference channels do not track each other perfectly because the cables , connectors ,
antennas , and other components in the test and reference paths are not identical. This causes
measurement errors because the receiver measures the response of the test setup along with
the performance of the device under test. Frequency response errors are the dierences in
phase and amplitude (between the test and reference paths), caused by the test setup itself.
Introduction to Time Domain RCS and
Antenna Measurements
13-9