QWhat are the general criteria for selecting one type of RF detector over
AThe type of RF signal to be measured is the most important determining factor
in the type of detector to use. For most general power measurement and control
applications, the log type is the most useful. For pulsed RF signals, the log
type is also best because of the fast response times available. In those
applications where the signal has a high crest factor or a widely varying crest
factor, the rms type
is generally better.
The crest factor is the ratio of the peak to rms value of the signal. For
example, higher-order quadrature amplitude modulation (QAM) signals (e.g.,
16QAM, 64QAM,and 256QAM) have high crest factors. In the case of
spread-spectrum signals such as those used in CDMA and WCDMA cellular systems
and orthogonal frequency-division multiplexing (OFDM) signals such as WiMAX and
WiBro, the high crest factor (typically 10 to 13 dB) will change dynamically.
In such applications, an RMS detector is generally more desirable.