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SYSTEM CONFIGURATIONS:
There are two basic types of impulse response measurement system. One processes the
received signal in the digital domain (digital sampling probe), the other processes it in the
analog domain (analog sliding correlator probe).
- Digital sampling probes:
These measurement systems digitize the received signal at an IF frequency and then
process the information in the digital domain. The advantages of these systems over the
analog system are several fold. The biggest advantage is that each impulse generated
represents the channel characteristics over a very short period of time, usually a few tens
of microseconds. Thus, the channel has very little chance to change during the time of
the impulse acquisition. The other advantages go along with all the benefits of digital
processing, namely increased precision of downconversion to baseband, unique
processing capabilities, system flexibility, greater stability with changes in time and
temperature, and the ability to post-process the data.
- Analog sliding correlator probe:
The analog probe processes the received signal using conventional analog processing
techniques. The impulse is produced by mixing (multiplying) a local copy of the PN
code with the received signal and then integrating (summing) the output. The local PN
code has a slightly different bit rate than the transmitted code so that the received code
and the local code slide past each other as they are being multiplied and summed, thus
producing an analog correlation. While this system has, in the past, been used for
wide-bandwidth, high-frequency measurements, it has been replaced by the more
flexible wide-bandwidth digital probe.
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