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 While this system has, in the past, been used for wide–bandwidth, high–frequency measurements, it has been replaced by the more flexible wide–bandwidth digital probe.