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4.3  Feedback bandwidth limitations

4.3.1  Feedback bandwidth

Fig. 4.22  Sensor noise effect at system's output

In Section 4.3 we will discuss physical constrains on the high-frequency loop gain. However, first we need to clarify the definitions of the term feedback bandwidth. In the literature and in the professional language of control engineers, this term may have any of the following three interpretations indicated in Fig. 4.21:

1. The crossover frequency f_b , i.e., the bandwidth of the loop gain exceeding 0 dB. In this book, this definition for feedback bandwidth is accepted.
2. The frequency f_M where |M| = 1, i.e., 20 log |M|  -3 dB. This frequency is the tracking system 3 dB bandwidth. The frequency is typically from 1.3f_b to 1.7f_b.
3. The frequency up to which the loop gain retains a specified value (e.g., the bandwidth of 30 dB feedback). This bandwidth is also called the bandwidth of functional feedback.