4.2 Frequency Resolution

Each filter is designed to pass only one small range of frequencies onto the detector. Thus, each filter/detector/meter combination displays the energy present over that particular range of frequencies. If two frequencies are present within the same filter, both of them will affect the meter reading. (The exact meter reading will depend on the type of detector used.) The analyzer cannot resolve two signals in the same filter; thus the filter bandwidth, BW _RES, determines the frequency resolution of the analyzer. BW _RES is called the resolution bandwidth.

For example, consider Figure 4-3. Frequency components f ₁ and f ₂ appear in the passband of the same filter. Therefore, they cannot be resolved (in frequency). Frequency components f ₃ and f ₄, on the other hand, do not appear within the same filter and each will be measured individually. The frequency of f ₃ and f ₄ are known to the extent that they are within the passband of their respective filters. Thus, their frequencies are known to within BW _RES.

Figure 4-3: The bank-of-filters analyzer response shown here with some representative spectral lines.

Assuming extremely sharp filters (with steep skirts) precisely positioned such that the edges of their passbands just touch, the resolution bandwidth of a bank-of-filters analyzer is given by

(4-1)

where

This equation can be used to explain the major limitation of...