Figure
2.26 shows a family of
feedback
combfilter
amplitude responses, obtained using a selection of feedback
coefficients.
Figure:
Amplitude response of the feedback
combfilter
(Fig.2.24 with and
) with and , , and . a) Linear
amplitude scale. b) Decibel scale.

Figure
2.27 shows a similar family obtained using
negated feedback coefficients; the opposite sign of the feedback
exchanges the peaks and valleys in the amplitude response.
Figure:
Amplitude response of the phaseinverted feedback combfilter, i.e., as in Fig.2.26 with negated
, , and .
a) Linear amplitude scale. b) Decibel scale.

As introduced in §
2.6.2 above, a class of feedback comb
filters can be defined as any
difference equation of the form
Taking the
z transform of both sides and solving for
,
the
transfer function of the feedback comb filter is found to be

(3.5) 
so that the amplitude response is
This is plotted in Fig.
2.26 for
and
,
, and
. Figure
2.27 shows the same case but with the feedback
signinverted.
For
, the feedbackcomb amplitude response
reduces to
and for
to
which exactly inverts the amplitude response of the
feedforward
comb filter with gain
(Eq.
(
2.4)).
Note that
produces resonant peaks at
while for
, the peaks occur midway between these values.
Next Section: FilteredFeedback Comb FiltersPrevious Section: Feedforward Comb Filter Amplitude Response