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Course by: Robert Nowak. E-mail the author

# Zero Locations of Linear-Phase FIR Filters

Module by: Ivan Selesnick. E-mail the author

Summary: (Blank Abstract)

## Zero Locations of Linear-Phase Filters

The zeros of the transfer function Hz H z of a linear-phase filter lie in specific configurations.

We can write the symmetry condition hn=hN(1n) h n h N 1 n in the ZZ domain. Taking the ZZ-transform of both sides gives

Hz=z(N1)H1z H z z N 1 H 1 z
(1)
Recall that we are assuming that hn h n is real-valued. If z 0 z 0 is a zero of Hz H z , H z 0 =0 H z 0 0 then H z 0 *=0 H z 0 0 (Because the roots of a polynomial with real coefficients exist in complex-conjugate pairs.)

Using the symmetry condition, Equation 1, it follows that H z 0 =z(N1)H1 z 0 =0 H z 0 z N 1 H 1 z 0 0 and H z 0 *=z(N1)H1 z 0 *=0 H z 0 z N 1 H 1 z 0 0 or H1 z 0 =H1 z 0 *=0 H 1 z 0 H 1 z 0 0

### note:

If z 0 z 0 is a zero of a (real-valued) linear-phase filter, then so are z 0 * z 0 , 1 z 0 1 z 0 , and 1 z 0 * 1 z 0 .

## ZEROS LOCATIONS

It follows that

1. generic zeros of a linear-phase filter exist in sets of 4.
2. zeros on the unit circle ( z 0 =ej ω 0 z 0 ω 0 ) exist in sets of 2. ( z 0 ±1 z 0 ± 1 )
3. zeros on the real line ( z 0 =a z 0 a ) exist in sets of 2. ( z 0 ±1 z 0 ± 1 )
4. zeros at 1 and -1 do not imply the existence of zeros at other specific points.

## ZERO LOCATIONS: AUTOMATIC ZEROS

The frequency response H f ω H f ω of a Type II FIR filter always has a zero at ω=π ω : hn= h 0 h 1 h 2 h 2 h 1 h 0 h n h 0 h 1 h 2 h 2 h 1 h 0 Hz= h 0 + h 1 z-1+ h 2 z-2+ h 2 z-3+ h 1 z-4+ h 0 z-5 H z h 0 h 1 z -1 h 2 z -2 h 2 z -3 h 1 z -4 h 0 z -5 H-1= h 0 h 1 + h 2 h 2 + h 1 h 0 =0 H -1 h 0 h 1 h 2 h 2 h 1 h 0 0 H f π=Hejπ=H-1=0 H f H H -1 0

### rule:

H f π=0 H f 0 always for Type II filters.
Similarly, we can derive the following rules for Type III and Type IV FIR filters.

### rule:

H f 0= H f π=0 H f 0 H f 0 always for Type III filters.

### rule:

H f 0=0 H f 0 0 always for Type IV filters.
The automatic zeros can also be derived using the characteristics of the amplitude response Aω A ω seen earlier.

Table 1
Type automatic zeros
I
II ω=π ω
III ω=0 or π ω 0
IV ω=0 ω 0

## ZERO LOCATIONS: EXAMPLES

The Matlab command zplane can be used to plot the zero locations of FIR filters.

Note that the zero locations satisfy the properties noted previously.

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