FIR lowpass filter Design with cutoff frequency 77e9

Dear All,

i am trying to design a FIR lowpass filter with cutoff frequency of 77e9 but i always ave unknown in the answer. Can anyone help me please?

Fc= 77e9; % cut off frequency

N= 120; % the filter order

Fs=2*Fc; % sampling frequency 80e9

Fp=8e3; % pass-band edge frequency 8e9

Ap=0.01; % pass-band ripple [dB]

Ast=80; % stop-band attenuation [dB]

% obtain the maximum deviation for the passband and stopband ripples in linear units.

Rp = (10^(Ap/20) - 1)/(10^(Ap/20) + 1);

Rst = 10^(-Ast/20);

filt = fdesign.lowpass('N,Fc', N, Fc);

lp = design(filt,'window','window',@hamming,'systemobject',true);

If i am trying to use measure(lp) then i have the following answer:

ans =

Sample Rate : N/A (normalized frequency)

Passband Edge : Unknown

3-dB Point : 0.39335

6-dB Point : 0.40001

Stopband Edge : Unknown

Passband Ripple : Unknown

Stopband Atten. : Unknown

Transition Width : Unknown