How to perform fft

I have 1000 samples from an experiment with frequency Fs=67890 Hz. How can I perform fft on them? I followed the guide here https://uk.mathworks.com/help/matlab/ref/fft.html but it seems that the dominant frequency is zero which has no physical meaning.

These are my data

x =

    0.3551
    0.2308
    0.3209
    0.4527
    0.4606
    0.4511
    0.4925
    0.4769
    0.5424
    0.4698
    0.4246
    0.4502
    0.5823
    0.5451
    0.4235
    0.4062
    0.3832
    0.2749
    0.1591
    0.1889
    0.4155
    0.3840
    0.3582
    0.1852
    0.2677
    0.2454
    0.2458
    0.2100
    0.1469
    0.1186
         0
    0.4154
    0.4807
    0.4320
    0.4072
    0.3352
    0.3357
    0.2770
    0.2801
    0.3578
    0.2703
    0.3626
    0.0998
    0.3656
    0.2590
    0.4388
    0.4144
    0.2631
         0
    0.1076
         0
    0.4535
    0.4626
    0.4159
    0.3686
    0.4763
    0.2582
    0.2961
    0.3691
    0.3860
    0.3875
    0.4018
    0.4292
    0.3921
    0.3128
    0.4884
    0.3153
    0.2672
    0.3448
    0.3787
    0.4799
    0.3870
    0.3534
    0.3968
    0.3006
    0.3119
    0.3585
    0.1352
    0.4154
    0.3323
    0.3733
    0.3232
    0.4116
    0.3276
    0.4852
    0.3715
    0.3991
    0.3766
    0.4866
    0.3483
    0.2736
    0.3153
    0.4049
    0.3774
    0.3071
    0.3831
    0.3992
    0.3661
    0.3337
    0.1616
    0.3305
    0.4556
    0.5053
    0.4209
    0.2868
    0.2666
    0.3057
    0.4016
    0.2579
    0.4286
    0.1672
    0.4614
    0.3814
    0.4272
    0.3374
    0.4215
    0.4788
    0.3943
    0.4097
    0.3937
    0.4230
    0.4981
    0.4821
    0.1748
    0.4015
    0.5066
    0.4959
    0.4267
    0.4692
    0.3354
    0.2919
    0.5676
    0.4875
    0.4957
    0.4122
    0.5627
    0.4573
    0.3724
    0.4320
    0.4127
    0.3655
    0.3132
    0.1982
    0.2905
    0.3757
    0.5282
    0.4584
    0.4669
    0.4059
    0.3229
    0.4696
    0.3960
    0.5024
    0.4505
    0.4084
    0.4720
    0.4251
    0.3683
    0.3791
    0.3650
    0.2426
    0.3169
    0.4405
    0.4129
    0.4839
    0.3578
    0.3550
    0.4090
    0.4063
    0.4497
    0.5195
    0.4645
    0.4514
    0.4375
    0.3405
    0.5263
    0.4195
    0.3746
    0.2887
    0.4121
    0.3987
    0.4428
    0.4065
    0.3340
    0.3511
    0.3328
    0.3698
    0.4988
    0.3478
    0.2817
    0.2795
    0.4926
    0.3976
    0.3728
    0.4816
    0.4690
    0.4328
    0.6150
    0.1455
    0.3981
    0.3184
    0.4321
    0.3678
    0.3407
    0.2930
    0.3325
    0.5747
    0.5205
    0.4418
    0.4604
    0.3597
    0.3404
    0.3153
    0.5779
    0.3666
    0.3215
    0.2842
    0.2314
    0.2940
    0.3745
    0.3215
    0.2711
    0.3004
    0.3946
    0.3942
    0.3256
    0.2587
    0.3177
    0.2474
    0.2057
    0.4025
    0.4435
    0.4262
    0.3123
    0.3033
    0.3595
    0.3224
    0.5162
    0.5210
    0.5185
    0.5082
    0.5174
    0.4634
    0.4224
    0.5525
    0.4637
    0.5132
    0.5806
    0.3519
    0.5952
    0.5103
    0.4021
    0.3890
    0.3671
    0.5863
    0.3946
    0.3198
    0.1014
    0.4934
    0.4089
    0.5890
    0.4601
    0.5628
    0.5392
    0.4553
    0.4755
    0.5468
    0.4478
    0.4900
    0.3323
    0.2200
    0.4340
    0.4119
    0.4075
    0.3577
    0.5101
    0.3585
    0.3939
    0.4366
    0.3738
    0.3934
    0.4416
    0.4464
    0.3510
    0.3791
    0.4289
    0.3966
    0.3113
    0.2998
    0.4251
    0.4033
    0.3393
    0.3843
    0.4246
    0.4224
    0.4072
    0.2900
    0.4400
    0.5314
    0.4580
    0.4382
    0.4118
    0.4298
    0.5275
    0.4492
    0.4100
    0.4098
    0.4530
    0.4531
    0.4177
    0.5175
    0.2001
    0.5330
    0.4534
    0.4613
    0.0637
    0.4619
    0.5318
    0.4129
    0.3292
    0.3293
    0.4428
    0.3560
    0.4558
    0.3736
    0.2481
    0.3881
    0.3586
    0.3284
    0.0465
    0.3070
    0.4227
    0.3891
    0.3911
    0.5650
    0.3529
    0.3481
    0.3482
    0.3682
    0.5319
    0.5387
    0.1824
    0.3062
    0.4315
    0.4625
    0.3685
    0.5253
    0.4801
    0.5584
    0.4634
    0.5326
    0.4494
    0.4534
    0.4064
    0.3226
    0.1444
    0.4603
    0.4277
         0
    0.3656
    0.4511
    0.5926
    0.4544
    0.4301
    0.3542
    0.3607
    0.3684
    0.4694
    0.5180
    0.3940
    0.4657
    0.3901
    0.4060
    0.3740
    0.3351
    0.3571
    0.3845
    0.3225
    0.4296
    0.3675
    0.4469
    0.3926
    0.3571
    0.3877
    0.2835
    0.4564
    0.4695
    0.3038
    0.4322
    0.3454
    0.4157
    0.4131
    0.3656
    0.3244
    0.3835
    0.3835
    0.3669
    0.3769
    0.3392
    0.4072
    0.4156
    0.4026
    0.4092
    0.3624
    0.4615
    0.3921
    0.4848
    0.4077
    0.2904
    0.3404
    0.3485
    0.4472
    0.4097
    0.3488
    0.3555
    0.2958
    0.1905
    0.2594
    0.5082
    0.3526
    0.5096
    0.2486
    0.3777
    0.3662
    0.4036
    0.4170
    0.4132
    0.4760
    0.4813
    0.2767
    0.4714
    0.3762
    0.3883
    0.2067
    0.1974
    0.3166
    0.3852
    0.2576
    0.3949
    0.2443
    0.3779
    0.4300
    0.3881
    0.3786
    0.3516
    0.4147
    0.3850
    0.4277
    0.4620
    0.4737
    0.4113
    0.3448
    0.3532
    0.3431
    0.2336
    0.4660
    0.4304
    0.4478
    0.2664
    0.3472
    0.3404
    0.3530
    0.5004
    0.4685
    0.4902
    0.5056
    0.4876
    0.3388
    0.3673
    0.4873
    0.3627
    0.3553
    0.3385
    0.3725
    0.5111
    0.4345
    0.3356
    0.3316
    0.3864
    0.3736
    0.3033
    0.4409
    0.4224
    0.3873
    0.3507
    0.3317
    0.3222
    0.2853
    0.3617
    0.4143
    0.4293
    0.3870
    0.3259
    0.4120
    0.3762
    0.3981
    0.4022
    0.3711
    0.3616
    0.4801
    0.3860
    0.2593
    0.5820
    0.4110
    0.4032
    0.4109
    0.3933
    0.4776
    0.2430
    0.4151
    0.4863
    0.3633
    0.1881
    0.1723
    0.4596
    0.3971
    0.3804
    0.4301
    0.2390
    0.4319
    0.3753
    0.4073
    0.4224
    0.4255
    0.4830
    0.3504
    0.3461
    0.1993
    0.4117
    0.4678
    0.4710
    0.3577
    0.3979
    0.3993
    0.3446
    0.3214
    0.3113
    0.3695
    0.3847
    0.4664
    0.4420
    0.3579
    0.5084
    0.4741
    0.4416
    0.4036
    0.3741
    0.4747
    0.5657
    0.4787
    0.4972
    0.3841
    0.2781
    0.4447
    0.5256
    0.4557
    0.4701
    0.4399
    0.3622
    0.3493
    0.3782
    0.3784
    0.6737
    0.5224
    0.4507
    0.2935
    0.4661
    0.3368
    0.3713
    0.4094
    0.3704
    0.4510
    0.3874
    0.4808
    0.3836
    0.4163
    0.2954
    0.4038
    0.3723
    0.3454
    0.3572
    0.2956
    0.3123
    0.3045
    0.3775
    0.3586
    0.3899
    0.3283
    0.2579
    0.3975
    0.3386
    0.3333
    0.3667
    0.2439
    0.3291
    0.4948
    0.4187
    0.4469
    0.3125
    0.2881
    0.1765
    0.3667
    0.4266
    0.4227
    0.4985
    0.3694
    0.3063
    0.3647
    0.3031
    0.4227
    0.4508
    0.3426
    0.2608
    0.3380
    0.4410
    0.2822
    0.3007
    0.2079
    0.3175
    0.2548
    0.2257
    0.2664
    0.2629
    0.3153
    0.2829
    0.1878
    0.2932
    0.4240
    0.3506
    0.3450
    0.3436
    0.3147
    0.4307
    0.3297
    0.3263
    0.2626
    0.3670
    0.3903
    0.3504
    0.3635
    0.3506
    0.3645
    0.3349
    0.3742
    0.4376
    0.3087
    0.1669
    0.5031
    0.4398
    0.3169
    0.1251
    0.3737
    0.4122
    0.3529
    0.3419
    0.3728
    0.3230
    0.3516
    0.3272
    0.4056
    0.4307
    0.4187
    0.3042
    0.3735
    0.3499
    0.4240
    0.1846
    0.2853
    0.2608
    0.3536
    0.3915
    0.4461
    0.4830
    0.4267
    0.2480
    0.4508
    0.1829
    0.2214
    0.3592
    0.4563
    0.2695
    0.3125
    0.2981
    0.4959
    0.3519
    0.1361
    0.3236
    0.3682
    0.3274
    0.4352
    0.3589
    0.3794
    0.3441
    0.4345
    0.3739
    0.3811
    0.3532
    0.3125
    0.4182
    0.2854
    0.3541
    0.3988
    0.4035
    0.3540
    0.3104
    0.4531
    0.5163
    0.5809
    0.3362
    0.4588
    0.4724
    0.4871
    0.4134
    0.4033
    0.3325
    0.4309
    0.3734
    0.3137
    0.3562
    0.4370
    0.2704
    0.3935
    0.3315
    0.3020
    0.3531
    0.2427
    0.3931
    0.3654
    0.3365
    0.5205
    0.3245
    0.6086
    0.4521
    0.3837
    0.4901
    0.3527
    0.4278
    0.2909
    0.3649
    0.3479
    0.2947
    0.5558
    0.4566
    0.5902
    0.4304
    0.5311
    0.5395
    0.3745
    0.5311
    0.3001
    0.4030
    0.4117
    0.3925
    0.4652
    0.3820
    0.2739
    0.4634
    0.3541
    0.3096
    0.3282
    0.3180
    0.2612
    0.2147
    0.4373
    0.4462
    0.4324
    0.4857
    0.2976
    0.3247
    0.3276
    0.3106
    0.5885
    0.5510
    0.3492
    0.3284
    0.4325
    0.4530
    0.5664
    0.5522
    0.4787
    0.4568
    0.4210
    0.5093
    0.4775
    0.4069
    0.4151
    0.4295
    0.4312
    0.3926
    0.3863
    0.3583
    0.4121
    0.3848
    0.3773
    0.3826
    0.3374
    0.3023
    0.3368
    0.4261
    0.2167
    0.4879
    0.3032
    0.2540
    0.5302
    0.4484
    0.4872
    0.3173
    0.3800
    0.4337
    0.3698
    0.3272
    0.2498
    0.3854
    0.4042
    0.4299
    0.4018
    0.3248
    0.3756
    0.3824
    0.4029
    0.4295
    0.3573
    0.3036
    0.0557
    0.4097
    0.5186
    0.4060
    0.3733
    0.2700
    0.4013
    0.2437
    0.4369
    0.3374
    0.3853
    0.4096
    0.3145
    0.3664
    0.4738
    0.2346
    0.3548
    0.2804
    0.4698
    0.4039
    0.4628
    0.4387
    0.3089
    0.3981
    0.4727
    0.4335
    0.3591
    0.4623
    0.3922
    0.4100
    0.3585
    0.4101
    0.3834
    0.2742
    0.2886
    0.4118
    0.4812
    0.4434
    0.4607
    0.3134
    0.0859
    0.1066
    0.3441
    0.2788
    0.3310
    0.4330
    0.3551
    0.4324
    0.4427
    0.3585
    0.4497
    0.1920
    0.3622
    0.4184
    0.4762
    0.4427
    0.4545
    0.4054
    0.4440
    0.3977
    0.5034
    0.5101
    0.3951
    0.5061
    0.4242
    0.4591
    0.5080
    0.4194
    0.6229
    0.3667
    0.4874
    0.4718
    0.4996
    0.2885
    0.4989
    0.5071
    0.4529
    0.5001
    0.4165
    0.4620
    0.4430
    0.3566
    0.3709
    0.4315
    0.4694
    0.3501
    0.3343
    0.4227
    0.3484
    0.3737
    0.1854
    0.4691
    0.4328
    0.4059
    0.4462
    0.4397
    0.3578
    0.3274
    0.4586
    0.4864
    0.5225
    0.3509
    0.4212
    0.4003
    0.4854
    0.1942
    0.4785
    0.4362
    0.4213
    0.4979
    0.4989
    0.3758
    0.4904
    0.6655
    0.4860
    0.4498
    0.4712
    0.3502
    0.3666
    0.3871
    0.5061
    0.3993
    0.2872
    0.3147
    0.3531
    0.4126
    0.4546
    0.4136
    0.4674
    0.4634
    0.4877
    0.4136
    0.3401
    0.4442
    0.3997
    0.3753
    0.4675
    0.3769
    0.3556
    0.3799
    0.5048
    0.3805
    0.4656
    0.4621
    0.3986
    0.2977
    0.3280
    0.4630
    0.4375
    0.3109
    0.3265
    0.4582
    0.4432
    0.3801
    0.4558
    0.4408
    0.4279
    0.3974
    0.3856
    0.4107
    0.4463
    0.4646
    0.3674
    0.4938
    0.3389
    0.4625
    0.3187
    0.3233
    0.4389
    0.3224
    0.3140
    0.4371
    0.3664
    0.4664
    0.4350
    0.4211
    0.3415

and here is the code I have used for the fft

fs=67890;
T = 1/fs;             % Sampling period
L = 1000;             % Length of signal
t = (0:L-1)*T;        % Time vector
y = fft(x);
P2 = abs(y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = fs*(0:(L/2))/L;
subplot(2,1,1), plot(t,x),title('original data'),ylabel('x'),xlabel('t')
subplot(2,1,2), plot(f,P1),title('fft'),ylabel('magnitude'),xlabel('frequency')

This gives me this image

Hope it's clear

[Rik]

To get rid of the 0Hz component, you can subtract the mean from your data, maybe that helps.

How does the result you are getting not work? Differently put: you ask how to perform fft, why is the answer 'with the function fft' incorrect?

[Adam]

The 0-frequency component is simply a measure of the average of your data and suggests it is not very 0-centred. If you have perfectly 0-centred data then the 0-frequency component will also be 0.

You haven't shown any example of what you have done though or what your data looks like so there's not much else to say since you have clearly looked at the fft documentation already and followed an example there.

[Efstathios Kontolatis]

I have edited the question hope now it's more clear.

[Adam]

So just do what Rik Wisselink suggests to zero-centre your data or simply remove the 0-frequency component from the final result and plot it without if you just want to look at the frequency spectrum.

[Efstathios Kontolatis]

Yes indeed that worked. Thank you very much. I would have accepted the answer if I knew how to do it.

Answer 1

[Rik]

[moved from comments]

To remove the 0Hz-component from the analysis, use y=fft(x-mean(x));

Answer 2

[Pooja Patel]

• amp1 = abs(fft(x1)); %Retain Magnitude
• % amp11 = amp1(1:Nsamps1/2); %Discard Half of Points
• % f11 = Fs*(0:Nsamps1/2-1)/Nsamps1; %Prepare freq data for plot
• f11 = 0:(fs1/Nsamps1):1000; %Prepare freq data for plot
• amp11 = amp1(1:length(f11)); % keep data till 1kHz
• plot(f11,amp11);