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How to perform fft

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Efstathios Kontolatis
回答： Rik 2017-2-23
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
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0.2426
0.3169
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0.4129
0.4839
0.3578
0.3550
0.4090
0.4063
0.4497
0.5195
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0.4514
0.4375
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0.5263
0.4195
0.3746
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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
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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
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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
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0.3841
0.2781
0.4447
0.5256
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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
  5 个评论
显示 3更早的评论
Adam
Adam 2017-2-23
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
Efstathios Kontolatis 2017-2-23
Yes indeed that worked. Thank you very much. I would have accepted the answer if I knew how to do it.

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采纳的回答

Rik
Rik 2017-2-23
[moved from comments]
To remove the 0Hz-component from the analysis, use y=fft(x-mean(x));

更多回答（1 个）

Pooja Patel
Pooja Patel 2017-2-23
  • 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);

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