Why is my FFT diagram has a slope approaching to zero?

Hello,

I don't know why am I not getting proper FFT diagram. I am getting a slope which goes to zero. Can someone help me?

Fs=1000;
T=1/Fs;
t1=readtable('t.xlsx')

t1 = 2×849 table
     Var1      Var2      Var3      Var4     Var5      Var6      Var7      Var8      Var9     Var10     Var11     Var12     Var13     Var14     Var15     Var16     Var17     Var18     Var19     Var20     Var21     Var22     Var23     Var24     Var25     Var26     Var27    Var28     Var29     Var30     Var31     Var32     Var33     Var34     Var35     Var36     Var37     Var38     Var39     Var40     Var41     Var42     Var43     Var44    Var45     Var46     Var47     Var48     Var49     Var50     Var51     Var52     Var53     Var54     Var55     Var56     Var57     Var58     Var59     Var60     Var61    Var62     Var63     Var64     Var65     Var66     Var67     Var68     Var69     Var70     Var71     Var72     Var73    Var74     Var75    Var76    Var77     Var78     Var79     Var80    Var81     Var82     Var83     Var84     Var85     Var86     Var87     Var88     Var89     Var90     Var91    Var92     Var93     Var94     Var95     Var96     Var97     Var98     Var99     Var100    Var101    Var102    Var103    Var104    Var105    Var106    Var107    Var108    Var109    Var110    Var111    Var112    Var113    Var114    Var115    Var116    Var117    Var118    Var119    Var120    Var121    Var122    Var123    Var124    Var125    Var126    Var127    Var128    Var129    Var130    Var131    Var132    Var133    Var134    Var135    Var136    Var137    Var138    Var139    Var140    Var141    Var142    Var143    Var144    Var145    Var146    Var147    Var148    Var149    Var150    Var151    Var152    Var153    Var154    Var155    Var156    Var157    Var158    Var159    Var160    Var161    Var162    Var163    Var164    Var165    Var166    Var167    Var168    Var169    Var170    Var171    Var172    Var173    Var174    Var175    Var176    Var177    Var178    Var179    Var180    Var181    Var182    Var183    Var184    Var185    Var186    Var187    Var188    Var189    Var190    Var191    Var192    Var193    Var194    Var195    Var196    Var197    Var198    Var199    Var200    Var201    Var202    Var203    Var204    Var205    Var206    Var207    Var208    Var209    Var210    Var211    Var212    Var213    Var214    Var215    Var216    Var217    Var218    Var219    Var220    Var221    Var222    Var223    Var224    Var225    Var226    Var227    Var228    Var229    Var230    Var231    Var232    Var233    Var234    Var235    Var236    Var237    Var238    Var239    Var240    Var241    Var242    Var243    Var244    Var245    Var246    Var247    Var248    Var249    Var250    Var251    Var252    Var253    Var254    Var255    Var256    Var257    Var258    Var259    Var260    Var261    Var262    Var263    Var264    Var265    Var266    Var267    Var268    Var269    Var270    Var271    Var272    Var273    Var274    Var275    Var276    Var277    Var278    Var279    Var280    Var281    Var282    Var283    Var284    Var285    Var286    Var287    Var288    Var289    Var290    Var291    Var292    Var293    Var294    Var295    Var296    Var297    Var298    Var299    Var300    Var301    Var302    Var303    Var304    Var305    Var306    Var307    Var308    Var309    Var310    Var311    Var312    Var313    Var314    Var315    Var316    Var317    Var318    Var319    Var320    Var321    Var322    Var323    Var324    Var325    Var326    Var327    Var328    Var329    Var330    Var331    Var332    Var333    Var334    Var335    Var336    Var337    Var338    Var339    Var340    Var341    Var342    Var343    Var344    Var345    Var346    Var347    Var348    Var349    Var350    Var351    Var352    Var353    Var354    Var355    Var356    Var357    Var358    Var359    Var360    Var361    Var362    Var363    Var364    Var365    Var366    Var367    Var368    Var369    Var370    Var371    Var372    Var373    Var374    Var375    Var376    Var377    Var378    Var379    Var380    Var381    Var382    Var383    Var384    Var385    Var386    Var387    Var388    Var389    Var390    Var391    Var392    Var393    Var394    Var395    Var396    Var397    Var398    Var399    Var400    Var401    Var402    Var403    Var404    Var405    Var406    Var407    Var408    Var409    Var410    Var411    Var412    Var413    Var414    Var415    Var416    Var417    Var418    Var419    Var420    Var421    Var422    Var423    Var424    Var425    Var426    Var427    Var428    Var429    Var430    Var431    Var432    Var433    Var434    Var435    Var436    Var437    Var438    Var439    Var440    Var441    Var442    Var443    Var444    Var445    Var446    Var447    Var448    Var449    Var450    Var451    Var452    Var453    Var454    Var455    Var456    Var457    Var458    Var459    Var460    Var461    Var462    Var463    Var464    Var465    Var466    Var467    Var468    Var469    Var470    Var471    Var472    Var473    Var474    Var475    Var476    Var477    Var478    Var479    Var480    Var481    Var482    Var483    Var484    Var485    Var486    Var487    Var488    Var489    Var490    Var491    Var492    Var493    Var494    Var495    Var496    Var497    Var498    Var499    Var500    Var501    Var502    Var503    Var504    Var505    Var506    Var507    Var508    Var509    Var510    Var511    Var512    Var513    Var514    Var515    Var516    Var517    Var518    Var519    Var520    Var521    Var522    Var523    Var524    Var525    Var526    Var527    Var528    Var529    Var530    Var531    Var532    Var533    Var534    Var535    Var536    Var537    Var538    Var539    Var540    Var541    Var542    Var543    Var544    Var545    Var546    Var547    Var548    Var549    Var550    Var551    Var552    Var553    Var554    Var555    Var556    Var557    Var558    Var559    Var560    Var561    Var562    Var563    Var564    Var565    Var566    Var567    Var568    Var569    Var570    Var571    Var572    Var573    Var574    Var575    Var576    Var577    Var578    Var579    Var580    Var581    Var582    Var583    Var584    Var585    Var586    Var587    Var588    Var589    Var590    Var591    Var592    Var593    Var594    Var595    Var596    Var597    Var598    Var599    Var600    Var601    Var602    Var603    Var604    Var605    Var606    Var607    Var608    Var609    Var610    Var611    Var612    Var613    Var614    Var615    Var616    Var617    Var618    Var619    Var620    Var621    Var622    Var623    Var624    Var625    Var626    Var627    Var628    Var629    Var630    Var631    Var632    Var633    Var634    Var635    Var636    Var637    Var638    Var639    Var640    Var641    Var642    Var643    Var644    Var645    Var646    Var647    Var648    Var649    Var650    Var651    Var652    Var653    Var654    Var655    Var656    Var657    Var658    Var659    Var660    Var661    Var662    Var663    Var664    Var665    Var666    Var667    Var668    Var669    Var670    Var671    Var672    Var673    Var674    Var675    Var676    Var677    Var678    Var679    Var680    Var681    Var682    Var683    Var684    Var685    Var686    Var687    Var688    Var689    Var690    Var691    Var692    Var693    Var694    Var695    Var696    Var697    Var698    Var699    Var700    Var701    Var702    Var703    Var704    Var705    Var706    Var707    Var708    Var709    Var710    Var711    Var712    Var713    Var714    Var715    Var716    Var717    Var718    Var719    Var720    Var721    Var722    Var723    Var724    Var725    Var726    Var727    Var728    Var729    Var730    Var731    Var732    Var733    Var734    Var735    Var736    Var737    Var738    Var739    Var740    Var741    Var742    Var743    Var744    Var745    Var746    Var747    Var748    Var749    Var750    Var751    Var752    Var753    Var754    Var755    Var756    Var757    Var758    Var759    Var760    Var761    Var762    Var763    Var764    Var765    Var766    Var767    Var768    Var769    Var770    Var771    Var772    Var773    Var774    Var775    Var776    Var777    Var778    Var779    Var780    Var781    Var782    Var783    Var784    Var785    Var786    Var787    Var788    Var789    Var790    Var791    Var792    Var793    Var794    Var795    Var796    Var797    Var798    Var799    Var800    Var801    Var802    Var803    Var804    Var805    Var806    Var807    Var808    Var809    Var810    Var811    Var812    Var813    Var814    Var815    Var816    Var817    Var818    Var819    Var820    Var821    Var822    Var823    Var824    Var825    Var826    Var827    Var828    Var829    Var830    Var831    Var832    Var833    Var834    Var835    Var836    Var837    Var838    Var839    Var840    Var841    Var842    Var843    Var844    Var845    Var846    Var847    Var848    Var849
    ______    ______    ______    ______    _____    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    _____    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    _____    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    _____    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    _____    ______    _____    _____    ______    ______    ______    _____    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    _____    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______    ______

     44469     44469     44469     44469    44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469    44469    44469     44469     44469     44469    44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469    44469      44469     44469     44469     44469    44469      44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469    44469      44469     44469     44469     44469     44469    44469      44469    44469      44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469      44469    44469      44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469    44469      44469     44469    44469     44469      44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469    44469      44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469     44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469    44469      44469     44469    44469     44469      44469     44469     44469     44469     44469     44469    44469      44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469     44469

L=numel(t1(1,:))

L = 849

t=t1(1,:);
S=t1(2,:);
plot(t,S)

Error using plot
Invalid subscript for Y. A table variable subscript must be a numeric array containing real positive integers, a logical array, a character vector, a string array, a cell array of character vectors,
or a pattern scalar used to match variable names.

f=Fs*(0:(L/2))/L;
Y=fft(S);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
plot(f,P1) 
title('Single-Sided Amplitude')
xlabel('f (Hz)')
ylabel('|P1(f)|')

Answer 1

[Star Strider]

It looks correct to me. I made three changes: indexing into the table and references to it, subtracted the mean from ‘S’ since it has a non-zero offset and the D-C compoonent was hiding the other peaks, and ‘zoomed’ the x-axis of the Fourier transform plot to make the details easier to see.

Fs=1000;
T=1/Fs;
t=readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1284000/t.xlsx');
L=numel(t(1,:));

tv=t{1,:};
S=t{2,:};
plot(tv,S)

f=Fs*(0:(L/2))/L;
Y=fft(S-mean(S));                                                       % Subtract The Mean (D-C Offset) To See The Other Peaks
P2 = abs(Y/L);
P1 = P2(1:fix(L/2)+1);
P1(2:end-1) = 2*P1(2:end-1);
plot(f,P1) 
title('Single-Sided Amplitude')
xlabel('f (Hz)')
ylabel('|P1(f)|')
xlim([0 150])                                                           % Optional

With those minor changes, your code appears to do what you want it to do.

.

[uzzi]

Thank you very much. It works. I don't know how to interpret the diagram. Do you have any idea?

[Star Strider]

As always, my pleasure!

‘I don't know how to interpret the diagram. Do you have any idea?’

I will do my best.

There appear to be three principal resonant frequencies, at about 2.4, 15.3, and 18.9 Hz. I am not certain what they mean, because I do not know what the signal represents. There is essentially no signal energy above about 50 Hz. That is all I can meaningfully say about it.

Fs=1000;
T=1/Fs;
t=readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1284000/t.xlsx');
L=numel(t(1,:));

tv=t{1,:};
S=t{2,:};
plot(tv,S)

f=Fs*(0:(L/2))/L;
Y=fft(S-mean(S));                                                       % Subtract The Mean (D-C Offset) To See The Other Peaks
P2 = abs(Y/L);
P1 = P2(1:fix(L/2)+1);
P1(2:end-1) = 2*P1(2:end-1);

[pks,locs] = findpeaks(P1, 'MinPeakProminence',0.05)

pks = 1×3

0.2105 0.1112 0.1446

locs = 1×3

3 14 17

plot(f,P1) 
title('Single-Sided Amplitude')
xlabel('f (Hz)')
ylabel('|P1(f)|')
xlim([0 150])                                                           % Optional
for k = 1:numel(locs)
    xline(f(locs(k)), '-r', sprintf('f = %.2f',f(locs(k))))
end

.

[uzzi]

Thank you very much. This is very helpful.

[Star Strider]

As always, my pleasure!

Answer 2

[Walter Roberson]

Fs=1000;
T=1/Fs;
t1 = readtable('t.xlsx');
L = width(t1);

t = t1{1,:};
S = t1{2,:};
plot(t,S)

f=Fs*(0:(L/2))/L;
Y = fft(S - mean(S));
P2 = abs(Y/L);
P1 = P2(1:floor(L/2+1));
P1(2:end-1) = 2*P1(2:end-1);
plot(f,P1) 
title('Single-Sided Amplitude')
xlabel('f (Hz)')
ylabel('|P1(f)|')

[uzzi]

Thank you for the answer. I don't know how to interpret the diagram. Do you have any ideas?