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Adding 4 sinewaves of different phase and amplitude

Former Member

Hello,

My project is basically modified SPWM which involves adding 4 sinewaves each of different frequency and amplitude to generate a reference wave. I am using MP Lab IDE v8.76 to program a dsPIC30f4011.

How do i add these using the software mentioned above?

I have a lookup table for the sinewaves.

signed  int lookup[256] ={0,807,1614,2420,3224,4027,4827,5624,6417,7207,7992,8773,9548,

   10317,11080,11837,12586,13328,14061,14786,15502,16208,16905,

   17592,18267,18932,19585,20226,20855,21472,22075,22665,23241,

   23803,24351,24883,25401,25903,26390,26860,27315,27752,28173,

   28577,28963,29332,29683,30016,30330,30627,30904,31163,31403,

   31624,31826,32008,32171,32315,32439,32543,32627,32692,32737,

   32761,32766,32751,32717,32662,32587,32493,32379,32246,32092,

   31920,31728,31516,31286,31036,30768,30481,30175,29851,29510,

   29150,28772,28377,27965,27536,27090,26627,26149,25654,25144,

   24619,24079,23524,22955,22372,21775,21165,20542,19907,19260,

   18601,17931,17250,16558,15856,15145,14425,13695,12958,12212,

   11459,10700,9933,9161,8383,7600,6813,6021,5226,4427,3626,2822,

   2017,1211,404,-404,-1211,-2017,-2822,-3626,-4427,-5226,-6021,

   -6813,-7600,-8383,-9161,-9933,-10700,-11459,-12212,-12958,-13695,

   -14425,-15145,-15856,-16558,-17250,-17931,-18601,-19260,-19907,

   -20542,-21165,-21775,-22372,-22955,-23524,-24079,-24619,-25144,

   -25654,-26149,-26627,-27090,-27536,-27965,-28377,-28772,-29150,

   -29510,-29851,-30175,-30481,-30768,-31036,-31286,-31516,-31728,

   -31920,-32092,-32246,-32379,-32493,-32587,-32662,-32717,-32751,

   -32766,-32761,-32737,-32692,-32627,-32543,-32439,-32315,-32171,

   -32008,-31826,-31624,-31403,-31163,-30904,-30627,-30330,-30016,

   -29683,-29332,-28963,-28577,-28173,-27752,-27315,-26860,-26390,

   -25903,-25401,-24883,-24351,-23803,-23241,-22665,-22075,-21472,

       -20855,-20226,-19585,-18932,-18267,-17592,-16905,-16208,-15502,

   -14786,-14061,-13328,-12586,-11837,-11080,-10317,-9548,-8773,

   -7992,-7207,-6417,-5624,-4827,-4027,-3224,-2420,-1614,-807, 0};

 

But i am a little confused as to how to take into account the phase difference and add them.

Thanks in advance.

  • 0

    Hi Dilip,

     

    Yes, it's a bit tricky, but hopefully not too hard. If you have four lookup tables containing your input signals (sinewaves) sampled at the same rate, then the four lookup arrays will be of different lengths (i.e. one will be of length 256 as in your example) but others will be of different length if you store up to a non-truncated signal (sinewave).

    Then, you could use four variables to store the current index into each lookup array, and increment each one up to the length of the array and then reset the index vars to zero independantly. Initial index value for any array would adjust your phasing.

    There are probably other ways to do it too (and I'm not familiar with your target device), this method should work with any processor.

  • 0 in reply to shabaz

    Hi shabaz, thanks for your idea.

    The thing is i'm having difficulty in finding the syntax that could get me to add them. Given that i will have to take into account the phase difference too. And i'm not too familiar coding in MP Lab sadly.

  • 0 in reply to Former Member

    It looks like your data is 16-bit with full amplitude, so it may need scaling to a lower amplitude before you store the lookup tables, if you want to stay in 16-bit arithmetic. Then, your four signals can literally be summed (i.e. dac_out=lookup0[idx0]+lookup1[idx1]+lookup2[idx2]+lookup3[idx[3] ). Then increment the idxn variables and set to zero if any have reached the limit.

  • 0 in reply to shabaz

    This is a part of my code where i calculate the amplitude values and duty cycle values for each wave.

     

    void __attribute__((__interrupt__)) _PWMInterrupt(void)

    {

      shift_0 = shift_0+freq;

      sample1 = shift_0>>8;

      sine_R1 = lookup1[sample1];

      REFERENCE1 = (sine_R1*(long)amp1)>>15;

      PDC1 = REFERENCE1+2000;

     

      shift_45 = shift_45+freq;

      sample2 = shift_45>>8;

      sine_R2 = lookup2[sample2];

      REFERENCE2 = (sine_R2*(long)amp3)>>15;

      PDC2 = REFERENCE2+2000;

     

      shift_90 = shift_90+freq;

      sample3 = shift_90>>8;

      sine_R3 = lookup3[sample3];

      REFERENCE2 = (sine_R2*(long)amp3)>>15;

      PDC3 = REFERENCE3+2000;

     

      shift_180 = shift_180+freq;

      sample4 = shift_180>>8;

      sine_R4 = lookup4[sample4];

      REFERENCE2 = (sine_R2*(long)amp4)>>15;

      PDC4 = REFERENCE4+2000;

     

    now i need to add these individual amplitudes to get my final amplitude which i will be controlling at run time.

  • 0 in reply to Former Member

    The last two paras are

     

    shift_90 = shift_90+freq;

      sample3 = shift_90>>8;

      sine_R3 = lookup3[sample3];

      REFERENCE3 = (sine_R3*(long)amp3)>>15;

      PDC3 = REFERENCE3+2000;

     

      shift_180 = shift_180+freq;

      sample4 = shift_180>>8;

      sine_R4 = lookup4[sample4];

      REFERENCE4 = (sine_R4*(long)amp4)>>15;

      PDC4 = REFERENCE4+2000;

  • 0 in reply to Former Member

    Scale the lookup tables beforehand appropriately so that you don't need to convert to long and right-shift by 15. I don't know what the 2000 offset is for by the way.

    Secondly, not sure what the code is doing to be honest - you'd need four sets of shift_90, shift_180 etc., one per lookup array since the array lengths will be different. Also not sure what freq is, and why the right-shift. Maybe it's simpler just incrementing the four variables as I mentioned.

    Finally, all you'd need to do is do an arithmetic sum of PDC1+PDC2.. etc.

  • 0 in reply to shabaz

    The shift_45, shift_90 and shift_180 represent the different sinewaves. And i am using four different lookup tables. 2000 is to set the duty cycle. if you refer to my earlier code, i'v made a small change.Not sure if its right.

    REFERENCE=REFERENCE1+REFERENCE2+...+REFERENCE4

    PDC1=REFERENCE+2000

     

    REFERENCE is my final resultant.

  • 0 in reply to shabaz

    The lookup table is the same. I'm just storing it in four different locations. One for each individual wave.