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  • Author Author: jc2048
  • Date Created: 26 Apr 2018 1:21 PM Date Created
  • Views 2556 views
  • Likes 9 likes
  • Comments 5 comments
  • r2r
  • arduino uno project
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Arduino: R-2R; "A Sweep is as Lucky, as Lucky Can Be..."

jc2048
jc2048
26 Apr 2018

I'm back with the test sketch and I've implemented a frequency sweep. This starts at 20Hz and

continuously changes the frequency until it arrives at 20kHz where it starts all over again.

The sweep is linear because that was what was easy to do (normally, for a frequency response

curve, you'd be looking at a logarithmic scale). I might have a go at a log version later if I

can get myself in the right frame of mind for some mathematics.

 

/* AD Test */
unsigned int tableOffset = 0;             // output count
unsigned int tableStep = 13;
//
//  --- Sine table - generated by sineTable.exe 
//
unsigned int sineTable[512] = {
   0x8000,0x8192,0x8324,0x84b6,0x8647,0x87d9,0x896a,0x8afb,0x8c8b,0x8e1b,0x8fab,0x9139,0x92c8,0x9455,0x95e2,0x976d,
   0x98f8,0x9a82,0x9c0b,0x9d93,0x9f19,0xa09f,0xa223,0xa3a6,0xa528,0xa6a8,0xa826,0xa9a3,0xab1f,0xac98,0xae11,0xaf87,
   0xb0fb,0xb26e,0xb3de,0xb54d,0xb6ba,0xb824,0xb98c,0xbaf2,0xbc56,0xbdb8,0xbf17,0xc073,0xc1ce,0xc325,0xc47a,0xc5cd,
   0xc71c,0xc869,0xc9b4,0xcafb,0xcc3f,0xcd81,0xcebf,0xcffb,0xd133,0xd269,0xd39b,0xd4ca,0xd5f5,0xd71d,0xd842,0xd964,
   0xda82,0xdb9d,0xdcb4,0xddc7,0xded7,0xdfe3,0xe0ec,0xe1f1,0xe2f2,0xe3ef,0xe4e8,0xe5dd,0xe6cf,0xe7bd,0xe8a6,0xe98c,
   0xea6d,0xeb4a,0xec24,0xecf9,0xedca,0xee96,0xef5f,0xf023,0xf0e2,0xf19e,0xf255,0xf307,0xf3b5,0xf45f,0xf504,0xf5a5,
   0xf641,0xf6d9,0xf76c,0xf7fa,0xf884,0xf909,0xf98a,0xfa05,0xfa7d,0xfaef,0xfb5d,0xfbc5,0xfc29,0xfc89,0xfce3,0xfd39,
   0xfd8a,0xfdd6,0xfe1d,0xfe5f,0xfe9d,0xfed5,0xff09,0xff38,0xff62,0xff87,0xffa7,0xffc2,0xffd8,0xffe9,0xfff6,0xfffd,
   0xffff,0xfffd,0xfff6,0xffe9,0xffd8,0xffc2,0xffa7,0xff87,0xff62,0xff38,0xff09,0xfed5,0xfe9d,0xfe5f,0xfe1d,0xfdd6,
   0xfd8a,0xfd39,0xfce3,0xfc89,0xfc29,0xfbc5,0xfb5d,0xfaef,0xfa7d,0xfa05,0xf98a,0xf909,0xf884,0xf7fa,0xf76c,0xf6d9,
   0xf641,0xf5a5,0xf504,0xf45f,0xf3b5,0xf307,0xf255,0xf19e,0xf0e2,0xf023,0xef5f,0xee96,0xedca,0xecf9,0xec24,0xeb4a,
   0xea6d,0xe98c,0xe8a6,0xe7bd,0xe6cf,0xe5dd,0xe4e8,0xe3ef,0xe2f2,0xe1f1,0xe0ec,0xdfe3,0xded7,0xddc7,0xdcb4,0xdb9d,
   0xda82,0xd964,0xd842,0xd71d,0xd5f5,0xd4ca,0xd39b,0xd269,0xd133,0xcffb,0xcebf,0xcd81,0xcc3f,0xcafb,0xc9b4,0xc869,
   0xc71c,0xc5cd,0xc47a,0xc325,0xc1ce,0xc073,0xbf17,0xbdb8,0xbc56,0xbaf2,0xb98c,0xb824,0xb6ba,0xb54d,0xb3de,0xb26e,
   0xb0fb,0xaf87,0xae11,0xac98,0xab1f,0xa9a3,0xa826,0xa6a8,0xa528,0xa3a6,0xa223,0xa09f,0x9f19,0x9d93,0x9c0b,0x9a82,
   0x98f8,0x976d,0x95e2,0x9455,0x92c8,0x9139,0x8fab,0x8e1b,0x8c8b,0x8afb,0x896a,0x87d9,0x8647,0x84b6,0x8324,0x8192,
   0x8000,0x7e6d,0x7cdb,0x7b49,0x79b8,0x7826,0x7695,0x7504,0x7374,0x71e4,0x7054,0x6ec6,0x6d37,0x6baa,0x6a1d,0x6892,
   0x6707,0x657d,0x63f4,0x626c,0x60e6,0x5f60,0x5ddc,0x5c59,0x5ad7,0x5957,0x57d9,0x565c,0x54e0,0x5367,0x51ee,0x5078,
   0x4f04,0x4d91,0x4c21,0x4ab2,0x4945,0x47db,0x4673,0x450d,0x43a9,0x4247,0x40e8,0x3f8c,0x3e31,0x3cda,0x3b85,0x3a32,
   0x38e3,0x3796,0x364b,0x3504,0x33c0,0x327e,0x3140,0x3004,0x2ecc,0x2d96,0x2c64,0x2b35,0x2a0a,0x28e2,0x27bd,0x269b,
   0x257d,0x2462,0x234b,0x2238,0x2128,0x201c,0x1f13,0x1e0f,0x1d0d,0x1c10,0x1b17,0x1a22,0x1930,0x1842,0x1759,0x1673,
   0x1592,0x14b5,0x13db,0x1306,0x1235,0x1169,0x10a0,0x0fdc,0x0f1d,0x0e61,0x0daa,0x0cf8,0x0c4a,0x0ba0,0x0afb,0x0a5a,
   0x09be,0x0926,0x0893,0x0805,0x077b,0x06f6,0x0675,0x05fa,0x0582,0x0510,0x04a2,0x043a,0x03d6,0x0376,0x031c,0x02c6,
   0x0275,0x0229,0x01e2,0x01a0,0x0162,0x012a,0x00f6,0x00c7,0x009d,0x0078,0x0058,0x003d,0x0027,0x0016,0x0009,0x0002,
   0x0000,0x0002,0x0009,0x0016,0x0027,0x003d,0x0058,0x0078,0x009d,0x00c7,0x00f6,0x012a,0x0162,0x01a0,0x01e2,0x0229,
   0x0275,0x02c6,0x031c,0x0376,0x03d6,0x043a,0x04a2,0x0510,0x0582,0x05fa,0x0675,0x06f6,0x077b,0x0805,0x0893,0x0926,
   0x09be,0x0a5a,0x0afb,0x0ba0,0x0c4a,0x0cf8,0x0daa,0x0e61,0x0f1d,0x0fdc,0x10a0,0x1169,0x1235,0x1306,0x13db,0x14b5,
   0x1592,0x1673,0x1759,0x1842,0x1930,0x1a22,0x1b17,0x1c10,0x1d0d,0x1e0e,0x1f13,0x201c,0x2128,0x2238,0x234b,0x2462,
   0x257d,0x269b,0x27bd,0x28e2,0x2a0a,0x2b35,0x2c64,0x2d96,0x2ecc,0x3004,0x3140,0x327e,0x33c0,0x3504,0x364b,0x3796,
   0x38e3,0x3a32,0x3b85,0x3cda,0x3e31,0x3f8c,0x40e8,0x4247,0x43a9,0x450d,0x4673,0x47db,0x4945,0x4ab2,0x4c21,0x4d91,
   0x4f04,0x5078,0x51ee,0x5367,0x54e0,0x565c,0x57d9,0x5957,0x5ad7,0x5c59,0x5ddc,0x5f60,0x60e6,0x626c,0x63f4,0x657d,
   0x6707,0x6892,0x6a1d,0x6baa,0x6d37,0x6ec6,0x7054,0x71e4,0x7374,0x7504,0x7695,0x7826,0x79b8,0x7b49,0x7cdb,0x7e6d};
void setup() {
  
  // set the digital pins as outputs:
  pinMode(0, OUTPUT);
  pinMode(1, OUTPUT);
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(A0, OUTPUT);
  pinMode(A1, OUTPUT);
  pinMode(A2, OUTPUT);
  pinMode(A3, OUTPUT);
  pinMode(8, OUTPUT);
 
  digitalWrite(A0, LOW);   
  digitalWrite(A1, LOW);   
  digitalWrite(A2, LOW);   
  digitalWrite(A3, LOW);   
  // timer 2 set up
  cli();                  // disable interrupts
  TCCR2A = 0;             // control register all 0
  TCCR2B = 0;             // control register all 0
  TCNT2 = 0;              // set count to 0
  OCR2A = 159;            // period = 160 x 1/16MHz = 10uS
  TCCR2A |= (1 << WGM21); // mode is clear on match
  TCCR2B |= (1 << CS20);  // no prescaler   
  TIMSK2 |= (1 << OCIE2A);  // enable interrupt on match
  sei();                  // enable interrupts
}
ISR(TIMER2_COMPA_vect) {
  tableOffset = tableOffset + tableStep;
  PORTD = (sineTable[tableOffset >> 7]) >> 8;
  tableStep = tableStep + 1;
  if(tableStep == 13107) {
    tableOffset = 0; 
    tableStep = 13;
    PORTB |= 0x01;   
    }
  else
    PORTB &= 0xFE;   
}
void loop() {
}

 

The code to do this is surprisingly simple. In the interrupt routine, I just add a constant to

the tableStep value (in this case incrementing by 1) until it reaches the value for 20kHz and

then reset it to the value for 20Hz. At the same time as I do the reset, I also set digital

output 8 on the Uno high to act as a trigger for the scope - it stays high for one timer

interrupt period [10uS]. I've done that by going straight for the port to keep the time to a

minimum (it's the lines referring to PORTB); the digitalWrite() function is slow and I didn't

want it clogging up the interrupt routine [but it does mean that this might not work on other

Arduino boards].

 

Here's what I see at the input to the filter:

 

image

 

The trigger point in the centre is at the transition from 20kHz back to 20Hz. The waveform to the left

of that point looks really ragged, but it's [mostly] what it should be. If I go in closer we see this:

 

image

 

The raggedness is down to the steps from the 100ksps sampling and is the reason why we need the filtering. The

good news is that the amplitude is maintained to 20kHz, so the falloff I was seeing in the previous

blog is coming from the filter and not the initial gain [really, attenuation] stage.

 

Here's the output from the filter:

 

image

 

here we can see that the response is rolling off from a couple of kilohertz and not the 20kHz

that it was meant to be. That might be me getting component values wrong or it could be the very

limited open loop gain that is available from the LM324 at those frequencies (as Gene usefully

pointed out in the comments to the previous blog).

 

 

Part one: Arduino: R-2R Experiment

Part two: Arduino: R-2R: Sine On You Crazy Diamond

Part three: Arduino: R-2R: Buffer, Attenuate, and Filter

Part four: Arduino: R-2R: "We Interrupt This Programme..."

Part five: Arduino: R-2R: "Resistance is..."?

Part six: Arduino: R-2R: Setting the Output Frequency

Part seven: Arduino: R-2R; "A Sweep is as Lucky, as Lucky Can Be..."

Part eight: Arduino: R-2R: Setting the Signal Amplitude

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Top Comments

  • jc2048
    jc2048 over 7 years ago in reply to jc2048 +5
    I've reworked the capacitor and this is what I now get. Here is what the simulator shows with it plotted linear rather than log (I don't know why it labels the vertical gain [dB]), so fairly similar. For…
  • DAB
    DAB over 7 years ago +2
    Nice post. DAB
  • jc2048
    jc2048 over 7 years ago in reply to genebren +2
    The filter circuit was in this blog: Arduino: R-2R: Buffer, Attenuate, and Filter I can see a mistake in my build. I didn't have a 820p capacitor so decided to make it from a 1.5n and 1.8n in series, both…
  • genebren
    genebren over 7 years ago in reply to jc2048

    Jon,

    Wow! That looks so much better.  Good find on the capacitor values.

    Gene

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  • jc2048
    jc2048 over 7 years ago in reply to jc2048

    I've reworked the capacitor and this is what I now get.

     

    image

     

    Here is what the simulator shows with it plotted linear rather than log (I don't know why it labels the vertical gain [dB]), so fairly similar.

     

    image

     

    For a flatter passband to 20kHz, the -3dB frequency would need to be pushed up to over 40kHz, though that would further reduce the filtering unless I increased the order of the filter or upped the sample rate.

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  • jc2048
    jc2048 over 7 years ago in reply to genebren

    The filter circuit was in this blog:

     

    Arduino: R-2R: Buffer, Attenuate, and Filter

     

    I can see a mistake in my build. I didn't have a 820p capacitor so decided to make it from a 1.5n and 1.8n in series, both of which I did have. Fairly predictably, by the time I came to build it I misremembered and put them in parallel, so I've got 3.3n in there. That will be some or all of the problem - I'll rectify that and see what happens.

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  • genebren
    genebren over 7 years ago

    Jon,

     

    That is really rolling off soon (<1KHz).  So how is that final stage wired?

     

    Gene

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  • DAB
    DAB over 7 years ago

    Nice post.

     

    DAB

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