Low cost audio function generator(DDS)

[Ingvar]

Hi Sougata,

I did some experimenting with DDS a few years ago, i wrote this piece of code. If my memory serves me right, it worked up to 20kHz with reasonable good looking sine on the output. I also did some testing with PWMoutput but never got good results with that, there may be some leftovers in the program from that ...... just remove.

'****************************************************************
'*  Name    : DDS16F.BAS                                        *
'*  Author  : Ingvar Gillholm                                   *
'*  Date    : 2004-03-11                                        *
'*  Version : 1.0                                               *
'*  Notes   : This program produces a sinewave through PortB,   *
'*          : which should have a R-2R leadder connected to it. *
'****************************************************************


@ device  pic16F876, hs_osc, wdt_off

define OSC 20
'DEFINE LOADER_USED 1

TonePort    VAR PortB
CheckNew    VAR PortA.3

Phase       VAR BYTE[3] BANK0
Freq        VAR BYTE[3] BANK0
Zero        VAR BYTE    BANK0

TempVar     VAR BYTE

ADCON1 = 7      'Porta is digital
TrisA = $ff     'All inputs
PortB = 0       'Start with all outputs low
TrisB = 0       'All outputs            
Zero = 0

PR2 = $3f
OUTPUT PortC.2
T2CON = %00000100
CCP1CON = %00001100
GOTO New

Main:
asm
;
;======================== MAIN LOOP =========================
;

    bcf STATUS,RP0
    bcf STATUS,RP1

Loop
    BTFSC   _CheckNew     ; Check for a new	frequency
    GOTO    _New          ; and get new frequency
                          ;
	MOVF	_Freq,W       ; Get the current frequency
	ADDWF	_Phase,F	  ; and increment the phase
	btfss STATUS,C        ;
    goto NoCarry          ;burn time if no carry
	incf _Phase+1         ;
	btfsc STATUS,Z        ;
	incf _Phase+2         ;
AfterCarry	              ;
	MOVF	_Freq+1,W     ;
	ADDWF	_Phase+1,F    ; 
	btfsc STATUS,C        ;
	incf _Phase+2	      ;
	MOVF	_Freq+2,W	  ;
	ADDWF	_Phase+2,F    ;

	MOVF	_Phase+2,W	  ;

	ANDLW	07Fh		  ;Reduce to 7 bits for 180 degrees
	CALL	SineTbl		  ; and get amplitude
    
	BTFSS  _Phase+2,7	;Is amplitude supposed to be negative?
	SUBWF  _Zero,w		;Change polarity of sine amplitude
   	
    MOVWF	_TonePort     ;Output to DAC
	GOTO	Loop		  ;

NoCarry
    goto AfterCarry

;*********** SINE TABLE ****************

SineTbl
	ADDWF  PCL
    retlw 080h
    retlw 083h
    retlw 086h
    retlw 089h
    retlw 08Ch
    retlw 090h
    retlw 093h
    retlw 096h
    retlw 099h
    retlw 09Ch
    retlw 09Fh
    retlw 0A2h
    retlw 0A5h
    retlw 0A8h
    retlw 0ABh
    retlw 0AEh
    retlw 0B1h
    retlw 0B3h
    retlw 0B6h
    retlw 0B9h
    retlw 0BCh
    retlw 0BFh
    retlw 0C1h
    retlw 0C4h
    retlw 0C7h
    retlw 0C9h
    retlw 0CCh
    retlw 0CEh
    retlw 0D1h
    retlw 0D3h
    retlw 0D5h
    retlw 0D8h
    retlw 0DAh
    retlw 0DCh
    retlw 0DEh
    retlw 0E0h
    retlw 0E2h
    retlw 0E4h
    retlw 0E6h
    retlw 0E8h
    retlw 0EAh
    retlw 0EBh
    retlw 0EDh
    retlw 0EFh
    retlw 0F0h
    retlw 0F1h
    retlw 0F3h
    retlw 0F4h
    retlw 0F5h
    retlw 0F6h
    retlw 0F8h
    retlw 0F9h
    retlw 0FAh
    retlw 0FAh
    retlw 0FBh
    retlw 0FCh
    retlw 0FDh
    retlw 0FDh
    retlw 0FEh
    retlw 0FEh
    retlw 0FEh
    retlw 0FFh
    retlw 0FFh
    retlw 0FFh
    retlw 0FFh
    retlw 0FFh
    retlw 0FFh
    retlw 0FFh
    retlw 0FEh
    retlw 0FEh
    retlw 0FEh
    retlw 0FDh
    retlw 0FDh
    retlw 0FCh
    retlw 0FBh
    retlw 0FAh
    retlw 0FAh
    retlw 0F9h
    retlw 0F8h
    retlw 0F6h
    retlw 0F5h
    retlw 0F4h
    retlw 0F3h
    retlw 0F1h
    retlw 0F0h
    retlw 0EFh
    retlw 0EDh
    retlw 0EBh
    retlw 0EAh
    retlw 0E8h
    retlw 0E6h
    retlw 0E4h
    retlw 0E2h
    retlw 0E0h
    retlw 0DEh
    retlw 0DCh
    retlw 0DAh
    retlw 0D8h
    retlw 0D5h
    retlw 0D3h
    retlw 0D1h
    retlw 0CEh
    retlw 0CCh
    retlw 0C9h
    retlw 0C7h
    retlw 0C4h
    retlw 0C1h
    retlw 0BFh
    retlw 0BCh
    retlw 0B9h
    retlw 0B6h
    retlw 0B3h
    retlw 0B1h
    retlw 0AEh
    retlw 0ABh
    retlw 0A8h
    retlw 0A5h
    retlw 0A2h
    retlw 09Fh
    retlw 09Ch
    retlw 099h
    retlw 096h
    retlw 093h
    retlw 090h
    retlw 08Ch
    retlw 089h
    retlw 086h
    retlw 083h

ENDASM
New:
' The frequency is calculated as FreqValue = DesiredFreq/((1/2^24)*(5*10^6/28))
' Same as FreqValue = DesiredFreq/((1/16777216)*(5000000/28))
' Same as FreqValue = DesiredFreq*16777216*28/5000000
' Same as FreqValue = DesiredFreq*93,9524096
' The resolution is 1/93,9524096 = 0,01064368656703404........Hz

' The eight frequencies selected for this test is........
'  67,0 = 001896
'  94,8 = 0022CA
' 118,8	= 002B99
' 146,2	= 0035A7
' 173,8	= 003FC8
' 199,5	= 004937
' 225,7	= 0052D5
' 254,1 = 005D41

    TempVar = PortA & %00000111     'Only lower 3 bits
'    LOOKUP TempVar,[$00,$00,$00,$00,$00,$00,$00,$00],Freq[2]    'Stuff Hibyte            
'    LOOKUP TempVar,[$1a,$25,$2e,$39,$44,$4e,$58,$63],Freq[1]    'Stuff Midbyte
'    LOOKUP TempVar,[$58,$47,$b7,$7d,$57,$72,$c0,$eb],Freq[0]    'Stuff Lowbyte
    LOOKUP TempVar,[$00,$00,$00,$00,$00,$00,$00,$00],Freq[2]    'Stuff Hibyte            
    LOOKUP TempVar,[$18,$22,$2b,$35,$3f,$49,$52,$5d],Freq[1]    'Stuff Midbyte
    LOOKUP TempVar,[$96,$ca,$99,$a7,$c8,$37,$d5,$41],Freq[0]    'Stuff Lowbyte    
    GOTO Main
END

Doing amplitde control is probably too much to ask from a 16F@20MHz but a 18F@40MHz might be able to pull it off.

/Ingvar

[sougata]

Hi All,

Thanks Ingvar at least you share the same vision as me.

Doing amplitde control is probably too much to ask from a 16F@20MHz but a 18F@40MHz might be able to pull it off.

Actually my client wants to put this module into a breadboard trainer to make it compete better. he does'nt want to increase the manufacturing cost however.

Because the amplitude control will also be used to modulte the base sinewave at a lower frequency it needs to be computed on the fly through a modulation index (I suppose).

Thanks Alain for the link but I cannot read German.(I am not sure even if it is German at all)

[Acetronics2]

Hi All,


Thanks Alain for the link but I cannot read German.(I am not sure even if it is German at all)

The only interesting info here is the price reached ... ~ 50 US $ ( sold ...) .

Alain

[sougata]

Hi,

How about making one for $50 ? Possible ?

[Acetronics2]

Here is the scheme ... and all you should know !!!

http://www.elv-downloads.de/service/...S-Board_km.pdf

and

http://www.elv-downloads.de/service/...M_G_041222.pdf


moreover ... all components are available at this address !!!

Alain

[sougata]

Hi Alain,

Thanks for the links.

[speck]

Hi Ingvar-

Could this be made to work using the HPWM output & a timer interrupt to output very low frequency sine waves? (< 30hz) Using a lookup table & a variable timer interrupt cycle to vary the sample rate/freq. of the sine? Perhaps amplitude could be varied by scaling the wavetable values?

Was it the case that the PWM output didn't give you a smooth sine at higher frequencies? Or it just didn't work well at all?

I did some experimenting with DDS a few years ago, i wrote this piece of code. If my memory serves me right, it worked up to 20kHz with reasonable good looking sine on the output. I also did some testing with PWMoutput but never got good results with that, there may be some leftovers in the program from that ...... just remove.
Doing amplitde control is probably too much to ask from a 16F@20MHz but a 18F@40MHz might be able to pull it off.

/Ingvar

[Archangel]

10-MHz-DDS-Funktionsgenerator mit PC-Softwaresteuerung-preis:69,95 euro
Das ist gut, Ya ?

[sougata]

Hi Speck,

It can always be done with a hardware PWM and low pass filter. See my sinewave inverter thread http://www.picbasic.co.uk/forum/showthread.php?t=1846 for a code snippet that does this at an update rate of 100Hz driving a half bridge. Scaling the sine table is impractical with only 8 bits of resolution. For example if you define your sinewave lookup table to produce the maximum peak value into an 8 bit one then you have to do fractional math.That is your modulation index (depth of modulation) is <= 1. (The dSPIC supports fractional math). So the trade-off is multiplying the lookup table with a 8bit value and shredding off the LSBs. This works okay for low fixed frequency. If you do a cycle by cycle computation then the overhead is manageable. But for a 20K function generator to work with a base frequency and modulating frequency it has to be updated every PWM interrupt cycle. That is too much you could ask for a 16F PIC @ 5 MIPS. The 18Fs have an extended instruction set featuring a single cycle hardware multiply that too @ 10MIPS. In fact you can use Darrel's Software PWM http://www.pbpgroup.com/modules/wfse...p?articleid=12 to make your life easy. It takes of all your pain in doing it in asm.

BTW the PIC Hardware PWM uses upto 10bits and it is a good practice to set your PR2 to $FF. That is where you get the maximum usable PWM resolution.

[Acetronics2]

10-MHz-DDS-Funktionsgenerator mit PC-Softwaresteuerung-preis:69,95 euro
Das ist gut, Ya ?

Hi,Joe

I've "built" ( laughs ...) the 20Mhz one ...

just need an extra output buffer for > 10 Mhz outputs ( there's a too low -pass response somewhere ! ) ... and because the HC132 is a bit weak on even small capacitive loads.

Otherwise it works really fine !!! ... and 4Mhz is the Max I really need ...as a variable PIC clock drive.

precision and resolution are really unbeaten for such price ...

Yawohl ... sehr gut !!!

Alain

[mister_e]

Je dit... c'est le excellent prix pour le excellent produit ... yah!

I don't know the following, and i don't know how good they are and are really far of the needed range here but...
http://www.amqrp.org/kits/dds60/
http://www.minikits.com.au/kits4.html

Kinda time saver i guess.

[Ingvar]

Hi Speck,

It should be possible to do with hardware PWM, espescially at low frequencies like that. I just never got it working very well at higher freqs, so i never really finished it off to see where the limits were. Feel free to carry out your own investigation cause it should be possible.

I think it was probably due to the fact that the loop runs at 5000000/28=178,571kHz and the highest (useable) PWM freq was around twice that ....... no resolution. However, like i said, i never took it all the way, i got the results i wanted with the R-2R ladder.

/Ingvar

[Acetronics2]

Hi,All

I saw one application in a the French "Electronique Pratique" magazine, issue 247, page 86 ...

But, halas, I didn't download the Pic ( Hex ) Program ... and it's no more available.

May be someone has it in his computer memory ???

Alain