AVR Freaks

Hi JC,

First, I like to thank you for the interesting analysis of the subject.

Second, you are totally right. Using DDS, R-2R ladder or PWM cannot help in this case. And, as you said, the output of the passive RC filter should be buffered. Actually, it will be followed by one (or two opamps) in order to control the end amplitude from 0V to Vmax (but this is another story  ).

So I had no choice but to think out of the box to fulfil the three conditions above.

After days of work (since here 6/24 of electricity only and no enough fuel to warm the room in which I work), I was able generating all 30 frequencies with rather very low THD  (condition 3). The signals at the output of the simple RC filter (condition 1) are like the sinewave on your scope (#11). I am trying now to decrease, as possible, the difference among the generated sinewave amplitudes since it is now around +/- 20% (not +/- 5%).

I have no doubt that if you found yourself facing the same conditions, you would surely think of the solution the way I did.

Kerim

My teacher?! PWM?! Aren't you kidding, Jim

But perhaps this is the first time you heard of me

If you are serious, you will real surprise me if you do think that, by using PWM (of ATmega8L), one can generate very low THD sinewaves for 18 kHz and 615 Hz (the two limits on the OP list) by using just one passive RC low-pass filter (of any degree).

By the way, this project is about the main part of one of my future products.

Kerim

Added: Please note the condition 2, on OP, concerning the output amplitudes.

Your suggestions are good, Wh0-me.

But the "necessity" here is that, besides the passive RC filter, there are ATmega8L and 8 MHz crystal only... nothing else

By the way, I tend to believe that the idea (I got while dreaming in bed  ) of the method\routine which made possible to build this digital audio sinewave generator with the OP constraints cannot be found on any website. The reason is simple; its seems no one in the world, but I, needed it in the first place

Yes, your two points are valid indeed.

But it happens that my routine/code lets just one passive RC filter do its job properly; by giving a low THD sinewave output for all frequencies on the list.

And this fact should be considered as an interesting hint to guess the routine/code which I had the chance to think of.

Brian, so you know already how an ATmega8L and a simple passive RC filter can generate sinewaves from 615b Hz up to 18 KHz with low THD (at any frequency)

Sorry for the delay. I attached here a few files concerning the new technique in generating periodic signals (now sinewaves only) that I mentioned and discussed on the previous posts. As you will see, its idea is based on using the MCU SPI function (MOSI pin only). So one passive RC filter (3rd degree) is enough to act as a low-pass filter for the high and low frequencies; 17544 Hz and  658 Hz.

The files are:

[1] SPIwaveGenerator.pdf , a Kicad schematic of the test circuit.

[2] SPI_bit_stream.zip , LTspice schematic that emulates the SPI output in the two cases.

[3] SPI_bit_stream.png , screenshot of LTspice emulation.

[4] SPIsine_2freq.hex , machine code (fuse settings included) for ATmega8L , to those who like testing it in real.

The main advantage of this technique is its simple hardware and its low cost as well.

But the creation of the data tables (one for each frequency that an application may require) is not a work for novice; mainly the tables that synthesizes the signals with minimum distortion (THD).

Sorry, I didn't know that Don Lancaster also thought of using the MCU SPI function.

On the other hand, I guess, from your comment, he also knew how to find out the data table for the actual SPI protocol although it hasn't a free running mode. After all, it wasn't made for such application in the first place.

I mean; you remind me two friends who went to watch a movie. When the great Lion of the Metro-Goldwyn-Mayer showed up on the screen, one of them said: "Oh, let us go out. We already saw this movie."

Speaking seriously, do you really believe that you can generate easily other frequencies (with low THD) just by reading Don Lancaster book.

Kerim

Sorry all, I am afraid that as long someone doesn't have time to try applying this technique in real, I can't take seriously his comments (and he will know why I am saying this now... from the first day he will decide to implement it as I did).

Kartman, so you think that with this simple hardware (nothing else), one can easily generate (or you may have done it already) sinewave signals in a rather wide spectrum (F_hi/F_lo equals about 30). Actually, by just inserting 3 more capacitors (using 3 NPN transistors), the spectrum could be extended to cover 20 Hz to 20 KHz.

By the way, when someone will find (using Google for example) an equivalent digital sinewave generator with the same simplicity (hardware) and cost (if not better, of course), the SPI technique, I presented to you, will be no more a novel idea 

That's not to undermine any one's work.

I simply had to think of and apply it because I needed such versatile digital generator (for my products) while I am not allowed to use any of the already known solutions due to... And, as you know, necessity is the mother of invention. I also liked it because it costs me less, besides its simplicity. But, truth be said, creating just the tables for 50 frequencies took me many weeks. Fortunately, the frequencies of the lower band (in the audio spectrum) can use these tables too.

In my actual application, it is a part of a product. It has to produce, if not halted, a continuous bit stream that generates a sinewave signal at a certain frequency (actually, in the audio spectrum). For the time being, there are 100 frequencies only (using 50 tables).

In my application, the frequency index (from 0 to 99) is sent via TWI. So when the bit stream is interrupted by TWI (as a slave) and TWDR is read (the frequency index), the starting address of the requested table and its length are read, in turn, from the general index table before re-entering the SPI loop.

Manually, 7 pins could specify the frequency index. When the state of a pin (or more) is changed, the SPI loop stops and the new frequency index could be read from the state of 7 pins.

Yes, the table data are based on Delta Modulation. I chose the SPI Prescaler of fosc/4. So the frequency of the SPI clock is 2MHz.

I wished there is a formula; this would have saved a lot of work. So the tables are placed in flash.

I write always my code in assembly. So I am not sure if, by using a C compiler, the exact timing could be achieved.

By the way, since the time of ZX Spectrum microcomputer, I was familiar with delta modulation for saving/playing speech signals (300 Hz up to 3.3 KHz) digitally. One of the project, as a hobby, was scrambling the voice (on phone) by reversing the bits every 100ms (if I remember well). But, by using the ICs that I was able to get at that time, its board had to be rather too big (not practical).

Now, the existing SPI protocol has a surprise to those who will try to use its output (MOSI) as if it were the output of a delta modulator. The surprise is that MOSI pin has a break/dead time (at a constant level) between every two consecutive transmissions. This timing is emulated on LTspice (#38) and could be seen if SPI_bit_stream.asc is simulated and the PLS1 or PLS2 is plotted and magnified.

To me in the least, I needed a reliable simple low-cost generator to produce audio sinewave signals with practical amplitudes (say around 300 mV if buffered).

There is another ATmega8 to play the role of ‘brain’ :)

For the time being, only one sinewave at a time is of interest to me. But the code can let it be for a sweep.

On the other hand, I believe that this technique could be applied to generate any periodic signal (below 100 KHz) in case it is not available already by one or more ready-made generators.

==============

By the way, the existing SPI protocol doesn’t have a free running mode (to eliminate the dead time after every byte transfer).

But this could be achieved by just adding a flag in SPSR and an 8-bit buffer register for SPDR in master mode (in this mode, MISO, SCK and SS could be used as I/O pins). So after transmitting the 8 bits of SPDR, the contents of its buffer will be loaded into it and the transmission goes on without interruption. It is up to the programmer to place, or not, new data in the SPDR buffer register before the end of the present transfer.

Edited:

But this could be achieved, by an MCU manufacturer, by just adding...

Thank you, sparrow2, for your interesting hints. I will revise Jespers DDS carefully to find out if it is better in term of simplicity and cost in my case (covering from 20 Hz to 20 KHz).

By the way, I usually try avoiding R2R DAC because the best tolerance of the available resistors here is 5% (if genuine); not suitable for producing many sets having similar low THDs. But this may not be a problem when the actual crisis will be over and I can produce my boards in China again.

I noticed now your add: , thank you.

By the way, I drew a schematic on LTspice to emulate DDS with 8-bit ladder. So now I can see the effects on the output THD when using real resistances having a certain tolerance for the R2R ladder. And it helps me adjust its output (as AC and DC) because I have to use, as a buffer, a low-cost opamp, as LM324/LM358, which is not rail-to-rail.

About R2R, I added a function which could be activated at boot to compensate, when necessary and to some degree, its non-linearity. This is done by using ADC to adjust the 256 entries of the ideal sine table. The new table is saved on the MCU EEPROM.

I guess the SPI technique is good only if applied by an MCU manufacturer as an added function (after giving it another name, I guess) since it uses just one pin. In this case, before enabling this function, the programmer needs just to set the first address (an internal memory space) and the data length that generates the required signal. The MCU will be free as before while it acts as a signal generator (though for relatively low frequencies). But perhaps, this was done already in a certain MCU family.