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AD9102 clock source and triangle wave generation

Question asked by BartSchroder on Nov 6, 2013
Latest reply on Dec 10, 2013 by danf

Hello,

I'm having two problems with the AD9102 DDS sig gen. Besides using it as Arbitrary Waveform Generator, I also want to use it as a standard function generator, outputting sine, square and triangle. I've got the sine and AWG working fine. However the Square and Triangle are a problem.

For the Square I planned to put low and high values into the AWG memory, set the pattern length to 2, and then clock it with the DDS_MSB. In this way I should have been able to make a variable frequency square wave generator (though with jitter of incoming clock period). But I couldn't get it to work as described in the data sheet. Secondly, I want a variable frequency Triangle wave. But I found the triangle wave can only be produced with just 64 frequencies, each a half of the previous frequency, and fixed to starting at CLK/32768 in frequency. This seems useless to almost everyone. So I must be doing something wrong. So to expand:

 

1. DDS Clock Source

I cannot make the DDS_MSB_EN in the DDS_CONFIG register work.

My assumption is that if I program the RAM to have two values, 0x1FFF <<2 and 0x2000 <<2 at 0x6000 on, I set Start_addr to 0 <<4 and Stop_addr to 1<<4, and Pattern_period to 2, then I should get a square wave. If the DDS_MSB_EN is 0, then the square wave frequency should be CLK/2 frequency. If DDS_MSB_EN = 1, then the square wave should be at the DDS/2 frequency - which is what I want.

 

However, the DDS_MSB_EN bit does not seem to have any effect.

 

2. Sawtooth Frequency

I cannot work out how to change the Sawtooth frequency. Set to Triangle, I see the generator outputting 16384 samples increasing from -max to + max, and then 16384 samples from +max to -max, and repeating this. This yields just one frequency at CLK IN/32768.

I can see that I can dwell at one sample for between 1 and 64 clock cycles, but that is a very coarse approach to setting the frequency. It means you can only get 64 frequencies - the next being half of the previous one. Kind of not very useful!  I had assumed (silly of me) that the part would use the phase accumulator with the linear to sine wave conversion turned off (just as the AD9834 does), and so I could get any frequency. However, I cannot figure out how to do this. I would be grateful for any help.

 

Can you clarify what I'm doing wrong?  Thanks.

 

Bart

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