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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Most of the commercial DDS function generator products from Agilent (HP) work pretty much as you described. The only difference is that they wrap up all the logic in a high-speed FPGA and communicate with that from a microcontroller that runs the display and GPIB interface etc.
"Fast" is definitely the order of the day, with modern DDS clocking speeds being in the high hundreds of MHz. At these speeds dedicated hardware or FPGA beats software hands down. But the software solution running on a low cost microcontroller or DSP is great for low frequency applications, particularly audio.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
There are many useful functions that can be added with relatively few lines of code. Voltmeter, 500 kHz oscilloscope, 30 MHz frequency counters, frequency and pulse generators, RS-232 decoders and most likely a few thousand other features.
It is even possible to do VGA resolution video and Ethernet in software using such a microcontroller and some very clever coding.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
BTW, Mike, regarding the squarewave output, the best way to do this is to pass the filtered sinewave through a fast comparator.
If you just take the MSB of the DAC output or use a lookup table for the squarewave it will suffer from horrendous phase-jitter at high frequencies. Lowpass filtering the sinewave removes this phase-jitter due to images of the sampling process. The comparator that follows can then produce a nice clean jitter free squarewave fit for whatever purpose you choose.
As an educational excercise it is interesting to compare the square waveform generated this proper way with that generated by taking the MSB of the DAC data. As you increase the output frequency towards the limit you will see that one remains clean and jitter free, and the other has 1 sample of jitter on the rising and falling edges.
Registered Member #58
Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317
GeordieBoy wrote ...
BTW, Mike, regarding the squarewave output, the best way to do this is to pass the filtered sinewave through a fast comparator.
If you just take the MSB of the DAC output or use a lookup table for the squarewave it will suffer from horrendous phase-jitter at high frequencies. Lowpass filtering the sinewave removes this phase-jitter due to images of the sampling process. The comparator that follows can then produce a nice clean jitter free squarewave fit for whatever purpose you choose.
As an educational excercise it is interesting to compare the square waveform generated this proper way with that generated by taking the MSB of the DAC data. As you increase the output frequency towards the limit you will see that one remains clean and jitter free, and the other has 1 sample of jitter on the rising and falling edges.
-Richie,
Ah, yes that would be a good idea for sure. I think Bjorn mentioned this to me before but I forgot about it. I will probably implement this later on and do as you suggest of looking at the comparator output against the MSB.
Now the only other thing I am thinking about with the function generator is a way to vary the duty cycle, I will have to research a bit to see how difficult it is to accomplish this in the method my generator is currently setup.
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Multiple Waveform Function Generator on an ARM7
