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y gI'd probably add the few lines of 2's complement trickery so only one quadrant of sine table is needed or even use a taylor series, on a cpu with mul-acc is takes very few cycles
but the point was really that even if you use float to generate the sine wave I'd still use integers for the phase accumulator
-Lasse
The sine table doesn't need to be that long. If every value of the DAC is used then you'd have a sine table which has the maximum accuracy you can ever achieve. With some math that comes to 2*pi* dac resolution. So a 10 bit dac should need a 1024*2*pi=6434 samples.
The idea behind it is that every DAC value should be used at least once to get the maximum accuracy and no values need to be interpolated. This means you need to know the angle which causes a difference of 1 LSB where a sine wave is the steepest (=in the zero crossing). A simple approximation is: lsb_angle=arcsin(1/dac resolution). The number of samples in the table is 2*pi/lsb_angle.
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Only if you're hurting for flash space. I once did a 16-in/16-out sine table with no folding, 128Kbytes of lookup table. Why not?
or even use a taylor
In practice, we'd make the table have 2^n entries, which might be 8192 for a
10-bit DAC. That's only 16K bytes, a small fraction of the flash space on an LPC1754.We've done DDS with table interpolation, but in FPGA hardware. It helps a little, sometimes.
Joerg schrieb:
Hello,
Agilent has a DDS generator with FM:
programmable via GPIB, LAN or USB.
Bye
Well, yeah, I am using something like that from a Chinese manufacturer right now. But that is not suitable because two of those would have to go into a smallish analog interface box. When you open up one of those generators the enclosure is quite full.
Currently I am hammering out an analog solution. Again.
So use the Armstrong method.
?-)
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They were sweet chips for their time, but now they are up against DDS chips with 14+ bit dacs for not all that much. They can't compete, except in the hobbyist market which way too small.
?-)
Well, FM and sweep seems to be a problem :-)
What, erythropoietin? Some kind of growth hormone?
:-)
I think ADI has a DDS chip that sweeps.
Why is it that ADI is the only outfit that makes DDS chips? I asked one of their guys, and he didn't know.
We mostly do DDS in FPGAs.
The usuakl reason would be some sort of patent protection. But that's just a guess.
But then they probably need 10x the power :-)
Here is a link to the demo software for the AD8950 that does sweep.
I doubt that he can download it and get it to run, like so much other software. I had it installed on another computer and it works. You can use the parallel port to connect the demo board to the PC and see that it works.
They have sample code for the 8051 processor
i have a DDS function generator that does all of that and it does not have any problems?
I think you'll find if you don't have a latency problem with the serial link to the chip, you should be able to do all the FMing you want..
Jamie
So do I and it dopesn't have modulation problems. But these are too big even if I'd remove the enclosure and power supply.
The problem with the uC on these little boards is not the serial link. Well, at least not if 100Hz sweeps suffice. The problem is that they are too small to calculate the numbers on the fly and usually have to little flash left to store them in a LUT. Plus that would be a major time-consuming hack.
If its software then it is not a real sweep but a stepped sweep. If you use that to test a resonator or a notch filter you might be in for a nasty surprise.
??ftp://ftp.analog.com/pub/www/techSupport/designTools/evaluationBoards/downloads/AD9850_51_V2.41_Win2KXP.exe?
That depends on the step size. Do you think modern network analyzers are all analog?
??ftp://ftp.analog.com/pub/www/techSupport/designTools/evaluationBoards/downloads/AD9850_51_V2.41_Win2KXP.exe?
No, but they won't use a dead slow 8051 either :-) Probably some FPGA or ASIC.
??ftp://ftp.analog.com/pub/www/techSupport/designTools/evaluationBoards/downloads/AD9850_51_V2.41_Win2KXP.exe?
You mean an 8051 doesn't have infinite MIPS? Really? You do know that an FPGA won't be perfect either?
??ftp://ftp.analog.com/pub/www/techSupport/designTools/evaluationBoards/downloads/AD9850_51_V2.41_Win2KXP.exe?
No specification was given for a sweep time, was it?
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