mo at ccr.org
Sat Jan 5 19:20:17 CST 2013
these days, the Cortext M3 parts are no more expensive than
the Arduino CPU parts and they have a lot more computrons
to show for the effort. There is at least one Arduino clone board
that uses a Cortex, I believe it's an STM32 family part. by "clone"
board, I mean that the cpu card is the same size and was designed
to use the Arduino "shields".
On 1/5/13 7:02 PM, wb4jfi at knology.net wrote:
> Hello there Karl. I did not see the LP filter design that you
> mentioned, but I believe the type of filter is an elliptical, or Cauer
> filter. You can use the free filter design and analysis program from
> aade.com to design them. I've used them for several years with DDS
> designs. The AD9850 is pretty old technology these days, but the
> price is pretty good. It's output DAC's resolution is not great, and
> it's a power hog. I was playing a lot with the AD9850, AD9851,
> AD9832, AD9833, AD9834, AD9835 for several years, driving them with
> PIC chips. The newer AD995x and AD9912 are more iinteresting, as they
> have better DAC resolution, higher clock rates, and lower power
> requirements. But, they have a harder package to use.
> Typically, you can design a Cauer filter to not only provide a
> traditional low-pass function, but you can often tune the response for
> maximum loss at the DDS clock frequency (sample rate). That helps to
> reduce the garbage seen from these older DDS devices.
> One thing to keep in mind is that programming these devices requires
> 32-bit and sometimes 64-bit math, and sometimes even floating point.
> The Arduino CPUs (except possibly the new Due) does not have a good
> math library. It only has "float" types, no doubles (which causes
> poor accuracy). Also, I've had problems (failures) in trying to use
> long-long (64-bit) ints, even though that is supposedly implemented.
> Using 32-bit long ints is problematic in doing the
> frequency-tuning-words (FTW). I'm having a similar problem
> programming the Si570 with an Arduino, I had to give up on precise
> calculations, and am using a "close-enough" approximation.
> Terry, WB4JFI
> *From:* Karl W4KRL <mailto:W4KRL at arrl.net>
> *Sent:* Thursday, January 03, 2013 1:54 PM
> *To:* tacos at amrad.org <mailto:tacos at amrad.org>
> *Subject:* idealCircuit simulator
> I ordered an AD9850 DDS Arduino shield on eBay for less than $6. It
> should be here from Thailand at the end of the month.
> A quick reading of the application notes suggests that the output
> should go through a 70MHz low pass filter for use in the 0 to 40MHz
> range. There are several nice on-line filter designers:
> But after looking at the low-res circuit jpg on eBay it turns out that
> it has an on-board filter. The values are vaguely similar to what the
> on-line designers calculate for a 7-pole Butterworth Pi but each
> inductor is shunted with a small capacitor. I thought it would be
> interesting to figure out what the filter was supposed to do and
> looked unsuccessfully for an on-line circuit simulator. There is a
> fair one sponsored by Digi-Key at http://www.partsim.com/simulator but
> I found it so awkward to use that I gave up after drawing the circuit.
> It did not produce any useful results.
> Then I came across idealCircuit at http://ic.sidelinesoft.com/
> The download is only 1.4MB and is free. There are probably a lot of
> limitations but it looks good enough for quick general use. One
> obvious drawback is the lack of part IDs. Maybe there is a work around
> in the manual.
> I drew the filter and got a voltage and phase graph in just a few
> minutes with zero frustration. The circuit and results are attached.
> It seems to be a 70MHz low pass with some ripple and peaks near
> cutoff. _Comments on the design and performance of the filter are
> welcome. Why did they shunt the inductors with caps?_
> Happy New Year
> Karl W4KRL
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> Tacos at amrad.org
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