idealCircuit simulator

[Karl W4KRL]

Tacoistas,

 

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: 

 

http://www.wa4dsy.net/filter/hp_lp_filter.html

http://www.calculatoredge.com/electronics/bw%20pi%20low%20pass.htm

 

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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