Re: Digital sine wave generation
- From: bill.sloman@xxxxxxxx
- Date: 30 Mar 2006 14:11:00 -0800
Tim Shoppa wrote:
While my obsession is still digital sine wave generation, I'd like to
note that the resistor values in AOE 2nd Ed, 9.38, for the twisted-ring
counter weighting resistors are not the best values to choose in the
E96 series. They're not bad but not the best.
If someone wants to check, I think Lancaster's _CMOS Cookbook_ has the
same resistor values as AOE. Can't find my copy of Lancaster's book at
this moment.
What is in AOE is:
Q1: 57.6K
Q2: 30.9K
Q3: 23.7K
Q4: 22.1K
Q5: 23.7K
Q6: 30.9K
Q7: 57.6K
My search has shown that you get 80% less sum-of-squares error if you
instead use these standard E96 values:
Q1: 30.1K
Q2: 16.2K
Q3: 12.4K
Q4: 11.5K
Q5: 12.4K
Q6: 16.2K
Q7: 30.1K
The matching with these values makes it worthwhile to use 0.1% or even
better resistors (which are actually affordable and available these
days!)
It appears to me that a generic 74HC595 output has an output impedance
of 40 or 50 ohms at room temperature and currents under 1mA and 5V Vcc,
and at these impedances it is actually very slightly better to use the
11.5K-12.4K-16.2K-30.1K range rather than the ten-times-higher
115K-124K-162K-301K.
I think that you are about to find about Gibbs oscillations. If your
values are sine x/x (sinc) you are going to find that you want to
applying a Hamming raised cosine window (which it looks as if Win and
Don have done) to eliminate the ringing in the frequency response that
you get by truncating the shift register (Gibbs oscillations).
http://www.daqarta.com/EEX06.HTM
--
Bill Sloman, Nijmegen
.
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