Re: DDS, again

On 12/11/24 03:38, john larkin wrote:

I have been unsuccessful in getting LT Spice to simulate a DDS
frequency generator. It's bad enough trying to make the NCO part, but
whenever I get close it stalls or throws convergence errors.
 So I wrote a PowerBasic program that's the 32-bit NCO...  GC_Num.exe.
 Making a proper .WAV file would be a nightmare, so it outputs as a
text file with just    time  data    per line, where  data  is the
selected number of MS bits of the phase accumulator.
 LT Spice can read the file, and then do whatever it wants: sine, DAC,
filter, comparator, FFTs.
 https://www.dropbox.com/scl/fo/o0mdxxqvxupg6ynz7i7rx/AMPMbv9NOY4mJFXggTGUqJ8?rlkey=9ecl38npbgy8kxuzd9bako4kr&dl=0
 Spice reads the file as a piecewise-linear thing, so wrecks the nice
MSB data steps. I had to fool it by outputting each clocked phase
accumulator value twice, as
 bits   time
bits   time+0.9*clock_period
 same data bits both lines. Looks pretty steppy.
 

On Linux, we have 'sox' that can convert between time-value
pairs and .wav in either direction. I suppose there must be
a Windows version. I suspect the piecewise-linear problem
will still be there.
As for the convergence problems: LTspice doesn't like
discontinuities. Jim Thompson approximated everything with
smooth functions. His favourite was tanh.
Jeroen Belleman