DDS clock generator in LT Spice...

[John Larkin]

https://www.dropbox.com/s/1xx7sz1e5rg6jsi/JLDDS_100M_4K.jpg?raw=1

https://www.dropbox.com/s/3859sc4qayv3jva/JLDDS_100M_4K_A.asc?dl=0

It would have been a horror to build a digital phase accumulator in LT
Spice, so I did it with the bootstrapped sample-and-hold. 1 LSB is 1
volt, making a 4 kilovolt sawtooth. Close enough.

We\'ll probably do this in a cute little efinix FPGA (digitally, and
not 4KV) which has a megabit of ram, so we can pull more bits out of
the phase accumulator and have a lot more entries in the sine table,
which will improve jitter at low frequencies.

We might even use a 14-bit DAC, if that helps much. The efinix RAM
comes in slices 5 bits wide, so 14 takes no more than 12. Might
explore 10 and see what happens.

Now I need a good way to measure peroid jitter, so I can play with
options.

keywords LT Spice DDS clock frequency synthesizer phase accumulator
jitter filter coffee yardwork sucks

 

[Ricky]

On Saturday, August 13, 2022 at 10:37:24 AM UTC-4, John Larkin wrote:

https://www.dropbox.com/s/1xx7sz1e5rg6jsi/JLDDS_100M_4K.jpg?raw=1

https://www.dropbox.com/s/3859sc4qayv3jva/JLDDS_100M_4K_A.asc?dl=0

It would have been a horror to build a digital phase accumulator in LT
Spice, so I did it with the bootstrapped sample-and-hold. 1 LSB is 1
volt, making a 4 kilovolt sawtooth. Close enough.

We\'ll probably do this in a cute little efinix FPGA (digitally, and
not 4KV) which has a megabit of ram, so we can pull more bits out of
the phase accumulator and have a lot more entries in the sine table,
which will improve jitter at low frequencies.

We might even use a 14-bit DAC, if that helps much. The efinix RAM
comes in slices 5 bits wide, so 14 takes no more than 12. Might
explore 10 and see what happens.

Now I need a good way to measure peroid jitter, so I can play with
options.

The main aspect of the DDS design to minimize jitter will be to increase the useful size of the phase accumulator, the NCO. If your DAC is only 14 bits, that will cause distortions, but mostly in easy to filter frequencies. Reducing your phase word size into the sine generator will generate close in spurs that are very hard to filter.

You can use a few multipliers and adders to extend the word width of your sine generator on the input side using a variety of techniques. It doesn\'t need to be one massive look up table. As you have pointed out, when examining a small enough region, the sine function is nearly linear.

--

Rick C.

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