[time-nuts] Triangle Waves

[Bob Camp]

Hi

Ok, next up on the dual mixer stuff is checking the limiter chain. To do that with any chance of the results meaning anything you need a good triangle wave. You certainly can build some pretty complex gizmos to make them. There also appears to be a fairly simple approach. 

If I take a fairly good 16 bit DAC that will accept a clock a bit above 1 MHz, I can feed a simple count up / count down into it. That should give me a triangle wave at (clock rate) / 2^32. Simply put, 1.3 MHz data gives me a 10 Hz triangle wave. The digital crud should be almost entirely up around the clock rate or higher and > 90 db down. That assumes that the DAC is a low clock feed through version and that it's got good linearity. 

A reasonable dual mixer or heterodyne system should have some kind of low pass filter built into it. Even a 150 Hz lowpass should knock the digital stuff down into a -160 noise floor. 

The gotcha seems to be flicker noise out of the DAC. There's no guarantee that the gizmo will have a 1nV/Hz class noise floor. The same sort of audio spectrum analyzers used for phase noise should be able to measure the noise coming out under various conditions. 

The nice thing about this gizmo is that it does not have to *only* put out a triangle wave. If you drive it with a micro, you can tell it to do all sorts of things. You might try a number of DC levels as you check for noise. You might also try various triangle wave levels to see how everything matches up. Slew rate limited square waves also sound interesting. 

There are a couple of other details like DC level shifting and driving it all with a decent clock. Both need to be done properly, but they don't appear to be the limiting factors in this kind of setup. 

I suspect this approach has been tried before. Any record of it out there?

Bob