[time-nuts] Triangle Waves
magnus at rubidium.dyndns.org
Wed Feb 3 00:03:40 UTC 2010
Bob Camp wrote:
> At least from the last time I tried it:
> If you use a sine wave input source, it's got to be an amazingly good 10 Hz sine wave. A normal audio generator will not produce a 10 Hz output with good enough short term stability / noise to give you useful data. Audio generators may be out there that will do the job, but I certainly don't have one, and have never come across one.
> Since the output of the mixer is basically a triangle wave, it makes sense to use that as your test source. A triangle wave also has the nice property that it's easy on the math. You don't have any approximation issues with the integers going into the DAC. That shoves the inevitable digital crud higher in frequency.
> Another nice thing about a pure digital approach is that it provides a clean trigger for the "start" channel of the counter you are testing things with. You can even set up the DAC to put out square waves to see just how good various bits of the chain are. Tough to do that with anything other than another arbitrary function generator.
> I agree that the reference is going to be an issue and that a LED stack may be the way to go. No matter how you generate the test tone, power supply noise will be an issue.
> The output amplifier on the DAC is my biggest worry. I could go with a current out DAC and something like an OP-27. That won't give me 1nV/Hz either, but it will at least be within shouting distance of it. Sigma deltas might be a third option. I have no idea what their low frequency flicker noise looks like.
> So, other than the noise issue (which obviously needs to be analyzed / tested / pounded on) any other issues with the approach?
> At least from what I have seen in the past, level sensitivity on the inputs shows up pretty fast in the output "beat note" as you vary the input signals that are supposed to be saturating the mixer. If they are doing their job, a 2 db level change produces a very small change in the output. If you have something amiss in that department, you will see it pretty fast. On that I'm pretty much in agreement with Rubiola's stuff.
> Since I intend to mate the isolation amps up directly on the same board as the mixer, there is no real need for a 50 ohm interface between them. If the mixer looks like 18.26 ohms, the amp output can be transformed to that level rather than 50 ohms. Everything is matched (over a 1/8" trace) and you don't burn up power in a bunch of resistors. How well that idea works - time will tell. It's easy to put the resistors in if it flunks out.
> So many things to try ....
Just a reality check question here... a simple triangle oscillator is
very easily created by two op-amps, one for an integrator and one for
Schmitt trigger operation. If you want better long-term stability open
the loop and insert a 10 Hz from your favourite divider chain of a
trusted 10 MHz or so. Would such a design be limiting your measurement
goals considerable, and would any flaws be reasonably to overcome by
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