[time-nuts] Triangle Waves
jfor at quik.com
Wed Feb 3 00:11:06 UTC 2010
If you use the integrator-hysteresis approach, make VERY sure the FB
capacitor has excellent dC/dV, otherwise the ramp will NOT be linear. I
built one ages ago, using a ceramic capacitor and it produced near a sine.
> 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
> better design?
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