[time-nuts] Triangle Waves
lists at cq.nu
Tue Feb 2 03:30:33 UTC 2010
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 ....
On Feb 1, 2010, at 9:48 PM, Bruce Griffiths wrote:
> Bob Camp wrote:
>> 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?
> You arent going to find a DAC with a 1nV/rtHz noise floor off the shelf due to the reference noise.
> Heroic filtering measures will be necessary to reduce the reference noise, then you will have to deal with the DAC component noise which will almost invariably be greater than 1nV/rtHz.
> If it has an external reference capability you could try using a series stack of leds as in this application the reference tempco shouldnt be too important.
> A Josephson junction stack would work as a DAC well with very low noise.
> NIST uses such JJ stacks as sigma delta DACs to calibrate the Johnson noise thermometers.
> Why can't you just use a sinewave test source?
> Only the part near the zero crossings is of any importance.
> Another effect to consider with diode mixers/phase detectors is that at 10MHz the amplitude sensitivity may be as high as 256ps/dB with both inputs ports saturated.
> Reducing the input port VSWR with a series resistor and attenuator pad can reduce this effect by a factor of 10 or more.
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