Hi The DAC will quite nicely produce a trapezoid (or clipped triangle wave). It's certainly on the list.
--- I've had a lot of lunch time discussions with the NIST guys about their "obsession" with input levels. About all I can say is that I don't see the same sensitivities they do. I suspect a lot of the issue is that we're not using the exact same circuits / components. ----- Matching the isolation amplifier to the mixer for efficient power transfer is something I do plan to look at. I suspect it's only good up to a certain point and then you get into trouble. ----- Lots of things to check ... Bob On Feb 2, 2010, at 12:06 AM, Bruce Griffiths wrote: > Bob Camp wrote: >> Hi >> >> 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. >> >> > When both the RF and LO ports are saturated, the mixer output waveform > depends on how the IF port is terminated. > The output is indeed approximately triangular with your IF port termination > method when both the RF and LO ports are saturated. > With the IF port terminated in a capacitor when both RF and LO ports are > saturated the output waveform is quasi trapezoidal. > When only the LO port is saturated the IF output is sinusoidal. >> 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. >> > Producing a high amplitude (eg 20V pp) output and attenuating it down to say > 2V pp or so typical of a mixer will significantly reduce the noise due to the > output amplifier. >> 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. >> >> > Yes but NIST used a saturated mixer and still found that the mixer phase > shift depended on how hard you drive the diodes. > Long term variations in isolation amplifier output due to temperature > variations may be significant. >> 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 .... >> >> Bob >> >> > You could also try driving the mixer ports from a highe impedance source (eg > transistor collector). > One early NIST paper advocated this. > > Bruce >> >> On Feb 1, 2010, at 9:48 PM, Bruce Griffiths wrote: >> >> >>> Bob Camp wrote: >>> >>>> 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 >>>> _______________________________________________ >>>> >>>> >>> Bob >>> >>> 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. >>> >>> Bruce >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
