Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Science
If you measure something often enough, it becomes science
I let the DDS cool down overnight, turned it on, and recorded the frequency offset as a function of temperature as it heated up again:
125 MHz Osc Freq Offset vs Temp – Quadratic – 29 – 43 C
The reduced spacing between the points as the temperature increases shows how fast the oscillator heats up. I zero-beat the 10 MHz output, scribbled the temperature, noted the offset, and iterated as fast as I could. The clump of data over on the right end comes from the previous session with essentially stable temperatures.
I only had to throw out two data points to get such a beautiful fit; the gaps should be obvious.
The fit seems fine from room (well, basement) ambient up to hotter than you’d really like to treat the DDS, so using the quadratic equation should allow on-the-fly temperature compensation. Assuming, of course, the equation matches some version of reality close to the one prevailing in the Basement Laboratory, which remains to be seen.
In truth, it probably doesn’t, because the temperature was changing so rapidly the observations all run a bit behind reality. You’d want a temperature-controlled environment around the PCB to let the oscillator stabilize after each increment, then take the measurements. I am so not going to go there.
The original data:
125 MHz Osc Freq Offset vs Temp – 29 – 43 C – data
DDS Oscillator Frequency Offset vs. Temperature – complete
Now, as it turns out, the one lonely little dot off the line happened just after I lit the board up after a tweak, so the oscillator temperature hadn’t stabilized. Tossing it out produces a much nicer fit:
DDS Oscillator Frequency Offset vs. Temperature
Looks like I made it up, doesn’t it?
The first-order coefficient shows the frequency varies by -36 Hz/°C. The actual oscillator frequency decreases with increasing temperature, which means the compensating offset must become more negative to make the oscillator frequency variable match reality. In previous iterations, I’ve gotten this wrong.
For example, at 42.5 °C the oscillator runs at:
125.000000 MHz - 412 Hz = 124.999588 MHz
Dividing that into 232 = 34.35985169 count/Hz, which is the coefficient converting a desired frequency into the DDS delta phase register value. Then, to get 10.000000 MHz at the DDS output, you multiply: 10×106 × 34.35985169 = 343.598517×106
Stuff that into the DDS and away it goes.
Warmed half a degree to 43.0 °C, the oscillator runs at:
125.000000 MHz - 430 Hz = 124.999570 MHz
That’s 18 Hz lower, so the coefficient becomes 34.35985667, and the corresponding delta phase for a 10 MHz output is 343.598567×106.
After insulating the DDS module to reduce the effect of passing breezes, I thought the oscillator temperature would track the ambient temperature fairly closely, because of the more-or-less constant power dissipation inside the foam blanket. Which turned out to be the case:
DDS Oscillator Temperature vs. Ambient
The little dingle-dangle shows startup conditions, where the oscillator warms up at a constant room temperature. The outlier dot sits 0.125 °C to the right of the lowest pair of points, being really conspicuous, which was another hint it didn’t belong with the rest of the contestants.
So, given the ambient temperature, the oscillator temperature will stabilize at 0.97 × ambient + 20.24, which is close enough to a nice, even 20 °C hotter.
The insulation blanket reduces short-term variations due to breezes, which, given the -36 Hz/°C = 0.29 ppm temperature coefficient, makes good sense; you can watch the DDS output frequency blow in the breeze. It does, however, increase the oscillator temperature enough to drop the frequency by 720 Hz, so you probably shouldn’t use the DDS oscillator without compensating for at least its zero-th order offset at whatever temperature you expect.
Of course, that’s over a teeny-tiny temperature range, where nearly anything would be linear.
The two knockoff Neopixeltestfixtures went dark while their USB charger accompanied me on a trip, so they spent a few days at ambient basement conditions. When I plugged them back into the charger, pretty much the entire array lit up in pinball panic mode:
WS2812 LED – test fixture multiple failures
Turns out three more WS2812 chips failed in quick succession. I’ve hotwired around the deaders (output disconnected, next chip input in parallel) and, as with the other zombies, they sometimes work and sometimes flicker. That’s five failures in 28 LEDs over four months, a bit under 3000 operating hours.
For lack of a better explanation: the cool chips pulled relatively moist air through their failed silicone encapsulation, quietly rotted out in the dark, then failed when reheated. After they spend enough time flailing around, the more-or-less normal operating temperatures drives out the moisture and they (sometimes) resume working.
Remember, all of them passed the Josh Sharpie Test, so you can’t identify weak ones ahead of time.
A slight modification spits out the (actual) frequency and dBV response (without subtracting the 108 dB intercept to avoid negative numbers for now) to the serial port in CSV format, wherein a quick copypasta into a LibreOffice Calc spreadsheet produces this:
Spectrum-32
Changing the center frequency and swapping in a 60 kHz resonator:
Spectrum-60
Much prettier than the raw scope shot with the same data, there can be no denyin’:
Log V vs F – 32766 4 Hz – CX overlay
I think the wobbulations around the parallel resonant dip come from the excessively hugely too large 10 µF caps in the signal path, particularly right before the log amp input, although the video bandwidth hack on the AD8310 module may contribute to the problem. In any event, I can see the log amp output wobbling for about a second, which is way too long.
Anyhow, the series-resonant peaks look about 1 Hz wide at the -3 dBV points, more or less agreeing with what I found with the HP 8591 spectrum analyzer. The series cap is a bit smaller, producing a slightly larger frequency change in the series resonant frequency: a bit under 2 Hz, rather than the 1 Hz estimated with the function generator and spectrum analyzer.
I still don’t understand why the parallel resonant dip changes, although I haven’t actually done the pencil pushing required for true understanding.
The rail trail recently sprouted white mile markers:
Rail Trail – Marathon 13 mile marker
This one stood out:
Rail Trail – Marathon 13.10938 mile marker
Not being a marathoner, I had the vague notion a marathonshould be an even number of kilometers, because it’s not an even number of miles, but nooooo it’s just an arbitrary distance everybody agreed would be about right for a good long run.
During the rest of the ride, I worked out that 1 micro mile = 5+ milli foot = 60+ milli inch, so the rightmost significant figure in that marker represents increments of, oh, a smidge under ¾ inch. Middle of the hash line marks the spot, perhaps?
I’ve seen similar markers along other courses, with varying numbers of ahem significant figures, and will not say how long it took me to recognize what it represented.
While tinkering with the SPI code for the AD9850 DDS module, I wrote down the ambient temperature and the frequency tweak required to zero-beat the 10 MHz output with the GPS-locked oscillator. A quick-n-dirty plot summarizing two days of randomly timed observations ensued:
AD9850 DDS Module – Frequency vs Temperature
The frequency offset comes from the tweak required to zero-beat the output by adjusting the initial oscillator error: a positive tweak produces a smaller count-per-hertz coefficient and reduces the output frequency. As a result, the thermal coefficient sign is backwards, because increasing temperature raises the oscillator frequency and reduces the necessary tweak. I think so, anyway; you know how these things can go wrong. More automation and reliable data would be a nice touch.
Foam sheets formed a block around the DDS module, isolating it from stray air currents and reducing the clock oscillator’s sensitivity:
AD9850 DDS module – foam insulation
I used the ambient temperature, because the thermocouple inside the foam (not shown in the picture) really wasn’t making good contact with the board, the readings didn’t make consistent sense, and, given a (nearly) constant power dissipation, the (average) oscillator temperature inside the foam should track ambient temperature with a constant offset. I think so, anyway.
The coefficient works out to 0.02 ppm/°C. Of course, the initial frequency offset is something like -400 Hz = 3 ppm, so we’re not dealing with lab-grade instrumentation here.
The Smell of Molten Projects in the Morning 