Tag: Improvements

Making the world a better place, one piece at a time

Crystal Modeling: Parasitic Capacitor Values

A Circuit Cellar reader asked for a better explanation of the parasitic capacitors inside a quartz crystal can than I provided in my April 2017 Circuit Cellar column. Here’s the schematic, with values for a 32 kHz tuning fork resonator and the original caption:

Figure 1 – The mechanical properties of quartz resonators determine the values of the “motional” electrical components in the bottom branch of its circuit model. These values correspond to a 32.768 kHz quartz tuning fork resonator: the 10.4 kH inductor is not a misprint!

I wrote this about the caps:

The value of C0 in the model’s middle branch corresponds to the capacitance between the electrodes plated onto the quartz. The 3.57954 MHz crystal in the title photo, with two silver electrodes deposited on a flat insulating disk, closely resembles an ordinary capacitor. You can measure a crystal’s C0 using a capacitance meter.
However, each of those electrodes also has a capacitance to the resonator’s metal case. The top branch of the model shows two capacitors in series, with Cpar representing half the total parasitic capacitance measured between both leads and the case. Grounding the case, represented by the conductor between the capacitors, by soldering it to the ground plane of an RF circuit eliminates any signal transfer through those capacitors. They will appear as shunt capacitors between the pins and ground; in critical applications, you must add their capacitance to the external load capacitors.

And this about the measurement technique, using a fixture on my AADE LC meter:

With the resonator case captured under the clip on the far right and both its leads held by the clip to the upper left, the meter measures Cpar, the lead-to-case parasitic capacitance. The meter will display twice the value of each parasitic capacitor, at least to a good approximation, because they are in parallel. The five resonators averaged 0.45 pF, with each lead having about 0.25 pF of capacitance to the case. Obviously, measuring half a picofarad requires careful zeroing and a stable fixture: a not-quite-tight banana jack nut caused baffling errors during my first few measurements.
With the resonator repositioned as shown in Photo 2, with one lead under each clip, the meter measures C0, the lead-to-lead capacitance. After careful zeroing, the resonators averaged 0.85 pF.
Although the parasitic lead-to-case capacitors are in parallel with C0, their equivalent capacitance is only Cpar/4 = 0.1 pF. That’s close enough to the measurement error for C0, so I ignored it by rounding C0 upward.

He quite correctly pointed out:

With both leads connected together on one side, then that essentially constitutes a single electrical element inside the case. And with the case being a single element, this configuration in the test fixture seems like a single capacitor with one lead being the case and the other lead being the pins, with a vacuum dielectric.

I would think the meter would display the total capacitance rather than twice the value … It makes sense to me to later say Cpar/2 when the leads are not connected together.

Here’s my second pass at the problem:

… the two-capacitor model comes from the common-case condition, where each lead displays a (nominally equal) parasitic capacitance to the case, because the crystal mounting is reasonably symmetric inside the can. It’s easiest to measure the total capacitance with the leads shorted together, because it’s in the low pF range, then divide by two to get the value of each lead-to-case cap.

[…]

For “real” RF circuits with larger (HC-49 -ish) crystals in parallel-resonance mode, you ground the case and subtract the parasitic capacitance at each lead from the external load capacitors. That’s the usual situation for microprocessor clock oscillators: the crystal sits across the clock amplifier pins, with two more-or-less equal caps from the pins to ground. You should subtract the internal parasitic caps from the clock’s specified load caps, but in practice the values are so small and the cap tolerance so large that it mostly doesn’t matter.

[…]

Un-grounding the case puts those two parasitic caps in series, just as with two discrete caps, so the lead-to-lead capacitance is (or should be!) half of each: 1/4 of the both-leads-to-case value.

Re-reading yet again says I glossed over the effect of having C0 in parallel with the Cpar/2 caps, but methinks dragging those complications into the model benefits only the theoreticians among us (or those working very close to the edge of the possible).

To make it worse, I also botched the QEX reference, which should be Jan/Feb 2016, not 2017. Verily, having a column go read-only makes the errors jump right off the page. [sigh]

At least I can point to this and amend as needed.

2017-07-10
AD9850 DDS Module: 125 MHz Oscillator vs. Temperature, Quadratic Edition

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

2017-07-06
AD9850 DDS Module: 125 MHz Oscillator vs. Temperature, Linear Edition

A day of jockeying the AD9850 DDS oscillator shows an interesting relation between the frequency offset and the oscillator temperature:

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.

Obviously, you need Pretty Good Precision in your arithmetic to get those answers.

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 original data:

DDS Oscillator offset vs temperature – 2017-06-24

2017-07-05
Canon LiDE 120 Scanner vs. SANE
I just replaced a cheap old Canon LiDE 30 flatbed scanner with a cheap new LiDE 120, only to get flat-black scans. The machinery worked (yes, I released the travel lock), everything seemed fine, the images were the proper size, but they were dead black.

Of course, the scanner worked OK on the Token Windows Box, but wow what crappy software they include.

Turns out the LiDE 120 requires the latest-and-greatest version 1.0.27 of the various SANE programs & libraries. Mercifully, getting those didn’t require compiling from source, just setting up the maintainer’s PPA of the most recent stable release:
```
sudo add-apt-repository ppa:rolfbensch/sane-release
sudo apt-get update
sudo apt-get upgrade
```
Which introduced circular dependencies with the distro-installed version 1.0.25 files, which I solved by ripping the entire SANE Thing out by the root(s) and reinstalling it to (re)synchronize All The Things:
```
sudo apt-get remove libsane:i386 sane sane-utils xsane libsane-common ia32-libs libsane
sudo apt-get install libsane:i386 sane sane-utils xsane libsane-common ia32-libs libsane
```
And then It Just Worked:

C-Note – detail

Of course, you must keep this WARNING in mind:

Canon LiDE 120 – Legal Issues Warning

Franklin didn’t know about scanners or color laser printers when he observed:

Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety

Of course, there’s more to the story, but one should:

Never let the truth get in the way of a good story.
2017-07-04
LM75A vs. SOIC Adapter: Mirror Imaging

After hairballing an LM75A I²C temperature sensor to verify at least one of the eBay lot worked, a bag of SOIC-to-DIP space transformers arrived, so I soldered up another LM75A:

LM75A on DIP8 adapter – top

The SOIC chip pattern sits at right angles to the DIP pins, which took some getting used to.

The slightly defocused wire connecting pin 4 (on the IC) to pins 5, 6, and 7 (on the PCB) selects address 0x48.

So I flipped it over, soldered four wires (+5 V, GND, SDA, SCL) to the numbered pins on bottom of the board, made up a little header for the other end, wired a socket strip on the crystal tester board, plugged it in, and … nothing worked.

Turns out that the other side of the board carries a TSSOP pattern, which I’d neatly masked off with a snippet of Kapton tape, surrounded by eight numbered pins. Of course, those pin numbers correspond to the TSSOP pattern facing you, so they’re mirror-imaged for the SOIC pattern on the other side.

Soooo, the proper wiring for the SOIC pattern as seen from the TSSOP side has the pin numbers exactly bass-ackwards:

LM75A on DIP8 adapter – bottom

The insulation looked a lot better the first time I soldered the wires to the PCB. Honest.

Anyhow, when correctly wired, the LM75A worked as it should:

LM75A Temperature Sensor – installed

It’s snuggled chip-down against the top of the 125 MHz oscillator can, with a dab of heatsink compound improving their thermal bond and a yellow cable tie around the foam holding them together. The socket header is wired pin-for-pin to the DAC I²C socket directly above it.

The OLED temperature display shows 28.250 °C, because the oscillator just started up in a cool basement. It’ll eventually settle around 39-ish °C, where its output should be pretty close to the 125 MHz – 344 Hz value hardcoded into the source.

Oh, that’s a 3 mm amber LED next to the relay can: much less glaring than the white LED, no matter what it looks like here.

2017-06-27

Mailing Tube End Caps: Screw-in Version

The mailing tube arrived with contents intact, although the USPS inlet scanning didn’t work and the tube pretty much teleported across several states without leaving any tracking data behind. The recipient suggested several modifications to the caps:

Review of user experience of tube end:
The ribs on the endcap are very good at holding the cap on, so much so that I had to use a prying implement to remove it, which cracked the flange.
Would consider less depth on the cap, and possibly another layer on the flange.

Some continuous process improvement (a.k.a OpenSCAD hackage) produced a swoopy threaded cap with thumb-and-finger grips:

Mailing Tube Screw Cap - top - Slic3r — Mailing Tube Screw Cap – top – Slic3r

The finger grips are what’s left after stepping a sphere out of the cap while rotating it around the middle:

Mailing Tube Cap - finger grip construction — Mailing Tube Cap – finger grip construction

That worked out surprisingly well, with the deep end providing enough of a vertical-ish surface to push against.

The two hex holes fit a pin wrench, because the grips twist only one way: outward. The wrench eliminates the need for a flange, as you can now adjust the cap insertion before slathering packing tape over the ends. Man, I loves me some good late binding action!

A three-start thread seemed like overkill, but was quick & easy. The “thread form” consists of square rods sunk into the cap perimeter, with one edge sticking out:

Mailing Tube Cap - thread detail — Mailing Tube Cap – thread detail

They’re 1.05 times longer than the cap perimeter facets to make their ends overlap, although they’re not tapered like the ones in the broom handle dingus, because it didn’t (seem to) make any difference to the model’s manifoldhood.

Not needing any endcaps right now, I built one for show-n-tell:

Threaded mailing tube end cap - installed — Threaded mailing tube end cap – installed

The OpenSCAD source code as a GitHub Gist:

	// Mailing tube end cap
	// Ed Nisley KE4ZNU – June 2017

	Layout = "Build";

	Model = "Screw";

	//- Extrusion parameters – must match reality!

	ThreadThick = 0.25;
	ThreadWidth = 0.40;

	function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);

	Protrusion = 0.1;

	HoleWindage = 0.2;

	//- Screw sizes

	inch = 25.4;

	TubeID = 2 * inch;
	TubeWall = 0.1 * inch;

	CapInsert = 15.0;
	CapRim = 6*ThreadThick;
	CapWall = 3*ThreadWidth;

	NumFlanges = 3;
	FlangeHeight = 3*ThreadThick;
	FlangeWidth = ThreadWidth/2;
	FlangeSpace = CapInsert / (NumFlanges + 1);

	ThumbHoleOD = 20.0;
	ThumbHoleAngle = 100;
	ThumbHoleSteps = 10;

	SpannerPinOD = 5.0;
	HelixOD = 4*ThreadThick;
	HelixHeight = 0.75*CapInsert;
	HelixAngle = atan(HelixHeight/(PI*TubeID));
	HelixStarts = 3;

	OAHeight = CapInsert + CapRim;

	NumRibs = 3*4;
	NumSides = 3*NumRibs;

	//- Adjust hole diameter to make the size come out right

	module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes

	Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);

	FixDia = Dia / cos(180/Sides);

	cylinder(r=(FixDia + HoleWindage)/2,h=Height,$fn=Sides);
	}

	module ScrewCap() {

	union() {

	difference() {
	cylinder(d=TubeID,h=OAHeight,$fn=NumSides);

	for (a=[0,180])
	for (i=[0:ThumbHoleSteps-1])
	rotate(a + i*ThumbHoleAngle/ThumbHoleSteps)
	translate([TubeID/4,0,-iThumbHoleOD/(2ThumbHoleSteps)])
	sphere(d=ThumbHoleOD);

	for (a=[0,180])
	rotate(a – 60)
	translate([0.75*TubeID/2,0,-Protrusion])
	rotate(0*180/6)
	PolyCyl(SpannerPinOD,0.75*CapInsert,6);
	}

	for (s=[0:HelixStarts-1])
	for (i=[0:NumSides-1])
	rotate(i360/NumSides + 180/NumSides + s360/HelixStarts)
	translate([TubeID/2 – 0.25HelixOD,0,iHelixHeight/NumSides + HelixOD])
	rotate([90 + HelixAngle,0,0])
	cylinder(d=HelixOD,h=1.05PITubeID/NumSides,center=true,$fn=4);
	}

	}

	module PushCap() {

	difference() {
	cylinder(d=TubeID,h=OAHeight,$fn=NumSides);
	translate([0,0,CapWall])
	cylinder(d=TubeID – 2*CapWall,h=OAHeight,$fn=NumSides);
	}

	for (i=[1:NumFlanges])
	translate([0,0,i*FlangeSpace])
	difference() {
	cylinder(d=TubeID + 2*FlangeWidth,h=FlangeHeight,$fn=NumSides);
	translate([0,0,-Protrusion])
	cylinder(d=TubeID – 2CapWall,h=FlangeHeight + 2Protrusion,$fn=NumSides);
	}

	for (i=[0:NumRibs-1])
	rotate(i*360/NumRibs)
	translate([0,-ThreadWidth,CapWall + ThreadThick])
	cube([TubeID/2 – CapWall/2,2*ThreadWidth,CapInsert + CapRim – CapWall – ThreadThick],center=false);

	translate([0,0,CapInsert]) {
	difference() {
	cylinder(d=TubeID + 2*TubeWall,h=CapRim,$fn=NumSides);
	translate([0,0,-Protrusion])
	cylinder(d=TubeID – 32CapWall,h=2*CapRim,$fn=NumSides);
	}
	}
	}

	//- Build things

	if (Model == "Push")
	if (Layout == "Show")
	PushCap();
	else if (Layout == "Build")
	translate([0,0,OAHeight])
	rotate([180,0,0])
	PushCap();

	if (Model == "Screw")
	if (Layout == "Show")
	ScrewCap();
	else if (Layout == "Build")
	translate([0,0,OAHeight])
	rotate([180,0,0])
	ScrewCap();

view raw Mailing Tube Cap.scad hosted with ❤ by GitHub

2017-06-26

Handbag Strap Rivet Repair

One of the leather strap anchors on Mary’s giant haul-everything-to-a-concert(*) handbag pulled its rivet through the canvas fabric:

Handbag – pulled-through rivet

We knotted the strap around the zippered opening and completed the mission.

Of course, it wouldn’t have pulled through if they’d splurged on washers, but noooo too expensive:

Handbag – intact rivet – inside

Some rummaging produced a pan-head M3 screw of suitable length:

Handbag – repaired – outside

A slightly battered acorn nut was a special treat for the inside, with another washer to keep me happy:

Handbag – repaired – inside

That was easy!

(*) At Tanglewood, where they don’t strip-search you on the way in, tow-behind coolers seemed de rigueur, and a good time was had by all.

2017-06-25