Archive for category Electronics Workbench

Byonics TinyTrak3+ vs. RFI

Some weeks ago, the APRS + voice adapter on my radio began randomly resetting during our rides, sending out three successive data bursts: the TinyTrak power-on message, an ID string, and the current coordinates. Mary could hear all three packets quite clearly, which was not to be tolerated.

I swapped radios + adapters so that she could ride in peace while I diagnosed the problem, which, of course, was both intermittent and generally occurred only while on the road. The TinyTrak doc mentions “… a sign of the TinyTrak3 resetting due to too much local RF energy”, so I clamped ferrite cores around All! The! Cables! and the problem Went Away.

Removing one core each week eventually left the last core on the GPS receiver’s serial cable, which makes sense, as it plugs directly into the TT3. The core had an ID large enough for several turns (no fool, I), another week established a minimum of three turns kept the RFI down, so I settled for five:

KG-UV3D APRS - ferrite on TT3 GPS cable

KG-UV3D APRS – ferrite on TT3 GPS cable

Prior to the RFI problem cropping up, nothing changed. Past experience has shown when I make such an assertion, it means I don’t yet know what changed. Something certainly has and not for the better.

I swapped the radios + adapters and all seems quiet.



Cylindrical Cell Adapter: 18650 to 3xAAA

Anker LC40 flashlights can use either one lithium 18650 cell or an adapter holding three AAA cells. I now prefer 18650 cells, but they’re nigh onto 4 mm smaller than the flashlight ID and rattle around something awful.

I can fix that:

Anker LC40 with 18650 cell adapter

Anker LC40 with 18650 cell adapter

Three new entries appear in the cell dimension table of my OpenSCAD inter-series battery adapter program:

NAME = 0;
ID = 0;       // for non-cell cylinders
OD = 1;

Cells = [
  ["AnkerLC40",23.0,55.0]           // Flashlight tube loose-fit for 3xAAA adapter

I took the opportunity of adding OpenSCAD Customizer comments, which means this now works:

OpenSCAD Customizer - dropdown selections

OpenSCAD Customizer – dropdown selections

The model looks about the same as before, although with a few more sides just for pretty:

AnkerLC40 vs. 18650 Sleeve - Slic3r

AnkerLC40 vs. 18650 Sleeve – Slic3r

That was easy …



LF Crystal Tester: OLED Noise vs. Log Amp

Having installed a cheap USB isolator to remove some obvious 60 Hz interference, the 100 Hz OLED refresh noise definitely stands out:

Log amp - xtal amp - OLED noise

Log amp – xtal amp – OLED noise

The bottom trace comes from the 100× = 40 dB MAX4255 amplifier boosting the crystal output to a useful level. The fuzz on the waveform is actually the desired (off resonance) 60 kHz signal at maybe 30 mVpp, so the input is 300 µVpp.

The worst part of the OLED noise looks like 100 mVpp, for about 1 mVpp at the crystal output, call it +10 dB over the desired signal. Some high-pass filtering would help, but it’s easier to just shut the display off while measuring the crystal.

The top trace is the log amp output at (allegedly) 24 mV/dBV. The input bandwidth obviously extends way too low, as it’s neatly demodulating the input signal: the peaks correspond to both the positive and negative signal levels, so reducing the 1 µF input coupling caps will be in order.

In between those 100 Hz groups, the input signal shines through to the log amp output at the V1 cursor. The peak noise rises 290 mV above that, so the log amp thinks it’s 12 dB higher. Pretty close to my guesstimated 10 dB, methinks.

So, turning off the OLED should help a lot, which is feasible in this situation. If you must run the display while caring deeply about signal quality, you must devote considerably more attention to circuit construction quality.


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Optiplex 980 Power Supply: Capacitor FAIL

Came up from the Basement Laboratory to find my Dell Optiplex 980 PC had failed, with the power button and diagnostic 1 + 3 LEDs blinking amber.  They built it back in June 2010, so section 3 of the Dell reference applies, the power supply status LED on the back panel was off, and, going straight to the heart of matter, I popped the top, disconnected the internal power supply cables, and poked the power supply test button:

Optiplex 980 Power Supply - rear panel test button

Optiplex 980 Power Supply – rear panel test button

… and it’s dead.

Inside, the system board sports a Mini-ATX power supply connector:

Optiplex 980 - Mini-ATX power connector

Optiplex 980 – Mini-ATX power connector

I originally hoped to swap a supply from an Optiplex 755 (also in a Small Form Factor case) residing on the recycle heap, but it has an ordinary ATX connector:

Optiplex 755 - ATX power connector

Optiplex 755 – ATX power connector

So I moved the 980’s SSD and dual-Displayport video card into the 755, fired that devil up, and … it worked!

With my desktop back in action, albeit somewhat slower, I popped the dead supply’s case by violating the Warranty Void If This Label Removed sticker to unscrew the last screw:

Optiplex 980 Power Supply - overview

Optiplex 980 Power Supply – overview

Notice anything?

The electrolytic capacitors over on the left look like this:

Optiplex 980 - Good capacitor

Optiplex 980 – Good capacitor

The cluster of caps on the upper right have bulged pressure-relief lids, like this:

Optiplex 980 - Bulging capacitor 1

Optiplex 980 – Bulging capacitor 1

And this:

Optiplex 980 - Bulging capacitor 2

Optiplex 980 – Bulging capacitor 2

And this:

Optiplex 980 - Bulging capacitor 3

Optiplex 980 – Bulging capacitor 3

None had ruptured, but they’re obviously feeling a bit nauseous.

Given the 980’s mid-2010 manufacturing date, this probably isn’t capacitor plague, just simple overheating from operating in a dead-air zone amid all those heatsinks and wires. Some of the Usual Unnamed Sources suggest overheating the capacitors is how manufacturers ensure their hardware doesn’t last forever, without being obvious about planned obsolescence; I’m loathe to ascribe to malice what can be explained by design desperation.

A Genuine Dell replacement supply from eBay ($25 delivered) came from yet another “small form factor” Dell chassis, so it isn’t quite the same size, lacks a supply test button / LED status light, and doesn’t quite fit:

Optiplex 980 - replacement supply misfit

Optiplex 980 – replacement supply misfit

Nothing a sheet metal nibbling tool can’t fix, though, given I haven’t developed a deep emotional attachment to the chassis. I gnawed off the left side of the frame and squared up the rim around the lower screw, after which the opening fit the supply pretty well, although the latching tab bent up from the bottom of the chassis didn’t quite engage the far end of the supply. No big deal: it’s not in a high-vibration environment.

The new-to-me supply also carries an ATX connector, but the eBay seller included a Mini-ATX adapter. Jamming the adapter + wires into the space available required concerted muttering, assisted by tucking the SSD under the DVD-RW drive. No pictures, as it’s a classic seven pounds in a five pound box situation.

And then It Just Worked again.


AT26 / TF26 Quartz Resonator Identification

There’s not much room on an AT26 / TF26 can for a readable label, unless one owns a metal-marking laser, but a simple bar code should let me identify each one:

Quartz Resonators - binary marking

Quartz Resonators – binary marking

The empty “0” slot down at the bottom will hold the crash-test dummy resonator I’ve been using to get the tester working.

The red-and-blue stripes from plain old fine-point Sharpie pens will rub off under duress, which I hope to avoid. After finishing up, I’m still not sure blue makes a better zero than red; you can make a convincing argument either way:

Binary marked AT26 Quartz Resonators

Binary marked AT26 Quartz Resonators

The bag allegedly contained 25 resonators, although I’m willing to agree the last one escaped into the clutter on or under the Electronics Workbench.



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:

Quartz resonator circuit model

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.



AD9850 Module: Oscillator Thermal Time Constant

With an LM75 atop the 125 MHz oscillator and the whole thing wrapped in foam:

LM75A Temperature Sensor - installed

LM75A Temperature Sensor – installed

Let it cool overnight in the Basement Laboratory, fire it up, record the temperature every 30 seconds, and get the slightly chunky blue curve:

125 MHz Oscillator - Heating Time Constant

125 MHz Oscillator – Heating Time Constant

Because we know this is one of those exponential-approach problems, the equation looks like:

Temp(t) = Tfinal + (Tinit - Tfinal) × e-t/τ

We can find the time constant by either going through the hassle of an RMS curve fit or just winging it by assuming:

  • The initial temperature, which is 22.5 °C = close to 22.7 °C ambient
  • The final temperature (call it 42 °C)
  • Any good data point will suffice

The point at 480 s is a nice, round 40 °C, so plug ’em in:

40.0 = 42.0 + (22.7 - 42.0) × e-480/τ

Turning the crank produces τ = 212 s, which looks about right.

Trying it again with the 36.125 °C point at 240 s pops out 200.0 °C.

Time for a third opinion!

Because we live in the future, the ever-so-smooth red curve comes from unleashing LibreOffice Calc’s Goal Seek to find a time constant that minimizes the RMS Error. After a moment, it suggests 199.4 s, which I’ll accept as definitive.

The spreadsheet looks like this:

T_init 22.5
T_final 42.0
Tau 199.4
Time s Temp °C Exp App Error²
0 22.250 22.500 0.063
30 25.500 25.224 0.076
60 28.000 27.567 0.187
90 30.125 29.583 0.294
120 31.750 31.317 0.187
150 33.250 32.810 0.194
180 34.375 34.093 0.079
210 35.375 35.198 0.031
240 36.125 36.148 0.001
270 36.813 36.965 0.023
300 37.500 37.668 0.028
330 38.125 38.273 0.022
360 38.500 38.794 0.086
390 39.000 39.242 0.058
420 39.500 39.627 0.016
450 39.750 39.959 0.043
480 40.000 40.244 0.059
510 40.250 40.489 0.057
540 40.500 40.700 0.040
570 40.750 40.882 0.017
600 41.000 41.038 0.001
RMS Error 0.273

The Exp App column is the exponential equation, assuming the three variables at the top, the Error² column is the squared error between the measurement and the equation, and the RMS Error cell contains the square root of the average of those squared errors.

The Goal Seeker couldn’t push RMS Error to zero and gave up with Tau = 199.4. That’s sensitive to the initial and final temperatures, but close enough to my back of the envelope to remind me not to screw around with extensive calculations when “two minutes” will suffice.

Basically, after five time constants = 1000 s = 15 minutes, the oscillator is stable enough to not worry about.