The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

  • Crystal Properties: Quick-and-Dirty

    Grab-bag 12 MHz crystals
    Grab-bag 12 MHz crystals

    Having cooked up a simpleminded 12.000 MHz crystal oscillator for the WWVB simulator and gotten something close-to-but-not-spot-on the right frequency, I thought it’d be interesting to measure some of my 12 MHz crystal grab-bag collection.

    This is in the nature of exploratory surgery to see if anything more complex is warranted.

    The general procedure is covered in excellent detail by K8IQY there. His method includes building a precision oscillator and a very nice test fixture, plus a bit of straightforward commercial instrumentation. Well worth perusing.

    I have the good fortune to own an HP8591E spectrum analyzer (hereinafter, the SA) with a tracking generator (ditto, TG) that can calibrate itself to a fare-thee-well. I had to let it warm up for a few hours, as the Basement Laboratory is a wee bit colder than it really likes. Heck, it’s a lot colder than I really like, for whatever that’s worth.

    Alligator-clip crystal connections
    Alligator-clip crystal connections

    I bodged up some random coax and a few clip leads into a fur-ball circuit in front of the SA. This is not the right way to do it; a fixture with mechanical stability is the first step toward solid numbers.

    The tracking generator & analyzer present 50 Ω impedances to the outside world, which is much too high for the expected crystal series resistance, but it’ll do for a start. You want to measure the crystal in an environment that’s pretty close to what it’ll be built into, so as to get meaningful numbers.

    The crystal sits in series between the TG and the SA, looking a lot like a simpleminded (and badly terminated) crystal filter. Write down the sum of the crystal’s source (TG) and load (SA) termination resistances: 50+50 = 100.

    12 MHz Crystal - Fs and Fp
    12 MHz Crystal – Fs and Fp

    Center frequency to 12 MHz (or whatever the crystal’s nominal frequency might be), span to 200 kHz, TG at -10 dBm, get a display of the series resonance peak and the parallel resonance dip, poke the Auto Tracking Adjust button to get the TG lined up with the SA. Span to 100 kHz, tune center frequency for best picture.

    That shows the series resonant peak and parallel resonance notch, but it’s way too broad for any decent resolution in the measurements.

    Soooo…

    Poke marker peak search to find the series peak, poke marker to center frequency to slap the peak to the middle of the screen.

    12 MHz Crystal - Fs Bandwidth
    12 MHz Crystal – Fs Bandwidth

    Span down to 5 kHz, which sets the resolution bandwidth to 100 Hz. Poke auto tracking adjust again, because it’ll be way off. Manual adjust moves in too-large steps.

    Marker peak search, marker to center frequency. Tick the reference level down enough to get the peak near the top of the graticule, set 3 dB/div to get enough vertical resolution. Another peak search, to center, then write down the peak frequency Fs. Single sweep to freeze the display.

    Marker delta, dial up the marker frequency, poke marker amplitude, set -3 dB, read off the marker delta frequency. Dial the marker frequency down to the other side of the peak, set amplitude -3 dB again, read off the frequency again. Compute the crystal’s 3-dB bandwidth BW as the sum of those two values.

    Marker normal, auto-sweep to get a live trace again, auto tracking adjust again as needed. Display line on, set to peak for amplitude reference.

    Replace the crystal with a 50-Ω (or so) non-inductive twiddlepot, twiddle to set level to the display line. Measure twiddlepot resistance, which will be equal to the crystal’s series resistance. Write down Rs (a.k.a, the ESR).

    Measure crystal capacitance: short both leads, measure to case. Write down C0. I used an Autek RF-1 with a homebrew fixture, which has 1 pF resolution at RF frequencies; obviously, you pick a frequency well below Fs.

    With all those numbers in hand, compute the crystal’s motional parameters…

    Lm = (100 + Rs) / (2π BW)

    Cm = 1 / ( (2π Fs)^2 Lm)

    Q = (2π Fs Lm) / Rs

    The three crystals from the grab bag are all reasonably close to what you’d expect.

    12 MHz Crystal Parameters 50 Ohm Term
    Marking Fs -3 dB Lo -3 dB Hi Fp-Fs Rs C0 BW Lm Cm Q

    		MHz Hz Hz kHz Ω pF Hz mH fF
    TEW 5C 11.996962 1030 920 24.8 8.9 3 1950 8.16 21.56 69133
    HCI 1200 11.997500 1087 1025 26.8 5.0 2 2112 7.54 23.35 113618
    ECS 12.00 11.999975 1162 1125 26.3 6.2 2 2287 6.96 25.28 84635

    Memo to Self:

    The Lm equation shows why you really need lower termination resistances. The K8IQY fixture involves 4:1 matching transformers on each side of the crystal to get the generator down to 12.5-ish Ω and the output back up to 50 Ω. Time to rummage through my pile of ferrite toroids.

    An accurate BW with excellent (1 Hz) resolution feeds directly into better Lm values. I’m not convinced I have the SA set up for that much resolution.

    Absolute Fs accuracy isn’t needed, but high resolution is. With that many digits, thermal drift is a real issue, hence the repeated TG tweakage.

    I also need better resolution for Rs and C0. The former needs a smaller twiddlepot. Both could use a better meter with more resolution and zero-offset for low values. Measuring pF caps requires a good fixture.

  • ICOM IC-Z1A Tone Squelch: Fixed?

    ICOM IC-Z1A HT with UT-93 Tone Board
    ICOM IC-Z1A HT with UT-93 Tone Board

    A few days ago I rode off to an eye doctor appointment and my ladies rode off later to meet me at the grocery store after they stopped in the garden to harvest root crops. This sort of thing is easy enough to synchronize with amateur radio, but this morning I didn’t hear a thing until they rolled up beside me in the store parking lot.

    It seemed they could hear each other and me, but I couldn’t hear either of them. We’re all on 144.39 MHz, the APRS data frequency, with 100 Hz tone squelch to keep the robots out of our ears. Our daughter has the GPS APRS tracker feeding data into the mic input, which is why we’re using a data channel for tactical comm.

    This has happened once or twice before, but it’s very intermittent. I now had sufficient motivation to disconnect the radio, an ancient ICOM IC-Z1A, from the bike and pith it on the Electronics Workbench for examination. The UT-93 Tone Squelch board is unplugged & flipped over, resting on the front half of the radio body at the lower-left of the photo.

    Turns out that there’s nothing visibly wrong in there. I suspect it’s a molecule or two of oxidation on the (gold-plated!) connector between the UT-93 and the main board, because the UT-93’s held firmly in position by the black foam square you can see in the lower-left of the photo. The small white plug near the top of the UT-93 mates with the equally small socket on the main board, just to the left of the lithium secondary cell in the middle.

    It’s all CMOS logic, of course, and there’s no actual load current involved. That’s the worst condition for contacts, as a dry connection simply doesn’t produce enough energy to burn through the least hint of oxidation. That’s why they use gold plating on connectors, but it’s been a long time since that board has moved at all; the foam square is deeply indented.

    So I wiggled & jiggled all the ribbon-cable connectors while I was in there, buttoned everything back up, and the tone decoding works again. I hope this will continue…

    Memo to Self: remove only the four black corner screws on the upper case, plus the two silver screws near the very bottom inside the battery compartment, and the two halves pop apart. No need to remove the mic and earphone plugs, whew!

  • VGA-grade Video Cable Connector

    VGA-class Video Cable Connector
    VGA-class Video Cable Connector

    Setting up a new (well, new to me, it’s that old GX270) PC gave me reason to rummage in the Video Cable box and come up with this VGA-class cable. Half the connector shell had worked its way off, giving a nice view of the handiwork. Easily snapped back on with no permanent damage.

    The cable works OK at 1280×1024, although the image seems a bit soft, and higher screen resolutions are out of its reach. There are no obvious signal reflections visible on the screen, so the impedance bumps are not as bad as you might think.

    The VGA connector includes common returns for the Red, Green, and Blue signals, and the two wires for each color should be twisted together. That obviously hasn’t happened here, but crosstalk doesn’t seem to be much of a problem.

    To their credit, they did solder the cable shield to the connector shell, which is a really nice touch. Alas, the impedance of a one-inch pigtail pretty much chokes off the high-frequency stuff you really want to drain to the shell.

    Memo to Self: One of these days, run a bandwidth check with the spectrum analyzer. Use 6-dB pads to get nice 75-Ω terminations.

  • SPICE Crystal Model

    Linear Technology’s LTSpice generic capacitor model has all the parts you need to synthesize a crystal, which is pointed out in the help file and various spots around the web. What’s missing is the relation between all the parts and the values you have in hand for an arbitrary crystal.

    SPICE Capacitor Model
    SPICE Capacitor Model

    The crystal capacitor model looks like this…

    Cpar (usually C0) along the right edge is the inter-electrode capacitance, on the order of a few pF.

    Rpar (usually R0) along the left edge is the parasitic resistance across the case, on the order of hundreds of MΩ.

    The RCL string in the middle is the “motional” part of the crystal model, generally found with a subscript “m” in the specs.

    • Rser (Rm or ESR) is on the order of 100 Ω
    • Capacitance (Cm) is the motional capacitance, on the order of fF (that’s femtofarad: 10-15)
    • Lser (Lm) is tens to thousands of mH
    • RLshunt is something I haven’t seen in any other model and, in fact, it doesn’t appear in the properties panel.
    Crystal Properties
    Crystal Properties

    Now, the part I screwed up is that the capacitor’s value (the number appearing on the schematic) is Capacitance (in the angle brackets that royally screw up WordPress HTML), not Cpar. So the crystal capacitor properties panel looks like this…

    That models a 10 MHz crystal, taken directly from a sidebar in Refinements in Crystal Ladder Filter Design by the legendary Wes Hayward W7ZOI, in the June 1995 issue of QEX.

    Guess what? Plug it into a model of his crystal-measuring circuit and it works exactly like he says it should. No surprise there…

    SPICE has a bit of trouble simulating high-Q oscillators; they tend to not start up properly. If nothing seems to be happening, wait for a few tens-to-hundreds of milliseconds before despairing. Try chopping Rser down by a factor of two or four to see if that improves its disposition.

    You could try injecting a few (hundred thousand) cycles of a kickstart signal, but that’s fraught with peril: you’re simulating something even further from reality than usual.

    Memo to Self: You can rename the cap from C2 (or whatever) to X1 (or whatever) and everything still works fine.

  • D-Cell Corrosion: Prepare for Liftoff!

    Corroded Alkaline C Cells
    Corroded Alkaline C Cells

    Surprisingly, the flashlight holding these cells wasn’t damaged.

    Judging from the position of the switch, my mother tried to turn the thing on, it didn’t light up, and she just dumped it back in the drawer. Time passed, corrosion never sleeps, and the weak link (fortunately, between the two cells) let the alkaline nastiness out.

    I gotta collect all these pix in one big pile…

    (These are not the D-cells from the Maglite adventure.)

  • Tenergy AA NiMH Cell Self-Discharge

    Got a set of Tenergy 2600 mAh AA NiMH cells a while ago and ran some self-discharge tests.

    Tenergy 8 AA NiMH Pack Self-Discharge
    Tenergy 8 AA NiMH Pack Self-Discharge

    The three curves on the far right (two overlap pretty closely) are freshly charged. All three packs exhibit pretty much the same 1.8 Ah capacity, about 1.8 Ah or 70% of the advertised value.

    I must say that Tenergy cells aren’t exactly Tier 1 quality.

    The little stubby red curve on the far left is the as-received capacity. They’re not advertised as ready-to-use and, for sure, they’re not. My analysis of some RTU cells is there.

    The two curves on the left of the main group, after six and seven weeks, are down to 1.4 Ah. That’s 78% of the actual fresh-charge capacity, a mere 54% of nominal.

    Condensing the endpoints into a table, we have:

    Pack Charge Date Discharge Date Storage Time Capacity
    A 09/04/09 09/04/09 0 1.82
    A 09/04/09 09/11/09 7 1.64
    B 09/04/09 09/24/09 20 1.58
    C 09/04/09 10/25/09 51 1.41
    A 09/11/09 10/25/09 44 1.41
    B 09/24/09 10/25/09 31 1.48

    A picture being worth a kiloword, a quick-and-dirty graph is in order:

    Tenergy AA NiMH Self-Discharge
    Tenergy AA NiMH Self-Discharge

    Ignoring the first and last points, the slope is more-or-less constant at 6 mAh/day.

    Call it 0.3%/day after the first week: store the cells for 50 days and you’re down 15%.

    Remember, that’s measured from the actual as-charged and as-measured capacity, not the nominal as-read-about-in-the-spec-sheet capacity.

  • Alpha-Geek Clock

    You’ve probably seen “ultimate geek clocks” floating around on the web, which seem to be Nixie tubes, binary readouts, or analog clocks with lightly encoded markings.

    Poseurs, all of them!

    If you’re an alpha geek, this is how you tell time…

    Absolute Geek Clock
    Absolute Geek Clock

    It’s a WWVB receiver wired up to a CR123A primary lithium cell. The time display is a single red LED, driven by a low-threshold FET. Yeah, you can package it up in a cute little box (which is the picture on hackaday.com), but this is the essence of the thing.

    Over the course of a minute, the LED blinks out the hour, minute, year, day-of-year, Daylight Saving Time, leap year, leap second, and some other stuff in binary-coded decimal.

    The key to the format is there and the bit format is straightforward:

    • Long = frame marker
    • Medium = binary 1
    • Short = binary 0

    You just watch the LED, catch the frame marker, decode BCD data on the fly, convert from UTC to local time, and that’s all there is to it.

    Sheesh, it’s only one bit a second: anybody can handle that, right?

    Truth to tell, I can hang on long enough to get the minute, but I taper off pretty quickly after that.

    Tech detail…

    Basically, you get the receiver and CR123 cell holder from DigiKey for maybe fifteen bucks. Wire up a FET (ZVNL110A or some such) to the receiver’s inverted-polarity output, so the LED is ON during the data bit’s active time (carrier drops 10 dB). I blobbed on a 300 ohm SMD resistor, so the total current is maybe 250 µA with the LED on. If you’re going crude, you can probably wire the LED & resistor directly to the receiver’s positive-polarity output.

    A primary CR123A is good for 1500 mAh and the average current is maybe 150 µA, so the clock will run for nearly a year. The LED is pretty dim, but perfect for late-night viewing.

    Reception is iffy during the day here in the Hudson Valley. At night it’s just fine. Interference from LCD panels with near-60-kHz refresh is a real problem, so it doesn’t play well near PCs.

    I put the clock on a shelf where I can watch it when I wake up in the middle of the night: it knocks me out again pretty quickly.

    In real life, I put this together to verify my WWVB simulator… but I might just box up a spare for the shelf, too.