Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
In the harsh light of the Electronics Workbench, you can see there’s less than meets the eye: a single knockoff Neopixel taped to the back side of the bulb just below the equator and a knockoff Arduino Pro Mini taped to the Mogul lamp socket:
500 W Incandescent – backlit light
The electrical box serves as a base and the cord doesn’t do anything in this incarnation.
The 5050 SMD LED package (inside an ugly 3D printed plate cap) looks enough like a point source to shadow the filament & support structure against the frosted bulb. The blurry upper part of the filament is closer to the LED, which isn’t really a point source and must fight its way through the frosting.
The Pro Mini runs the same firmware as the Bowl o’ Fire floodlamp, of course, dialed back for slow fades.
The topic of function generators came up at Squidwrench a while ago (Sophi was tinkering with LCD shutters) and I finally picked up one of those JYE Tech FG085 DDS function generators to see how they work:
FG085 Fn Gen – in case
Short answer: adequate, if you’re not too fussy.
The board arrived with a bizarre solder defect. It seems a solder stalk yanked one terminal off a ceramic SMD caps:
FG085 – Solder stalk – C26
The schematic and adjacent parts suggested the victim was a 10 uF cap, so I replaced it with one from my stash that worked fine.
However, after soldering enough of the switches to do something useful, the board wouldn’t power up. With a bit of poking around, I discovered the power jack had +15 V from the wall wart, but the center terminals on the DPDT power switch that should have been connected to the jack showed maybe 0.3 V. Jumpering around the failed via and a short trace on the bottom surface let the board power up correctly:
FG085 – Jumpered power trace
If you’re building one of these, solder one pin of each switch, push all the switch caps in place, shove the faceplate over all of them, tape it to the PCB, make sure all the switches are push-able, then solder the remainder of the switch pins. If you do them one by one, you’re certain to end up with a few mis-aligned switches that will either prevent the faceplate from sliding over them or wedge firmly against the side of their assigned hole. Just sayin’.
I tweaked the dimensions slightly to fit the (slightly larger, possibly new, maybe tolerance-eased) front panel, but the bottom mounting screw hole spacing depends on the front panel size, not a specific set of dimensions, leading me to relocate those holes by abrasive adjustment. I didn’t bother with the lid (which doesn’t clear the BNC jack anyway) or the printed plastic feet (having a supply of silicone rubber feet).
The fancy vent gridwork along the sides printed surprisingly well, even in PETG. I’d have gone with larger slots, although I doubt the thing really needs vents in the first place.
The DDS sine wave output is rough, to say the least:
FG085 Fn Gen – 60 kHz sine
The spectrum shows oodles of harmonic content:
FG085 Fn Gen – 60 kHz sine – spectrum
A closer look:
FG085 Fn Gen – 60 kHz sine – spectrum – detail
Stepping back a bit shows harmonics of (and around) the 2.5 MHz DDS sampling frequency:
FG085 Fn Gen – 60 kHz sine – spectrum – 10 MHz
For comparison, my old Fordham FG-801 analog function generator has nice smooth harmonics:
FG-801 Fn Gen – 60 kHz sine – spectrum
Closer in:
FG-801 Fn Gen – 60 kHz sine – spectrum – detail
Of course, that crusty old analog dial doesn’t provide nearly the set-ability of a nice digital display.
I stuck some old 12 V 7 A·h batteries in my homebrew power supply for the HP 3801A GPS Time / Frequency Standard, fired it up, put the antenna where it could see a good chunk of the sky, gave it a day to warm up / settle out, and it’s perfectly happy:
------------------------------- Receiver Status -------------------------------
SYNCHRONIZATION ............................................. [ Outputs Valid ]
SmartClock Mode ___________________________ Reference Outputs _______________
>> Locked to GPS TFOM 3 FFOM 0
Recovery 1PPS TI -38.3 ns relative to GPS
Holdover HOLD THR 1.000 us
Power-up Holdover Uncertainty ____________
Predict 366.2 us/initial 24 hrs
ACQUISITION ............................................ [ GPS 1PPS CLK Valid ]
Satellite Status __________________________ Time _____ +1 leap second pending
Tracking: 4 Not Tracking: 6 UTC 18:22:19 22 Jul 2016
PRN El Az SS PRN El Az 1PPS CLK Synchronized to UTC
3 34 104 48 * 1 36 48 ANT DLY 0 ns
17 62 308 103 6 27 220 Position ________________________
19 39 281 50 11 21 58 MODE Hold
28 80 133 64 *22 Acq .
24 12 319 LAT N 41:39:32.328
30 15 191 LON W 73:52:26.733
ELEV MASK 10 deg *attempting to track HGT +82.87 m (MSL)
HEALTH MONITOR ......................................................... [ OK ]
Self Test: OK Int Pwr: OK Oven Pwr: OK OCXO: OK EFC: OK GPS Rcv: OK
scpi >
The FFOM 0 entry says the Frequency Figure Of Merit is “within specifications” of 10-9, averaged over one day. That means the actual frequency should be within 0.010 Hz of 10 MHz.
Feeding the 10 MHz frequency reference into the (equally warmed up) HP 8591E spectrum analyzer and selecting an absurdly narrow span produces a comforting sight:
HP Z2801A GPS Receiver – 10 MHz ref – HP 8591E
Given the horizontal resolution, that’s dead on 10 MHz.
Two of the external Li-Ion battery packs I’m using with the bike radios seemed to fail quickly after being charged, so I sawed them open to check the state of the cells. This time I used the fine-tooth cutoff blades, rather than a coarse slitting saw:
Li-Ion pack – sawing case
As before, a 2 mm depth-of-cut, done 0.25 mm per pass after the first millimeter, seems about right. I didn’t saw the front of the case near the jack, which proved to be a mistake; the interlocked case halves need cutting.
No cell trouble found, which leads me to suspect an intermittent short in the battery-to-radio cable that trips the battery protection circuit. The spare cables went into hiding during the shop cleanout, so I can’t swap in a known-good cable just yet; of course, the existing cable behaves perfectly on the bench. The suspect cable is now on my bike and, if the problem follows the cable, further surgery will be in order.
The object of soldering all 40 wires in the 5 m hank of ribbon cable in series is to build a 40 turn loop antenna to receive LF radio signals like WWVB at 60 kHz. The antenna, being basically a big coil of wire, will have an inductance that depends on its layout, so putting a capacitor in parallel turns it into a resonant tank circuit. Given a particular layout (and, thus, an inductance), you can choose the capacitor to make the antenna resonant at whatever frequency you need (within reason).
With the joints soldered & reinforced with epoxy, the inductance across all 40 turns:
535 µH – rolled into a compact bundle
6.66 mH – vaguely circular loop on the concrete floor
5.50 mH – lumpy rectangle on the concrete floor
Back in a slightly different circular layout on the floor:
6.8 mH – across all 40 turns, as above
2.0 mH – across either set of 20 turns from the center tap
Given that inductance varies as the square of the number of turns, you’d expect a factor of four between those two inductances, but that’s not how it worked out.
Hanging the loop from a pair of screws in the floor joists to make a droopy rectangle-oid shape and driving it from a 600 Ω signal generator through a 10 kΩ resistor, it’s self-resonant at 213 kHz. Repeating that with a 470 kΩ resistor drops the resonance to 210 kHz, which isn’t different enough to notice and surely has more to do with my moving the loop while dinking with resistors.
Adding parallel capacitance (measured with an LCR meter, just to be sure) changes the resonance thusly:
9.9 nF → 20 kHz
900 pF → 64 kHz
400 pF → 87 kHz
250 pF → 108 kHz
none → 213 kHz
Because the resonant frequency varies inversely as the square root of the capacitance, halving the resonant frequency means you’ve increased the capacitance by a factor of four. Because 250 pF halves the frequency (mostly kinda sorta close enough), the loop’s stray capacitance must be about 1/3 of that: 83 pF.
Yeah, 1/3, not 1/4: the additional capacitance adds to the stray capacitance, so it goes from 83 pF to 250 + 83 pF = 333 pF, which is four times 83 pF.
The self-resonant frequency of 213 kHz and the 83 pF stray capacitance determines the loop inductance:
L = 1/((2π · 213 kHz)^2 · 83 pF) = 6.9 mH
Pretty close to the measured value from the floor, I’d say.
To resonate the antenna at 60 kHz, the total capacitance must be:
60 kHz = 1/(2π · sqrt(6.9 mH · C)) → C = 1050 pF
Which means an additional 1050 – 83 = 970-ish pF should do the trick, which is about what you’d expect from the 64 kHz resonance with the 900 pF cap above. I paralleled pairs of caps until it resonated at 59.9 kHz.
The -3 dB points (voltage = 1/sqrt(2) down from the peak) turned out to be 58.1 and 60.1 kHz, so my kludged caps are slightly too large or, once again, I nudged the loop.
Figuring Q = (center frequency) / bandwidth = 59.1 / 2 = 30, which works out close enough to Q = X / R = 2600 / 80 = 33 to be satisfying. Using standard 26-ish AWG ribbon cable, rather than crappy 31-ish AWG eBay junk, would double the conductor area, halve the series resistance, and double the Q. Faced with that much resistance, I’m not sure better caps would make any difference.
Attaching the spectrum analyzer through a 470 Ω resistor to reduce the load:
Loop – 40T 1nF – spectrum
I’d love to believe that big peak over on the left at 57.1 kHz is WWVB, but it’s not.
What’s more important: the broad hump between 56 and 62 kHz, where the increased amount of background hash suggests the antenna really is resonant, with a center frequency around 59 kHz. The -3 dB points might be 57 and 61 kHz, but at 10 dB/div with 5 dB of hash, I’d be kidding myself.
Dang, I love it when the numbers work out!
It’s faintly possible the spectrum analyzer calibration is off by 2.5 kHz at the low end of its range. The internal 300 MHz reference shows 299.999925 and it puts FM stations where they should be, but the former could be self-referential error and the latter lacks enough resolution to be comforting. I must fire up the GPS frequency reference, let it settle for a few days, see whether it produces 10.000000 MHz like it should, then try again.
Given five meters of 40 conductor ribbon cable, the object is to make a 40 turn five foot diameter loop antenna by soldering the ends together with a slight offset. After squaring off, marking, and taping the cable ends, I stripped the wires:
LF Loop Antenna – wire stripping
Twirling those little snippets before pulling them off produced nicely twisted wire ends with no few loose strands. Separate the individual wires, wrap with transformer tape to prevent further separation, run a flux pen along the wire ends, tin with solder, repeat on the far end of the cable.
Tape one end to the ceramic tile. Align the other end with a one-wire lateral offset and the stripped sections overlapping, then tape it down. Slide a paper strip between the ends, passing under every other wire, to separate the top pairs from the bottom pairs, then tape the strip in place:
LF Loop Antenna – wire prep
Grab each left wire with a needle point tweezer, forcibly align with the corresponding right wire, touch with the iron, iterate:
LF Loop Antenna – top solder joints
The red wire trailing off to the left will become the center tap.
Slide a strip of the obligatory Kapton tape underneath the finished joints, slobber on enough clear epoxy to bond the insulation on both sides of the joints into a solid mass, squish another strip atop the epoxy, smooth down, wait for curing.
Untape from the tile, flip, re-tape, solder the bottom joints similarly, add Kapton / epoxy / Kapton, and that’s that:
LF Loop Antenna – complete joint
Prudence dictates checking for end-to-end continuity after you finish soldering and before you do the Kapton + epoxy thing, which is where I discovered I had 80 Ω of distributed resistance along 200 meters of cable. A quick check showed 40 Ω at the center tap and 20 Ω at the quarters (the black wires on the left mark those points), so it wasn’t a really crappy joint somewhere in the middle.
The joint and its dangly wires cry out for a 3D printed stiffener which shall remain on the to-do list until I see how the loop tunes up.
What’s wrong with this picture? (clicky for more dots)
eBay – 40 pin IDC cable – header
Not obvious?
Here’s the description, slightly reformatted for clarity:
New 5m IDC Standard 40 WAY 1.8” Multi-Color Flat Ribbon Cable Wire Connector
Description
Type: IDC standard.
10 colors, 4 group, total 40 pcs cables per lot
5 meter per lot.
width: 4.7 cm / 1.8 inch
Package content: 5M Flat Color Ribbon Cable
If you divide the 1.8 inch cable width by its 40 conductors, you find the wires lie on a 45 mil pitch. If you were expecting this “IDC standard” cable to fit in standard insulation displacement cable connectors with a 50 mil pitch, you’d be sorely disappointed. You can get metric ribbon cable with a 1 mm = 39 mil pitch, but this ain’t that, either.
Here’s what an individual eBay wire (black jacket) looks like, compared to a wire from a standard ribbon cable (red jacket):
Ribbon cable – 26 AWG – eBay vs standard
A closer look at the strands making up the wires:
Ribbon cable – 26 AWG – eBay vs standard – strands
As nearly as I can measure with my trusty caliper, the eBay ribbon cable has wire slightly smaller than 30 AWG, made up of seven 40 AWG strands, as opposed to standard 26 AWG wire made of seven 34 AWG strands. The good stuff might be 28 AWG / 7×36 AWG, but I was unwilling to break out the micrometer for more resolution.
I’d like to say I noticed that before buying the cable, but it came to light when I measured the total resistance of the whole cable: 80 Ω seemed rather high for 200 meters of 26 AWG wire. The wire tables say that’s about right for 31 AWG copper, though.
Changing the AWG number by three changes the conductor area by a factor of two, so you’re getting less than half the copper you expected. Bonus: it won’t fit any IDC connectors you have on the shelf, either.
Turns out a recent QEX article suggested building an LF loop antenna from a ribbon cable, so I was soldering all the conductors in series, rather than using connectors, and it should work reasonably well despite its higher DC resistance.
The Smell of Molten Projects in the Morning 