Archive for category Oddities
Rolling into Vassar Farms, we encountered a Canadian Goose family:
The gander pulled straight up and hissed as we rolled by at what we thought was a respectful distance:
Their little fuzzballs retreated in good order under the fence toward the pond; they don’t need much survival training.
Word has it a goose family (perhaps this one) built their nest near a path around the ponds and defend their turf with sufficient resolve to deter even singletrack bikers.
I occasionally see snakes along the way, but none that hiss:
We approach rail-trail curves with a bit more caution than some folks; I’m at about the spot where that rider began losing control and didn’t quite wipe us out.
Mary spotted this critter atop the roof and, much to my surprise, it waited courteously until I deployed the camera:
It looks, walks, and acts just like a pigeon:
… but we’ve never seen one with those feather patterns & colors. It’s not in any of our books, so it may be an escaped domestic pigeon.
Those feathers require plenty of body maintenance:
As nearly as we can tell, it’s wearing a green leg band with three digits that might be 904:
If this was your bird, it flew through Red Oaks Mill NY just after noon on 1 May 2017 …
Compare this picture:
… with any of the doc for the generic AD8950/51 DDS modules you’ll find out on the Interwebs. This snippet from the seller’s schematic will suffice:
Here’s a closer look at the 2×7 header in the upper left corner:
Don’t blame me for the blur, the schematic is a JPG.
Compared it with the board in hand:
Yup, the D7 and GND pins are reversed.
Some careful probing showed the silkscreen is correct: the pins are, in fact, correctly labeled.
Should you be laying out a PCB in the expectation of using any DDS module from the lowest-price supplier, remember this high truth: Hell hath no fury like that of an unjustified assumption.
Fortunately, I’m hand-wiring the circuit and caught it prior to the smoke test.
A strangely equipped van-like object emerging from Vassar Farms waited entirely too long for me to ride past:
The signage on the rear quarter panel read “Apple Maps / maps.apple.com” and a search with the obvious keywords produced a much better picture from the good folks at Adafruit in NYC of what might be the very same vehicle:
The Apple Maps schedule says nothing about being in Dutchess County this month. Maybe they’re lost?
Not being an Apple kind of guy, let me know if you see me riding by …
The Zire would power on whenever the switches clicked or that little joystick moved, which happened regularly enough to be annoying.
Mary made a small case that matched the other pouches I carry around:
She made the case to fit an HP48 calculator, but it was close enough for the Zire.
Time passed, the Zire died, I started carrying a Kindle Fire in another pocket, but the ABS slab provided a convenient stiffener between some Geek Scratch Paper and the various pencils / pens / markers / screwdrivers / flashlight filling the available space.
Unfortunately, minus the backup of an electronic slab, the protector finally failed along an obvious stress riser:
I cut a similar rectangle from a sheet of unknown flexy plastic, rounded the corners, clipped the pencils & whatnot to it, and maybe it’ll survive for a while.
The label atop a generic AD8310 Log Amp module seemed unambiguous:
Firing the HP 8591 tracking generator into the InHi SMA, terminating InLo (not shown above, for reasons you’ll see below), connecting the Out SMA to the scope’s Trace 1, and the spectrum analyzer’s sweep output to Trace 2 produced an oddity:
The upward-sloping ramp (lower trace) shows the HP 8591’s horizontal sweep, with the tracking generator tuning from 100 kHz to 500 MHz during the 20 ms sweep. The log amp output (upper trace) drops more-or-less linearly with increasing frequency, which seems odd. The tracking generator signal should be pretty much level and the log amp’s output should be more-or-less flat.
My oscilloscope tops out at 150 MHz. The displayed RF is down by 3 dB = 0.6 div at 1.5 division = 190 MHz into the sweep:
However, the RF looks pretty much flat up to 125 MHz and it’s still visible beyond 400 MHz, so I think the tracking generator is doing what it’s supposed to. If the RF were decreasing, then the trace would look different, methinks.
The response to a 60 kHz sine wave doesn’t look quite right:
Eyeballometrically, it might be a log response to the absolute value of the derivative: kinda flat on the ups-and-downs, kinda zero-ish at the tops-and-bottoms. Or maybe it’s the log response to a phase-shifted version of the input, with the lows corresponding to the zero crossings.
Documentation for the circuit seems nonexistent, because eBay. Fortunately, one can pop the top to reveal the straightforward PCB layout:
A closer look:
A capacitance meter says input capacitors C5 and C7 are both 10 nF.
A sketch of the circuitry:
The datasheet puts the terminating resistor on the other side of the input caps, where it surely belongs:
Achtung: the solder blob just to the left of C7 grounds the signal pin on the InLo SMA. Don’t connect anything to InLo which might take offense at having its output shorted to ground; the SMA terminator I used had no effect whatsoever.
The AD8310 chip (assuming that’s what it really is) has a differential input resistance = 1 kΩ and capacitance = 1.4 pF in parallel with R3, the 52.3 Ω terminating resistor, making the net resistance just under 50 Ω.
At 60 kHz, the input caps have a reactance of 270 Ω apiece, which means the “terminating” resistor is maybe 10% of the mostly capacitive input impedance seen at the InHi connector. That means the AD8310 inputs see maybe 10% of the input signal.
In fact, if you regard those three parts as an RC high pass filter and merge the caps into a single 5 nF unit, it rolls off at 620 kHz = 1/(2π · 52 · 5 pF). Obviously, it’ll be a fine differentiator at 1/10 the breakpoint frequency.
A simulation shows it in action (clicky for more dots):
The two 1 MΩ resistors provide a balanced DC path-to-ground for R3 to keep the simulator happy.
The (+) input tends toward 0 dB as C5 tends toward a short, the (-) input tends toward ground as C7 does likewise, but their difference isn’t a constant value. Seeing as how a log amp should respond to small differences, methinks it’s hard at work.
The AD8310 data sheet says the scale factor is about 24 mV/dB between 10 MHz and 200 MHz, with no frequency dependence worth mentioning. Eyeballometrically, the output has a 240 mV = 10 dB straight-line decrease over the entire frequency range of that scope shot. It drops by 220 mV = 9.2 dB in the decade from 50 to 500 MHz, half of the 20 dB you’d expect from a first-order filter response.
The AD8310 has an internal 2 MHz high pass feedback loop to suppress low frequency input offset voltages. The doc recommends a 1 µF cap from OLFT to ground for frequencies down in the low audio range. One might solder the cap across the convenient pads labeled C8 below the chip.
Rearranging the input circuitry seems in order:
- Move R3 outside C5 and C7, per the datasheet
- Increase C5 and C7 to 1 µF -ish
- Add 100nF – 1 µF bypass cap at C8
I have the uneasy feeling I’m overlooking something obvious …
Jessica: Hi! I am a Verizon specialist, can I help you today?
You (that would be me = Ed): Verizon has announced gigabit Internet service for $70/month. That isn’t listed as one of the my “upgrade” options. Is it available in this area? If not, why do the 25 and 50 Mb/s services cost 90 and $100/month?
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However, you can make the upgrade for Verizon Fios Internet up to 100/100 Mbps and above speed plan.
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You: That’s the bait-and-switch tactic I’m /not/ interested in; DO NOT change my service. Verizon tacks on a few bucks a month for a “Municipal Construction Charge” without actually building anything. Let me know when you can offer me a gigabit for $70, then we can talk. Before then, DO NOT CHANGE ANYTHING. Thanks …
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You: Do you have access to the previous half hour of chat before Agent Jessica was mysteriously disconnected?
Carl: It seems that the previous agent lost connection.
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Carl: I read that you wish to check the availability of Gigabit speeds for your home. Correct?
You: That’s what I asked, half an hour ago, and was told it’s not available, but I /can/ pay more than that (minus a teaser discount) for 10% of the bandwidth. If that’s still the best you can do, it’s not what I want.
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You: OK, we’re going in circles. Let me know when you can deliver what Verizon offers to other FiOS customers. Thanks …
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You: Nope, we’re off to a concert. Have a good rest of the evening!
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