The heron hurked the fish down, with the abrupt right-angle bend in its neck marking the fish’s current location:
A bit of wiggling & jiggling put the meal in the right place and the bird relaxed:
A postprandial flight around the pond apparently settled the fish:
It landed on a snag a few dozen feet from where it started, then proceeded to look regal:
Those things really do look like pterodactyls in flight!
After adding a MAX4165 buffer amp to drive the crystal test fixture at 1 µW and a MAX4255 to amplify the 1 mV crystal output by 40 dB, then removing the AD8310 log amp module’s 50 Ω terminator to better match the MAX4255’s output drive ability, this happened:
- A 32.768 kHz quartz resonator
- A ±2 Hz span centered on 32.766 kHz
- 0.10 Hz frequency steps
- The 22 pF cap out / in circuit (left & right peaks, respectively)
- Log amp output at 24 mV/dBV, with a nominal -108 dBV intercept at 0 V
With a 4 Hz span and 0.1 Hz steps, you get only 41 samples along the X axis: it’s supposed to look spotty.
The 2.2 V response at the top of the left peak corresponds to 2.2 / 24 mV/dBV = 91.7 dBV, then you knock off the -108 dBV intercept to get -16.3 dBV. The valley at 1.88 V is 78.3 – 108 = -29.7 dBV, down about 13 dBV from the corresponding peak. The peak-to-baseline over on the right looks like 200 mV = 8 dBV.
The AD8310 datasheet uses “intercept” in a manner I had not previously encountered. They plot the AD8310 output in volts against the input signal level in dBV, with the “intercept” marking the extrapolated point where the straight line with slope 24 mV/dBV crosses the X axis: the equation is volts = slope*(input dBV – intercept dBV). Back in the day, I learned the intercept was where the line crossed the Y axis at X=0, so the straight-line equation was simply y = slope*x + intercept. Took me a while to figure that out.
Then subtract the 40 dB gain from the crystal output to the log amp to get -56 dbV = 1.6 mV. That’s close enough to the 1 mV before adding the MAX4255. All those numbers seem slightly squishy, but they’re close enough.
The peaks are 13-ish spots apart, which corresponds to 1.3 Hz, which is roughly the 1 Hz I measured with the HP8591 spectrum analyzer, The baseline is down 8 dBV, not quite as much as the analyzer’s 13 dB at 1 Hz offset from the peaks.
What’s not right: the parallel-resonant dip to the right of each peak should be at the same frequency for both traces, because it doesn’t vary with added series capacitance, but it’s pretty much tracking the series-resonant peak frequency.
The amount of noise on the log amp output looks like 50 mV = 2 dBV. That’s a lot, compared to the 13 dBV response, but some judicious averaging may save the day.
The 22 MHz GBW of the MAX4255 rolls off the high end at 220 kHz. I AC coupled the signal chain with 10 µF dipped tantalum caps from my lifetime supply, which may pass entirely too much of the low end; the settling time is way too long. This probably requires smaller caps and maybe an actual bandpass filter.
The 50 mV-ish noise on the DAC output driving the X axis suggests my proto board layout isn’t up to the demands of this circuit: there shouldn’t be any noise in that direction.
Some poking around suggests the OLED display is way noisier than you’d (well, I’d) expect. The faded-out lower section in the picture below suggests it’s refreshing one line = 128 pixels at a time. More study is indicated.
But, if you squint hard enough, this lashup produces numbers in the right ballpark. Given that it’s a collection of cheap-as-dirt eBay modules flying in formation, that’s nothing to sniff at:
Those “gold tone” SMA connectors really make it look like serious RF hardware, don’t they? [grin]
The round twiddlepot floating on the white pillow trims the DDS output voltage by a factor of two = 6 dB. Combined with the 0-6-12-18 dB gain steps provided by the header in front of the MAX4165 (to the right of the pillow), you can set the drive voltage so the crystal gets (roughly) its rated 1 µW maximum drive power.
Improving the crystal tester’s (nonexistent) grounding requires a band of copper tape around the inside of the proto board holder. Rather than cut the tape lengthwise to fit the holder, a new one will be just tall enough:
While I was at it, I deleted the washer recesses, because those didn’t work out well, and fiddled the screw holes to put the inserts in from the bottom:
Although the overhang inside the holes will be ugly, I’ll epoxy the inserts flush with the bottom and nobody will ever know.
The copper tape now makes a tidy ground strap:
With a gap in the front to eliminate the obvious loop:
The OpenSCAD source code as a GitHub Gist:
A batch of 1.3 inch white I²C OLED displays arrived from halfway around the planet, so I figured I could run a quick acceptance test by popping them into the socket on the crystal tester proto board:
The first one flat-out didn’t work, as in not at all. The original display continued to work fine, so I compared the old & new displays:
Yup, swapped VCC and GND pins. I should be used to that by now.
I rewired the socket, tried the new displays, undid the change, popped the original display in place, and all is right with the world. Somewhat to my surprise, all five new displays worked, including the one I’d insulted with reversed power.
I watched the Canada Goose family paddling around the pond:
A hiker on the trail around the pond brought them to DEFCON 4:
The little ones aren’t triphibans yet, but they know the drill:
Maybe he only does that when Mom’s not watching?
The under-cabinet LED strips work wonderfully well, except that the IR sensor seemed rather hypersensitive, so I added a small reflector made of shiny steel:
Even though I rounded those corners and deburred the edges, it does look a bit threatening, doesn’t it?
It moves the sensor’s hotspot back about half a foot, which seems Good Enough to eliminate false triggering from normal activity over the cutting board.
The first pass at the crystal tester used a manual jumper to switch the 33 pF series capacitor in / out of the circuit:
With an Arduino close at hand, however, a relay makes somewhat more sense. For long-forgotten reasons, I have a small fortune in Teledyne 732TN-5 relays intended for RF switching:
The 7820 date code on the side suggests they’ve been in the heap basically forever, although some fractions of Teledyne still exist and you can apparently buy the same relay today at 50 bucks a pop. It’s definitely overqualified for this job and you can surely get away with an ordinary DIP DPDT (or, heck, even SPST) relay.
It seems I picked a hyper-bright white LED: the red ink tones it down a bit. Black might be more effective. A diffused LED may be in order.
The “TN” suffix indicates a built-in transistor driver with a catch diode on the relay coil, so the relay needs power, ground, and a current drive into the transistor’s base terminal:
Even with the internal catch diode, I ran the +5 V power through a 12 Ω resistor to a 10 µF cap in hopes of isolating the inevitable switching transients from the DDS and log amp. As a result, the turn-on transient isn’t much of a transient at all:
The 560 mV drop suggests a 47 mA coil current through the 12 Ω resistor, just about spot on for a 100 Ω coil.
The energy stored in the coil makes the turn-off transient much steeper:
Note the 1.5 µs delay from the falling control input to the relay opening. Granted, it’s running at 4.7 V, not the rated 5 V, but that’s still rather peppy. The turn-on delay seems to be about the same, making the datasheet’s “6 ms nominal” operating time look rather conservative.
Dang, that’s a nice gadget!