Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
I’d added Mad Phil’s trusty Circuitmate to the tool kit I carry along to Squidwrench:
Beckman DM73 – new ground clip
Over the last few months it became increasingly erratic, eventually got to the point where slight pressure on the case would blank the display, and finally didn’t turn on at all. Yes, I replaced the batteries.
So I took it apart:
Beckman DM73 Circuitmate – case open
Nothing seemed particularly broken and, even after resoldering all the joints, it continued to not work at all:
Beckman DM73 Circuitmate – PCB
If you want to try your hand at instrument rehabilitation, let me know.
… 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:
AD9850 module schematic – cropped
Here’s a closer look at the 2×7 header in the upper left corner:
AD9850 module schematic – J5 detail
Don’t blame me for the blur, the schematic is a JPG.
Compared it with the board in hand:
AD9850 DDS Module – swapped GND D7 pins – detail
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.
Adding a 56 nF cap across the C6 terminals (just above the AD8310) lowered the VBW to about 1 kHz:
AD8310 Log Amp module – VBW rolloff cap
Which flattened that sucker right out:
AD8310 – 1 kHz VBW cap – 60 kHz 1.394 V
The ripple for an absurdly high amplitude 32 kHz signal amounts to 36 mV:
AD8310 – 1 kHz VBW cap – 32 kHz – VBW ripple
Firing the tracking generator into the input with a frequency sweep from 100 kHz to 250 MHz shows the low end looks much better:
AD8310 – 1 kHz VBW cap – 100 kHz 250 MHz – 0 dB atten
There’s a slight droop across the sweep that might amount to 50 mV = 2 dB, which I’m inclined to not worry about in this context.
Applying the attenuators once again produces a scale factor of 23.5 mV/dB across 30 dB of RF, but this time the 60 kHz output makes sense, too.
Using the typical output curve from AN-691, that 2.0 V output corresponds to -13 dBm, which sounds about right for the tracking generator (which might really be -10 dBm).
I must calibrate the log amp output to find the actual intercept point (nominally -95 dBm, but could range from -86 to -102) at 60 kHz. The intercept is the extrapolated RF input producing 0 V out, which then acts as an offset for the voltage-to-dBm calculation; you start by finding the slope of the voltage vs. dBm line at some convenient power levels, then solve for dBm with V=0.
So a cheap AD8310 Log Amp module from eBay can work in the LF band, after you rearrange the input circuitry and tweak the chip’s filters. At least now I have a better understanding of what’s going on …
Given the intended LF crystal-measurement application, a hulking 51 Ω metal film resistor sprawled across the ground plane will work just fine. All three ceramic caps measure a bit under 1 µF; I intended to solder the input caps atop the existing 10 nF caps, but that didn’t work out well at all.
I should harvest the InLo SMA connector to prevent anyone from mistaking it for an actual input.
With that in place, the log amp output makes more sense:
AD8310 – modified – 100 kHz 150 MHz – 0 dB atten
That trace tops out at 150 MHz, not the previous 500 MHz, but now the response is flat all the way out. The log amp generates plenty of hash when the tracking generator isn’t producing a valid signal.
The 60 kHz response looks different:
AD8310 – modified – 60 kHz 1Vpp
So it’s really the log amp response to the absolute value of the sine wave (or, more accurately, to the sine wave re-zeroed around Vcc/2), with minimum output at the input’s zero crossings. At 500 mV/div, the log amp says the input varies by 42 dB = 1000 mV/(24 mV/dB), which might actually be about right for a zero-crossing (or zero-approaching absolute value of a) signal; logarithms don’t deal well with zeros.
The AD8310 datasheet and AN-691 suggest the 2.5 V output corresponds to +10 dBm = 12.5 Vrms input, which flat-out isn’t the case. However, the actual 500 mVpeak = 350 mVrms input is 2.5 mW = +4 dBm, so maybe it’s within spitting distance of being right.
AN-691 recommends 10 µF input caps for “low frequency” use, showing results down to 20 Hz; 1 µF seems to get the circuit close enough to the goal for use near 60 kHz.
It also recommends a cap on the BFIN pin (pin 6) to reduce the output stage bandwidth = “video bandwidth” and improve the overall accuracy, which remains to be done. The datasheet suggests rolling VBW off at 1/10 the minimum input frequency, which would be around 3 kHz for use with 32.768 kHz crystals. The equation, with reference to the internal 3 kΩ bias resistor:
CFILT = 1/(2π 3 kΩ VBW) – 2.1 pF = 18 nF
For a bit more margin, 1 kHz would require 56-ish nF.
The PCB has a convenient pair of pads labeled C6 for that capacitor. This may require protracted rummaging in the SMD capacitor stash.
Rolling off the VBW should reduce the hash on the 100 kHz end of the frequency sweep and filter the 60 kHz response down to pretty much a DC level.
Applying the 10 dB and 20 dB SMA attenuators to the input from the tracking generator and recording the log amp output voltage produces this useful table:
AD8310 Log Amp – mods and log response
With the terminating resistor on the correct side of the input caps, the log amp seems to be working the way it should, with an output varying a bit under the nominal 24-ish mV/dB over a 30 dB range.
We need caps! Lots of caps!
A quick search with the obvious keywords suggests nobody else has noticed how these modules work over a reasonable bandwidth. Maybe I’m the first person to use them in the LF band?
Once again, the single moving part on my first-generation Kindle Fire stopped working. As before, the switch contacts accumulated enough fuzz & contamination to prevent any current flow, but this time the (soft) solder joints attaching the switch body to the PCB failed:
Kindle Fire power switch – failed anchor
My joint cleaning & fluxing wasn’t up to contemporary standards, as shown by the obviously un-fused footprints left in the upper pads:
Kindle Fire power switch – failed anchor joints
The switch frame seems to be unplated steel, which shouldn’t be an excuse.
So I dismantled the switch, cleaned the contacts and tactile bump plate, put it all back together, and did a much better job of surface preparation:
Kindle Fire power switch – rebuilt – right anchor
The other joint:
Kindle Fire power switch – rebuilt – left anchor
And, for completeness, the switch leads:
Kindle Fire power switch – rebuilt – switch pads
I don’t like the way the joint on the right looks, either, but we’ll see how long the whole affair holds together.
This may be the last time I can repair the Kindle, as a bypass cap came loose while I was working on the PCB, the screen has been accumulating dust at an increasing pace, and several latches securing the back of the case have cracked.
Methinks it’s getting on time for a new pocketable memory device; if only Pixel XL phablets had a bigger screen and didn’t cost night onto a kilobuck.
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:
AD8310 Log Amp – 100 kHz 500 MHz
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:
AD8310 Log Amp – 100 kHz 500 MHz – RF 50 ohm term
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:
AD8310 Log Amp – 60 kHz 1 Vpk
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:
AD8310 Log Amp module – uncovered
A closer look:
AD8310 Log Amp module – PCB detail
A capacitance meter says input capacitors C5 and C7 are both 10 nF.
A sketch of the circuitry:
AD8310 Log Amp module – input circuit
The datasheet puts the terminating resistor on the other side of the input caps, where it surely belongs:
AD8310 Datasheet – Basic Connections diagram
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):
AD8310 Log Amp module – input circuit simulation
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 …
A quick-and-dirty test routine showed the sticks start out close to VCC/2:
Welcome to minicom 2.7
OPTIONS: I18n
Compiled on Feb 7 2016, 13:37:27.
Port /dev/ttyACM0, 10:23:45
Press CTRL-A Z for help on special keys
Joystick exercise
Ed Nisley - KE4ZNU - May 2017
00524 - 00513 - 1
That’s from minicom on the serial port, as the Arduino IDE’s built-in serial monitor ignores bare Carriage Return characters.
The joystick hat tilts ±25° from its spring-loaded center position, but the active region seems to cover only 15° of that arc, with a 5° dead zone around the center and 5° of overtravel at the limits. This is not a high-resolution instrument intended for fine motor control operations.
The analog input values range from 0x000 to 0x3FF across the active region. Aim the connector at your tummy to make the axes work the way you’d expect: left / down = minimum, right / up = maximum.
The delay(100) statements may or may not be needed for good analog input values, depending on some imponderables that seem not to apply for this lashup, but they pace the loop() to a reasonable update rate.
Pushing the hat toward the PCB activates the simple switch you can see in the picture. It requires an external pullup resistor (hence the INPUT_PULLUP configuration) and reports low = 0 when pressed.
Those are 0.125 inch (exactly!) holes on a 19.5×26.25 mm grid in a 26.5×34.25 mm PCB. Makes no sense to me, either.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The Smell of Molten Projects in the Morning 