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Archive for category Amateur Radio

SRAM Shift Indicator Repair: Polypropylene Sheet

Over the course of a few weeks, both of the indicators in the SRAM grip shifters on my bike snapped off. Having recently touched my parallel jaw clamp assortment, it occurred to me I could mold snippets of polypropylene sheet (saved from random clamshell packages for just such a purpose) around the nose of a clamp and come out pretty close to the final shape:

SRAM Shift Indicator - shaped replacements

SRAM Shift Indicator – shaped replacements

A hot air gun set on LOW and held a foot away softened the polypro enough so a gloved thumb could squash it against the jaw. Too much heat shrinks the sheet into a blob, too little heat lets the sheet spring back to its original shape.

The flat tab of the original indicator is about 1 mm thick. I found a package of 47 mil = 1.2 mm sheet with one nice right-angle bend and ran with it.

Because I expect sunlight will fade any color other than black, that’s the Sharpie I applied.

They don’t look as awful as you might expect. The rear shifter, minus the cover:

SRAM Shift Indicator - rear detail

SRAM Shift Indicator – rear detail

The front shifter, with cover installed and HT PTT button below the still-good Kapton tape:

SRAM Shift Indicator - front assembled

SRAM Shift Indicator – front assembled

The transparent covers press the OEM indicators down and do the same for my homebrew tabs. I expect the Sharpie will wear quickly at those contact points; next time, I should tint the other side.

They’re rather subtle, I’ll grant you that.

Now, to see if they survive long enough to make the worry about a brighter color fading away a real problem…

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60 kHz Preamp: Filtering and Rebiasing

The LT1920 instrumentation amplifier now sports two silver-mica caps on its inputs as a differential-mode input filter cutting back strong RF signals (clicky for more dots):

60 kHz Preamp Schematic - DM filter inst amp - BP filter rebias - 2017-09-22

60 kHz Preamp Schematic – DM filter inst amp – BP filter rebias – 2017-09-22

In principle, a DM filter should eliminate RF rectification from out-of-band signals, although I think the attic is quiet enough to not need any help. The caps form a simple RC LP filter rolling off at 5.490 kΩ × 150 pF → 193 kHz, high enough above the 60 kHz signal to not make much difference down there.

The silver-mica caps come from the Big Box o’ Caps, which contained an envelope with a few large 150 pF ±1% caps and a bag stuffed with similar 147 pF ±1% caps. Mixed in with the latter were some smaller 147 pF caps (*) of no particular tolerance (perhaps 5%), from which I neurotically matched a pair to 0.05 pF without too much effort. Doesn’t matter, given the other tolerances and suchlike, but it was amusing.

I’d inadvertently grounded the cold end of the 330 Ω input resistor in the LM353 bandpass filter, now properly tied at the Vcc/2 virtual ground to take the DC load off the LT1920 output: a 100 nF cap (27 Ω at 60 kHz) stores the bias level without messing up the filter shape.

A similar cap rebiases the protected resonator at the LT1010 buffer input:

60 kHz Preamp Schematic - protected resonator - output rebias - 2017-09-22

60 kHz Preamp Schematic – protected resonator – output rebias – 2017-09-22

The new caps aren’t all that visible and the resonator vanishes in the clutter:

60 kHz Preamp - protected resonator filter - overview

60 kHz Preamp – protected resonator filter – overview

Next: find out how well it works!

(*) Yes, there were two envelopes between 150 pF and 147 pF:

Silver-mica caps

Silver-mica caps

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LF DDS Sine Generator With 0.1 Hz Steps

Gutting the crystal tester program and grafting a simple joystick interface onto the corpse produces an LF sine wave generator with 0.10 Hz steps:

FG085 vs AD9850 DDS frequencies

FG085 vs AD9850 DDS frequencies

The FG085 function generator shows 60000 Hz and the AD9850 shows 60001.58 Hz, but they’re running at exactly the same frequency:

DDS 1.58 FG085 0.0

DDS 1.58 FG085 0.0

I trust the AD9850 readout, because I just finished zero-beating it against the GPS-locked 10 MHz frequency reference: it’s dead on. The scope’s frequency measurement is clearly out of its depth at this resolution.

The “user interface” doesn’t amount to much. The DDS starts at 60.000 kHz, as defined by a program constant. Push the joystick left-right to step by 0.1 Hz (actually, multiples of 0.0291 Hz, so either 0.087 or 0.116 Hz, whichever makes the answer come out closer to the next multiple of 0.1 Hz). Push it up-down to step by 1.0 Hz (insert similar handwaving here). Push the button inward to reset to 60.000 kHz.

The OLED displays the frequency (in big digits), the output of the log amplifier (which isn’t hooked up here) in dB (over 4 μV), the DDS clock oscillator temperature, and a few lines of static prompting. The camera shutter blanked the last line, which should read “Button = reset”.

There’s no amplitude adjustment, other than the DDS current-control twiddlepot and the buffer amp’s gain-setting jumpers, but I (think I can) gimmick up an adequate inductive kicker for the fake preamp antenna circuit.

Not much to look at, but now I can (manually) probe the frequency response of the 60 kHz preamp with sufficient resolution to figure out if / how the tuning fork resonator filter behaves.

The Arduino source code as a GitHub Gist:

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60 kHz Preamp: Power Supply Noise

This took entirely too long to figure out:

Ground noise - 24 VDC wall wart - probe on gnd lug

Ground noise – 24 VDC wall wart – probe on gnd lug

That’s with the scope probe ground clip connected to the wall wart coax connector barrel and the scope probe tip on the ground clip. It’s not the noise on the 24 VDC supply, it’s the noise injected into the ground connection!

Huh. Makes it tough to sort out low-level signals, it does indeed.

Consider one of my bench power supplies at 24 V:

Ground noise - bench supply 24 V - probe on gnd lug

Ground noise – bench supply 24 V – probe on gnd lug

Nice & quiet, the way power should be. One might quibble about the residual noise, but at least it’s not blasting out horrific bursts at 120 Hz.

For completeness, the PCB inside the offending SMAKN 24 V wall wart:

SMAKN 24 VDC wart - PCB

SMAKN 24 VDC wart – PCB

“High Quality Commercial Grade” my aching eyeballs.

[Update: Edits based on eagle-eyed observations in the comments. ]

Not as many missing components as I expected, though, if the truth be told. The missing transformer common-mode choke seems odd and, AFAICT, the resistor inductor angling out from the R1 callout doesn’t connect to anything, connects directly to the AC line because  C5 is missing and the pad joining them doesn’t go anywhere else it replaces the jumper (?) to the bottom-left pad and the missing parts. The red LED in the upper right isn’t visible through the black case, although it might serve as a voltage regulator.

Over on the far right, beyond the transformer and between the two capacitor cans, is a component marked C9 with an oddly angled part. Seen from the other end, it’s a ferrite bead:

SMAKN 24 VDC wart - output ferrite

SMAKN 24 VDC wart – output ferrite

I don’t know why that spot has an inductor symbol with a capacitor part callout.

The other side of the PCB looks clean:

SMAKN 24 VDC wart - PCB solder side

SMAKN 24 VDC wart – PCB solder side

It’ll probably serve well in a noise-tolerant application, maybe an LED power supply.

FWIW, the UL mark seems conspicuous by its absence:

SMAKN 24 VDC wart - label

SMAKN 24 VDC wart – label

Not sure what I’ll replace it with, although a small 24 V power supply brick may suffice.

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60 kHz Tuning Fork Resonator: Maximum Overdrive

Datasheets loosely associated with the tuning fork resonators in hand suggest 1 μW maximum drive power, which works out to maybe 100 mVrms = 150 mVpk at about 10 kΩ ESR. If you inadvertently apply 500 mVpk = 375 mVrms, the resulting 14 μW does this:

Broken 60 kHz Tuning Fork Resonator - overview

Broken 60 kHz Tuning Fork Resonator – overview

I was applying a precisely tuned 60 kHz sine wave to the first pass at a crystal filter grafted onto the loop antenna preamp and wasn’t paying attention to the amplitude. For all I know, though, the poor thing died from a power-on transient. I’m pretty sure I didn’t break it during extraction, because it stopped being a resonator while in the circuit.

The missing tine fell out of the can:

Broken 60 kHz Tuning Fork Resonator - tine detail

Broken 60 kHz Tuning Fork Resonator – tine detail

Laser trim scars form a triangle near the tip, a T a bit further down, a slot just above the nicely etched gap.

A closer look at the fractured base:

Broken 60 kHz Tuning Fork Resonator - detail

Broken 60 kHz Tuning Fork Resonator – detail

The metalization appears black here and gold in person.

So, yeah, one down and 49 to go …

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LF Crystal Tester: 60 kHz Resonator Frequency Distribution

Histogramming all 50-ish resonator frequencies shows reasonably good distributions:

Notably, there’s no obvious suckout in the middle, as with those eBay Hall-effect sensors.

60 kHz Resonant Frequencies - CX 24 pF - histogram

60 kHz Resonant Frequencies – CX 24 pF – histogram

I don’t know what to make of the difference between the parallel series-capacitor and basic serial resonant frequencies for each tuning fork:

60 kHz Resonant Frequencies - CX 24 pF - delta histogram

60 kHz Resonant Frequencies – CX 24 pF – delta histogram

Perhaps each resonator’s frequency depends on its (laser-trimmed) tine mass and follows a more-or-less normal distribution, but the parallel-serial difference series capacitor changes the frequency based on (well-controlled) etched dimensions producing quantized results from three different masks / wafers / lots, with the motional inductance and capacitance incompletely modeling the physics?

For reference, the resonators look like this:

Quartz resonator - detail

Quartz resonator – detail

Producing the histograms uses the LibreOffice frequency() array function, which requires remembering to whack Ctrl-Shift Enter to activate the function’s array-ness.

[Update: Faceplant about “parallel” resonance, which is actually the shifted resonant peak due to the 24 pF series cap. Apparently I typo-ed the second histogram subheading and ran with the error; the figures are now correct.]

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LF Crystal Tester: Resonance Frequencies vs CX

Adjusting the series capacitor produces pretty much the expected results, with the parallel resonance still tracking the series peak.

CX = 19.3 pF
Fs peak: 59996.18 Hz 80.4 dbV
Fc peak: 59998.19 Hz 78.2 dbV
Delta frequency: 2.01

60 kHz Quartz Resonator 0 - CX 19.3 pF

60 kHz Quartz Resonator 0 – CX 19.3 pF

CX = 9.9pF
Fs peak: 59996.19 Hz 79.4 dbV
Fc peak: 59999.97 Hz 75.8 dbV
Delta frequency: 3.78

60 kHz Quartz Resonator 0 - CX 9.9 pF

60 kHz Quartz Resonator 0 – CX 9.9 pF

CX = 6.8 pF
Fs peak: 59996.10 Hz 80.3 dbV
Fc peak: 60001.48 Hz 74.6 dbV
Delta frequency: 5.38

60 kHz Quartz Resonator 0 - CX 6.8 pF

60 kHz Quartz Resonator 0 – CX 6.8 pF

At the frequency resolution of these graphs, none of the standard equations are helpful; this is definitely a “tune for best picture” situation.

So, assuming the same general conditions apply in a filter, a series capacitance around 10 pF should pull the resonant peak to 60.000 kHz. Unfortunately, the cheery 76 dB level is relative to the AD8310‘s nominal -108 dBV intercept at 4 μV: the log amp sees 25 mV after the MAX4255 op amp applies 40 dB (×100) of gain to the 250 μV coming from the resonator. The resonator drive is 1 μW = 150 mV, so the resonator produces a 55 dB loss for a signal dead on frequency.

The off-peak attenuation looks like a mere 7 dB, although I hope plenty of noise masks the true result in this circuit.

Phew & similar remarks.

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