The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

60 kHz Preamp: Power Supply Noise

This took entirely too long to figure out:

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

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

“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

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

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

As pointed out in the comments, there’s a UL mark:

SMAKN 24 VDC wart – label

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

2017-09-26
60 kHz Preamp: Tuning Fork Resonator Protection

Limiting the resonator drive to about 1 μW in the face of wildly varying RF from the antenna (or the occasional finger fumble) requires brute force. A nose-to-tail pair of Schottky diodes seems to do the trick:

Tuning Fork Resonator Filter – protection and biasing

The 100 Ω resistor blunts the drive from the LM353 op amp (implementing a bandpass filter) when the signal peaks exceed 200-ish mV in either direction from the Vcc/2 bias stored in the 10 μF cap.

The 11.5 kΩ resistor downstream of the resonator isolates it from the Vcc/2 bias, with the 100 nF cap sinkholing the signal and the 4.7 kΩ resistor preventing feedback into the bias supply. The cap looks like 26 Ω at 60 kHz, so the feedback runs -52 dB from the output and the bias supply knocks it down a bit more. The preceding amps apply 40-ish dB of gain from the antenna terminals, so the loop gain looks OK.

It’s another few components on the board:

LF Crystal Tester – resonator protection

The blue twiddlecap should allow pulling the tuning fork’s series resonance upward to exactly 60 kHz.

Applying way too much signal to the antenna terminals in order to get 1 Vpp from the LM353 shows the limiter in action:

BP and Xtal filter out – 10.0 v sine 10 Meg xfmr

The resonator sees no more than 200 mV in either direction from the bias level, so it’s all good.

On the low end, the diodes have no effect:

BP and Xtal filter out – 1.1 v sine 10 Meg xfmr

Pay no attention to all that noise.

My first thought was to put the diodes across the resonator, a Bad Idea: straight up, doesn’t work. The 1N5819 datasheet shows they have about 300 pF of junction capacitance at zero bias and a pair of ’em will swamp the resonator’s internal 0.8 pF parallel capacitance and punch it out of the circuit.

2017-09-25
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

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

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

The metalization appears black here and gold in person.

So, yeah, one down and 49 to go …

2017-09-20
Amazon Packaging: PAR30 LED Bulb

The second incandescent bulb over the kitchen sink popped and a replacement LED bulb arrived with the by-now-familiar homeopathic Amazon padding:

Amazon Packaging – Satco LED bulb

Turns out the new bulb is slightly brighter than the old one:

Satco S9415 LED PAR30 bulbs

Oh, and it’s three bucks cheaper, too.

Eyeballometrically, 5% makes no difference whatsoever, even in a side-by-side comparison.

Life is good.

2017-09-18
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

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

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

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.]

2017-09-14
LF Crystal Tester: Grounded CX Case

The usual model for a quartz resonator apportions half the measured both-leads-to-case capacitance to each lead:

AT26 crystal capacitance fixture – Cpar detail

These AT26 / TF26 cases run around 0.6 pF, so each parasitic capacitor is 300 fF:

60 kHz Quartz Resonator – model

For ordinary quartz crystals, you solder the case to the ground plane to get rid of the sneak path around the central capacitor (normally C0, but labeling it properly in LTSpice just isn’t happening), but those little aluminum cans aren’t solderable. One could blob some Wire Glue over them, but …

So I just wrapped a wire around the case and soldered it to a convenient ground point under the board:

LF Crystal Tester – grounded TF26 case

Aaaand ran the obvious measurements:

60 kHz Quartz Resonator 0 – CX 6 pF – grounded vs float

Solid lines = case ungrounded. Dotties = case grounded.

Grounding the case knocks the off-peak response down by less than 1 dB. The on-peak response remains about the same, so eliminating the series capacitance does reduce the blowthrough.

With the case grounded and CX = 6 pF in the circuit, the peaks over on the right seem ever so slightly lower in frequency, which suggests a slightly higher motional capacitance. There’s not much to write home about, though, so I’d say there’s very little effect, even on this scale.

2017-09-13
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

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

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

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.

2017-09-12