Archive for category Amateur Radio
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):
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:
The new caps aren’t all that visible and the resonator vanishes in the clutter:
Next: find out how well it works!
(*) Yes, there were two envelopes between 150 pF and 147 pF:
The FG085 function generator shows 60000 Hz and the AD9850 shows 60001.58 Hz, but they’re running at exactly the same frequency:
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.
The Arduino source code as a GitHub Gist:
This took entirely too long to figure out:
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:
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:
“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:
I don’t know why that spot has an inductor symbol with a capacitor part callout.
The other side of the PCB looks clean:
It’ll probably serve well in a noise-tolerant application, maybe an LED power supply.
FWIW, the UL mark seems conspicuous by its absence:
Not sure what I’ll replace it with, although a small 24 V power supply brick may suffice.
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:
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:
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:
The metalization appears black here and gold in person.
So, yeah, one down and 49 to go …
Histogramming all 50-ish resonator frequencies shows reasonably good distributions:
I don’t know what to make of the difference between the
parallel series-capacitor and basic serial resonant frequencies for each tuning fork:
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:
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.]
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
CX = 9.9pF
Fs peak: 59996.19 Hz 79.4 dbV
Fc peak: 59999.97 Hz 75.8 dbV
Delta frequency: 3.78
CX = 6.8 pF
Fs peak: 59996.10 Hz 80.3 dbV
Fc peak: 60001.48 Hz 74.6 dbV
Delta frequency: 5.38
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.
Isolating the USB port from the laptop eliminated a nasty ground loop, turning off the OLED while making measurements stifled a huge noise source, and averaging a few ADC readings produced this pleasing plot:
Those nice smooth curves suggest the tester isn’t just measuring random junk.
The OLED summarizes the results after the test sequence:
Collecting all the numbers for that resonator in one place:
- C0 = 1.0 pF
- Rm = 9.0 kΩ
- fs = 59996.10 Hz
- fc = 59997.79 Hz
- fc – fs = 1.69 Hz
- Cx = 24 pF
Turning the crank:
I ripped that nice layout directly from my November Circuit Cellar column, because I’m absolutely not even going to try to recreate those equations here.
Another two dozen resonators to go …