Archive for category Amateur Radio
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 …
Some weeks ago, the APRS + voice adapter on my radio began randomly resetting during our rides, sending out three successive data bursts: the TinyTrak power-on message, an ID string, and the current coordinates. Mary could hear all three packets quite clearly, which was not to be tolerated.
I swapped radios + adapters so that she could ride in peace while I diagnosed the problem, which, of course, was both intermittent and generally occurred only while on the road. The TinyTrak doc mentions “… a sign of the TinyTrak3 resetting due to too much local RF energy”, so I clamped ferrite cores around All! The! Cables! and the problem Went Away.
Removing one core each week eventually left the last core on the GPS receiver’s serial cable, which makes sense, as it plugs directly into the TT3. The core had an ID large enough for several turns (no fool, I), another week established a minimum of three turns kept the RFI down, so I settled for five:
Prior to the RFI problem cropping up, nothing changed. Past experience has shown when I make such an assertion, it means I don’t yet know what changed. Something certainly has and not for the better.
I swapped the radios + adapters and all seems quiet.