Archive for category Amateur Radio
The DSO150 oscilloscope’s specs give a 200 kHz bandwidth, so a 50 kHz sine wave looks pretty good:
A 100 kHz sine wave looks chunky, with maybe 25 samples per cycle:
The DSO150 tops out at 10 µs/div, so you can’t expand the waveform more than you see; 25 samples in 10 µs seems to be 2.5 Msample/s, exceeding the nominal 1 Msample/s spec. I have no explanation.
A 10 kHz square wave shows a blip just before each transition that isn’t on the actual signal:
At 50 kHz, there’s not much square left in the wave:
And, just for completeness, a 200 kHz square wave completely loses its starch:
A 10% (-ish) duty cycle pulse at 25 kHz has frequency components well beyond the scope’s limits, so it’s more of a blip than a pulse:
The pulse repetition frequency beats with the scope sampling and sweep speeds to produce weird effects:
Tuning the pulse frequency for maximum weirdness:
None of this is unique to the DSO150, of course, as all digital scopes (heck, all sampled-data systems) have the same issues. The DSO150’s slow sampling rate just makes them more obvious at lower frequencies.
Key takeaway: use the DSO150 for analog signals in the audio range, up through maybe 50 kHz, and it’ll produce reasonable results.
Using it for digital signals, even at audio frequencies, isn’t appropriate, because the DSO150’s low bandwidth will produce baffling displays.
Baofeng UV-5 radios can (mostly) eliminate the loud hiss heard at the end of a transmission before the squelch kicks in after the received carrier drops:
Menu → 34 STE → ON. A detailed description of the option suggests it’s a 55 Hz subaudible tone sent for 250 milliseconds after the sender releases the PTT and before the transmitter stops sending, with the receiver muting its audio during the tone. Obviously, this requires a Baofend radio at each end of the conversation, which applies to our bikes.
Saying “laaaa” while kerchunking (into a smaller dummy load than the hulk) with STE OFF:
Compared to the received audio, the squelch tail hiss is really really loud.
Then with STE ON:
You can see the STE tone reception start about 250 ms before the audio cuts off, although it’s not at all clear the audio is muted on either end. In any event, there’s no squelch tail worth mentioning, even if there’s an audible tick when the STE tone starts.
Saying nothing with STE ON:
It’s unlikely the audio output would include the subaudible tone, but you might convince yourself something happens in the 250 ms between the STE blip near midscreen and the final pop (now clipped) as the audio drops.
All in all, a definite improvement!
Perhaps because we’re using better quality earbuds, the Baofeng UV-5 radios on our bikes produce extremely loud audio, even with the volume knob just above its power-on click. Reducing the volume requires a series resistor downstream of the diodes clipping the pops:
The color codes come from previous work.
Because we have different earbuds and different hearing, my radio has a 140 Ω resistor and Mary’s has a 430 Ω resistor. Getting the right value requires a few iterations of on-road testing, but it’s not particularly critical; the volume knob should end up roughly in the middle of its range.
For now, all the “circuitry” lives among layers of Kapton tape:
Speaking of volume knobs, Baofeng radios have large flat-top cylindrical knobs (unlike Wouxun’s fluted knobs), so I added a pointed snippet of reflective tape to make the position visible:
The flash lights it up, but there’s enough backlighting behind your (well, my) head to make it easily visible under normal conditions. Once you figure out the proper volume, it’s easy to set the pointer in that direction before every ride.
To the road!
Our first ride with the Baofeng UV-5 radios subjected us to loud pops around each transmission. Back on the bench, this is the signal applied to the earbud during a no-audio simplex kerchunk:
The small noise burst to the right of the center, just before the downward pulse, happens after the carrier drops and before the squelch closes; it’s familiar to all HT users.
The huge pulses, upward at the start and downward at the end, cause the pops. They’re nearly 3 V tall, compared with the 300-ish mV squelch noise, and absolutely deafening through an earbud jammed in my ear. Mary refused to listen, so we finished the first ride in companionable silence.
I think the radio switches the audio amp power supply on and off to reduce battery drain. It’s obviously a single-supply design, so we’re looking at a hefty DC blocking capacitor charging and discharging through the earbud resistance. I suppose that’s to be expected in a $25 radio.
The obvious solution: clamp the audio signal to something reasonable, perhaps with a pair of nose-to-tail Schottky diodes across the earbud. Rather than using axial diodes, along the lines of the 1N5819 diodes in the WWVB preamp, I used a BAT54S dual SMD diode as a tiny clamp:
No pix of the final result, but it’s basically two wires soldered alongside the SMD package, surrounded by a snippet of heatstink tubing to stabilize the wires and protect the SMD leads. It might actually survive for a while, even without the obligatory epoxy blob.
The BAT54S clamps the pops to 200-ish mV, as you’d expect:
That’s a kerchunk at twice the vertical scale. The very thin spike at the start of each pop isn’t audible, as nearly as we can tell, and I’ve cranked up the audio gain to make the squelch noise more prominent. Your ears will determine your knob setting.
With the audio amp applying 3 V to the diodes at the start of each pop, you’re looking at an absurdly high pulse current. I’m sure the radio exceeds the BAT54 datasheet’s 600 mA surge current limit by a considerable margin, but I’m hoping the short duration compensates for some serious silicon abuse.
Tamping those pops down made the radios listenable.
I’ve often observed that Baofeng radios are the worst HTs you’d be willing to use.
Rather than 3D print and hand-wire a plug adapter to fit the socket around the Baofeng UV-5 mic and speaker jacks, I cheated:
Un-wearably bad Baofeng headsets now cost just over a buck apiece in lots of five, delivered halfway around the planet, and provide:
- A compatible molded mic+speaker plug
- A decent length of four-conductor cable with solder-meltable insulation
- An unlistenably bad earbud on a stick
- A lump with an electret mic and PTT switch
- Various junk I’ll never use
The “hook earpiece” seems to have been designed by someone who had read the specs for a human head, but had never actually met a human being.
The wire colors from the dual plug, along with the wire colors for the repurposed USB cable to the headset, and the PTT connection:
Then wire it up accordingly:
The small heatstink tubing surrounding each connection isn’t easily visible, which, in the case of the ground / common lump, is a Good Thing. I chivied a strip of Kapton under the whole mess, folded it over on top, squished it together, then secured it with 1/4 inch tape extending over the plate edges. The cable ties stick out far enough to keep the joints from rubbing on anything; it’s not built to last for a thousand years, but should let us hear how this lashup works.
Now, to the bikes:
I’m convincing myself a little supporting ring under the SMA-to-UHF adapter won’t actually stabilize the precarious-looking joint.
My venerable amateur radio HT APRS-voice interfaces have recently begun failing and, given poor APRS coverage in Poughkeepsie due to having two iGates shut down (due to the aging radio geek population), I decided it’s time to simplify the radio interface. Given that HTs are designed to run with an external electret mic and earbud, the “interface” becomes basically some wires between the radio’s jacks, a repurposed USB plug on the bike helmet, and the PTT switch on the handlebar.
I expected to add a resistive attenuator to the earbud, but it wasn’t clear whether the mic would need an amplifier similar to the one in the APRS interface, so I decided to start as simply as possible.
The general idea is to anchor all the cables to a plate on the back of the radio, interconnect as needed, then “protect” everything with tape. The pocket clip has M2.5 screws on 26 mm (not 25.4, honest) centers, so that’s how it started:
The four holes beside the tabs will serve as starting points for rectangular notches holding cable ties lashing the wires to the plate:
That’s hot and nasty, straight from the bandsaw.
After some edge cleanup, add obligatory Kapton tape to insulate stray wires from the aluminum:
The alert reader will note beveled corners on one plate and square corners on the other; think “continuous product improvement”.
The big rectangular gap in the middle of the plate provides (barely enough) finger clearance to push the battery release latch.
Now, to wire it up …
The dimensions of the recess surrounding the jacks on the Baofeng UV-5, just to have them around:
Which came from measurements of both the Wouxun and Baofeng radios:
Ex post facto notes from the third Squidwrench Electronics Workshop.
Exhibit various 50 Ω resistors, including my all-time favorite, a 600 W 3 GHz dummy load:
… down to a 1/8 Ω metal film resistor.
The dummy load’s N connector triggered a regrettable digression into RF, belatedly squelched because I wasn’t prepared to extemporize on AC concepts like reactance which we haven’t covered yet.
Discussion of resistor applications, power handling, power derating with temperature, etc:
Why you generally won’t find 50 Ω load resistors in Raspberry Pi circuits. Cartridge heaters for 3D printers, not aluminum power resistors, although everyone agrees they look great:
Discussion of voltage vs. current sources, why voltage sources want low internal resistances and current sources want high resistances. Bungled discussion of current sources by putting diodes in parallel; they should go in series to show how added voltage doesn’t change current (much!) in sources driven from higher voltages through higher resistances:
Use Siglent SDM3045X DMM in diode test mode to measure forward drop of power / signal / colored LEDs, discuss voltage variation with color / photon energy. Measure 1.000 mA test current for all forward voltages.
Compute series resistor (500 Ω) to convert adjustable power supply (the digital tattoo box, a lesson in itself) into reasonable current source; roughly 10 V → 20 mA. Find suitable resistor (560 Ω) in SqWr
junk box parts assortment, digression into color band reading.
Wire circuit with meters to measure diode current (series!) and voltage (parallel!), measure same hulking power diode (after discovering insulating washers now in full effect) as before in 1 mA steps to 10 mA, then 15 and 20 mA, tabulate & plot results:
Discover warm resistor, compute power at 20 mA, introduce cautionary tales.
Lesson learned about never returning parts to inventory, with 560 Ω resistor appearing in diode drawer. Cautionary tales about having benchtop can of used parts as front-end cache for inventory backing store.
Another intense day of bench work!