Secrets from the EE Scrolls - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2025, 02:35:16 pm
content: #001 - Clip a coil to a DSO on Normal, take a camera, and make a close up photo with flash.Big L want to keep their secrets!They don't want you to know this.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 06, 2025, 03:59:54 pm
content: Looks like you have (re)discovered a lightning detector.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 06, 2025, 07:32:15 pm
content: Cool! So what is going on?

author: jpanhalt
author_rank: Super Contributor
timestamp: June 06, 2025, 08:13:25 pm
content: Grok explains it better than I could (fewer spelling erors).QuoteLightning does too. The AMS AS3935 is a single chip detector (http://media.digikey.com/pdf/Data%20Sheets/Austriamicrosystems%20PDFs/AS3935.pdf). I used an 8-bit PIC to read it. Ebay has devices, including watches much cheaper).An emulator (from AMS ) was fun to build and play with (attachment).

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 06, 2025, 08:19:13 pm
content: Hmm could I use either one of these for this purpose? Those aliexpress high voltage modules or arc based candle lighters?

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 06, 2025, 08:19:27 pm
content: Yes, looks like you're ready for a collaboration with Chet.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: June 06, 2025, 09:46:01 pm
content: Pretty cool, I'm able to get these even when the spark gap is a feet away from the freshly wound coils. I don't want to move the voltage multiplier closer to my scope but I have no doubt it'll work even better.Quite apparent in the captures which is from the coil with more turns, and one with less.Let me see if I can find a higher value coil

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 07, 2025, 03:51:57 pm
content: In the 1980s, two-stroke snowmobiles (spark plugs) running within 100 meters of TV antennae caused visible snow. No need for fancy equipment to discover the spark gap radio transmitter. Bugged the heck out of me as a kid.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2025, 04:06:41 pm
content: The technical term for the output of a spark transmitter is “evanescent wave”, as opposed to the “continuous wave” produced by an oscillator or alternator.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 07, 2025, 04:52:59 pm
content: You can get that with a piezo gas lighter. These can even be used as a poor man's ESD gun.

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 07, 2025, 05:09:22 pm
content: Quote from: TimFox on June 07, 2025, 04:06:41 pmI've never seen any actual research into which part in those two-stroke engines caused the actual interference, especially because some channels (frequency bands) were more susceptible than others. I'm basically just assuming that it's the spark gap more than e.g. the alternator –– these typically used gasoline fuel but no battery, starting with a pull cord. The "snow" noise effect corresponds to bursts of radio frequency noise at a few tens Hz to a couple of hundred Hz, which corresponds to the engine firing rates, which kinda-sorta points to the spark gap as the reason.Interestingly, the difference to cheap automobiles at the same period was mostly that cars had a metal chassis (non-grounded Faraday cage of sorts) whereas snowmobiles only had a lightweight thin fiberglass cover.That said, assuming I understood correctly, TimFox's point is that a spark gap transmitter range of effect is very limited, essentially dropping of exponentially as the distance increases, because the spark itself does not generate propagating radio waves similar to e.g. an antenna. I do agree, of course.Interestingly, there was also a directional component. The range was typically on the order of 50 meters or perhaps a dozen wavelengths (lowest frequencies having wavelengths of a couple of meters), but during cold dry spells with lots of static charges in air and on surfaces, could reach for at least half a kilometer if between the sending and receiving antennae. This, too, indicates it was an "evanescent wave" rather than "continuous wave" issue, although the exact details of how and why it occurred is a bit of a mystery –– although everybody knew even back then that enclosing the engine inside a thin metal mesh or sheeting does basically eliminate it (just didn't bother).For me, even as a small kid, this always meant that "noise", especially electromagnetic noise, can be a problem even when books and common wisdom say it shouldn't. Only a good practical measurement in controlled conditions can verify that, because things can interact and easily make the actual physical situation different than one thinks.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2025, 05:25:32 pm
content: Not exactly. Evanescent wave (as opposed to continuous wave) refers to the exponential decay of the amplitudein time, after the spark.The rotating spark gaps (commonly found in Frankenstein movies) sparked faster than the duration of a Morse dot or dash, so the receiver did not need a BFO to hear audio (audio frequency determined by the spark rate).Evanescent wave transmissions (usually called spark transmitters) were banned due to their broad frequency range causing interference, but the CW that replaced it needed a BFO or similar in the receiver.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 07, 2025, 05:37:38 pm
content: Quote from: SteveThackery on June 06, 2025, 07:32:15 pmThe camera flash is in fact a spark, a discharge in the gases of the photo blitz. This is a sharp spike of current running through the blitz and through the wires inside the camera, which creates an electromagnetic spike. An EMP!The electromagnetic pulse eventually pass through the coil under test, and a little bit of the EMP energy remains trapped in the coil. Each coil has also a parasitic capacitance, and a parasitic resistance, so it's in fact an RLC circuit. The bit of energy captured by the coil will start to slosh back and forth between the L and the parasitic C, like in any oscillant circuit.At each oscillation, a certain percent will be lost as heat in the parasitic R of the coil, and another bit will be lost as radio waves (EM radiation resistance of the coil). The ratio between the total energy and the losses is the quality factor, Q, and if anybody is brave enough to do the math (I've copied the formula), should notice that the amplitude of oscillations goes to half in a certain number of periods. Or else said, Q can be determine by counting the number of oscillations needed for the oscillations to go to half.\$Q=\frac{\pi}{\ln(2)}N\$, where \$N\$ is the number of osc periods that halve the amplitudeNotice how \$\frac{\pi}{\ln(2)}\$ is a constant, about 4.53, so Q=4.53*N. Also, notice how it doesn't matter from which moment you start to count. The counting doesn't necessarily have to be from the very beginning. You can count from any point, until half of that amplitude, and that's N (because it's an exponential decay, the half-life is a constant, specific only to the circuit's Q, and not to the initial amplitude or to the segment where you count).Because nowadays the digital oscilloscope is the norm, it is enough a single pulse to memorize the oscillation and count them, and such the trick with the spark flash. Same, a single EMP is enough to find out the self-resonance, or the frequency of an LC. In the past one would have used a generator to repeat the spikes in order to create a stable display on an analog oscilloscope without memory.Anyway, looks like the main attraction was the flash, and not the way to measure the self resonant frequency, or to measure the Q by the "ring down method" (counting the periods taken for the amplitude to become 50%, and multiply that with 4.53 to find Q - some are multiplying by 5, to ease the calculations).

author: RoGeorge
author_rank: Super Contributor
timestamp: June 07, 2025, 06:12:03 pm
content: Quote from: Nominal Animal on June 07, 2025, 05:09:22 pmMe neither, but as an approximation, here's some spectral measurements of a spark gap transmitter byjoeqsmith.https://www.eevblog.com/forum/rf-microwave/fly-swatter-ham-radio-transmitter/About the evanescent waves, in my mind that term summons the image of water waves left behind a moving ship, or an exponential decay, like the oscilloscope print-screens attached in the opening post, though the decay doesn't have to be mandatory exponential, but it has to go smaller and smaller. Not sure if this is the proper meaning, only saying what my mind makes out of it.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2025, 06:38:15 pm
content: The literal meaning of the English word "evanescent" is "quickly fading or disappearing" (temporal), but physics also uses it to describe the penetration (in distance) of an EM wave into a metal (skin effect).There was an infamous patent medicine that correctly advertised "quick, evanescent relief" from pain.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 08, 2025, 08:16:11 pm
content: Quote from: TimFox on June 07, 2025, 06:38:15 pmThat sounds like the sign P.T. Barnum was supposed to have put up in one of his exhibits saying "This Way To The Egress".

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 09, 2025, 02:30:12 am
content: Quote from: TimFox on June 07, 2025, 06:38:15 pmLanguage is hard. Another I still haven't figured out (a key connotation of) isto emerge(and derivative terms): does the emerging thing exist in its current form before becoming observable, or not? Depending on the dictionary you use, the definition usually impliesyes, but many practical uses implyno. Go figure. Me fail English anyway...

author: jpanhalt
author_rank: Super Contributor
timestamp: June 09, 2025, 09:23:46 am
content: RE: emergeGenerally, as a verb, it existed before, like a snake emerged from under a rock.https://dictionary.cambridge.org/us/dictionary/english/emergeHowever, when used as a noun, e.g., emerging, emergence, it can mean developing (as in new), such as an emerging technology. That sense is commonly used in marketing. According to OED (https://www.oed.com/dictionary/emerge_n), even "emerge" was used as a noun in about 1878. In my experience, that is uncommon.. Unfortunately, OED has begun requiring an account to get any more information.

author: tom66
author_rank: Super Contributor
timestamp: June 09, 2025, 11:00:26 am
content: Related to the spark-gap noise problem, apparently a major reason you will not find an AM radio on an electric vehicle is it is far too difficult to make that work when the drive inverter and other power electronics produce noise in a similar operating band to the AM radio frequency. The noise is within FCC limits, but the antenna on the car is typically located towards the rear of the vehicle (for aerodynamics) but most EVs are rear-wheel drive as it is easier to package.

author: bdunham7
author_rank: Super Contributor
timestamp: June 09, 2025, 12:08:31 pm
content: Quote from: tom66 on June 09, 2025, 11:00:26 amMy EV has an AM radio but I don't recall using it. I'll give it a try in a few days when I'm home.

author: bdunham7
author_rank: Super Contributor
timestamp: June 09, 2025, 12:23:29 pm
content: Quote from: Nominal Animal on June 07, 2025, 05:09:22 pmIt is the extremely rapid discharge of the charge accumulated in the spark plug cable that corresponds with most of the radiated energy in those older systems. Resistor spark plug wires reduced the intensity of that noise by reducing the peak discharge current.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 09, 2025, 02:25:11 pm
content: There are no am transmissions where I am, ie Ireland and they are disappearing everywhere else in Europe.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 09, 2025, 03:20:31 pm
content: Quote from: bdunham7 on June 09, 2025, 12:23:29 pmAnd also damping the high frequency secondary oscillations.

author: jmelson
author_rank: Super Contributor
timestamp: June 09, 2025, 03:29:21 pm
content: Quote from: bdunham7 on June 09, 2025, 12:08:31 pmMy Honda Civic hybrid has an AM/FM radio that works VERY well. However, they did move the antenna to the back of the roof to get away from the drive electronics. I converted a VW beetle in about 1980 to electric, and was surprised the AM only radio still worked!Jon

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 09, 2025, 03:46:00 pm
content: I had an acquaintance that was much into tuning muscle cars.He insisted that the resistive spark plug wire would rob him of a couple of horsepower. So he replaced them with copper ones. He also installed a capacitive discharge ignition to have a “hotter spark”.His words, not mine.If I was listening to AM radio, I would know he was driving in the vicinity. Some moments later I would then hear his loud mufflers.

author: coppercone2
author_rank: Super Contributor
timestamp: June 14, 2025, 08:47:49 pm
content: IIRC that wire is current limiting 15kV to like 1kOhmthe peak current would be 15 amps. Does more really get you something?How long do the spark plugs last lolBridgewire engine detonatorgasoline is one of the most ignitable things ever, sensitive to tiny static sparks, requiring ESD protection

author: Nominal Animal
author_rank: Super Contributor
timestamp: June 17, 2025, 05:56:29 am
content: Snowmobiles, quads, et cetera tend to be much better nowadays, at least hereabouts.These days, I tend to mostly notice the poorly shielded audio stuff (cheap external computer speakers in particular) playing thetratta-tratta-trahwhenever a cellphone is looking for a base station nearby.(I do like my quiet. It does annoy me that in my quietude I can hear the USB HID event reports from mice/trackpads causing audible ticking. Some nice soft classical music is a good antidote, but I never got into a habit of collecting the good stuff, so I tend to be at the mercy of online/web radios for that, and they do repeat a lot of the same stuff.)

author: RoGeorge
author_rank: Super Contributor
timestamp: July 04, 2025, 12:35:03 pm
content: #002- Take an emitter follower, and add a capacitor between the emitter and ground. What do you have now?(ɐu ǝɯᴉʇʇǝɹ pǝʇǝɔʇoɹ' oɹ ∀W pǝɯopnlɐʇoɹ)

author: SteveThackery
author_rank: Super Contributor
timestamp: July 05, 2025, 11:09:37 am
content: Quote from: RoGeorge on July 04, 2025, 12:35:03 pmThat's clever - how did you get the upside-down text?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 05, 2025, 11:41:53 am
content: Search for 'upside down text generator', or alike terms (flip, mirror text), there are many online generators like that. They can flip, mirror, upside down or backward a normal text, or do combinations of those. Then, copy paste your transformed text from there, back into the forum. Random example:https://upsidedowntext.top/Should work with any webpage or program that supports Unicode text encoding, and Unicode is supported nowadays almost everywhere, certainly supported here, on EEVblog.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 05, 2025, 02:33:21 pm
content: For those who don't have Unicode support in their web browser, the obfuscated text reads "an emitter detector, or AM demodulator", like this:The additional capacitor Ce has an unexpected effect, it rectifies the output (the trace in red).In green is the input AM signal fed to both transistors. In yellow is the response of a normal emitter-follower BJT, and in red is what happens if an additional capacitor is added between emitter and GND (C5): The emitter-follower becomes an emitter-detector (AM demodulator).The BE junction of Q1 works as a detection diode. The advantage of this over a classical diode detector is that the BE diode is already forward polarized, so it can demodulate just as well AM signals much smaller than Vf (starting from zero), and can do that with less distortions than a diode detector (linear response for the emitter-detector, vs quadratic response for a simple diode detector).Another plus is that it can work up to very high frequencies. The following example is from a satellite-finder (a device used to orient a satellite dish for max signal amplitude), and Q1 is used as an amplitude detector for up to 2.3GHz. Q1 and Q2 are twoBFG590/X(NPN transistors 5GHz):Fromhttps://www.eevblog.com/forum/rf-microwave/schematic-for-parksidelidl-sattelite-finder/msg5201925/#msg5201925

author: Analog Kid
author_rank: Super Contributor
timestamp: July 06, 2025, 03:13:28 am
content: Quote from: RoGeorge on July 05, 2025, 02:33:21 pmNo, it doesn't rectify the signal; it just filters out the carrier.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 06, 2025, 09:57:27 am
content: That's not just a filter. Q2 really detects/extracts only the positive peaks, like a rectifying diode would do, except this Q2 "diode" opens from 0V (Vf is 0, not 0.6V like in a simple diode, because Q2 BE junction is already forward polarized).Here's how it works:- once the signal goes above zero, it is repeated to the emitter, and the capacitor in emittercharges(follows the raising voltage of the AM signal)- once the input signal starts to decrease, the BE of Q2 suddenly becomes reversed polarized. The transistor switches off, and it is no longer a voltage follower, it's a peak detector, and if it weren't for the resistor in the emitter to discharge the emitter capacitor, it will remember the max voltage.The transistor+C_emitter act together like a switch+filter, similar with how a diode+C acts as a switch+filter.That's a tricky one. I failed to see that at first, had to double check the schematic when I've first encounter the emitter-detector (in that hand drawn satellite finder schematic posted before).Even ADI failed to identify the rectification effect hidden in an emitter follower with an output to ground capacitor, and added a useless diode in the base of Q1 with a wrong explanation of how it works:Fromhttps://wiki.analog.com/university/courses/alm1k/circuits1/alm-cir-envelope-detector, "Figure 8. Biased Envelope Detector circuit" (at the end of the Analog Devices web page).It's not the diode D1 who does the envelope detection there, it's the Q1, because of C2.To prove the emitter-detector acts as a diode, and not as a filter, I've prepared an AM signal where the positive and the negative modulation envelopes are different (the trace in blue):On the right hand side of the schematic there are 3 circuits, all 3 fed with the same AM signal in which the positive and the negative modulation envelopes are different (the blue trace).There are 3 outputs:- poz-detector (the red trace, notice Q1 is NPN), to detect only the positive voltage, like a diode- neg-detector (the magenta trace, notice Q2 is PNP), to detected only the negative, like an inversed diode- low-pass filter (the grey trace), to remove only the carrier (no diode effect)Notice how by filtering out the carrier with a low-pass filter (R7+C7) the output grey trace Vlow-passis a mixture of both positive and negative input signals (grey trace is a mixture of the green and yellow traces).By contrast with that behaviour, the NPN Q1 can detect only the positive peaks of the AM carrier, like a diode. Similar, the PNP Q2 is also as a diode (mounted in reverse), detecting only the negative peaks of the AM carrier. No mixing of positive and negative sides, as a filter would do.

author: paul cotter
author_rank: Regular Contributor
timestamp: July 09, 2025, 09:53:40 am
content: For this to work the transistor would have to be biased on the "edge" of conductance but in that case the output, in the case where no emitter-ground cap is present , would not be a faithful reproduction of the input waveform as is shown on the scope shots. Or am I missing something?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 09, 2025, 11:43:11 am
content: The transistor is polarized normally, with 0.6V on its BE junction, and has the same CE current as a normal emitter follower would have. With or without the capacitor, the operation point bias remains the same. At rest, the emitter-follower and the emitter-detector are polarized the same.The difference between emitter-follower and emitter-detector becomes visible only when the input signal changes.When the signal increases, the capacitor C2 of the Q1 emitter-detector charges, too. Then, when the signal decreases, the Vbe of Q1 decreases from the "normal" 0.6V to zero. During the Vbe 0.6V to 0V decrease, the current through the transistor decreases exponentially with Vbe, and eventually the transistor is turned off. Then, eventually Vbe reverses becoming reverse polarized.Maybe I shouldn't have used the words "suddenly" in the "suddenlybecomes reversed polarized" a couple of posts ago, sorry. Not suddenly, but pretty fast when compared to how slow the envelope changes.The Q1+C2 acting as a detection diode comes from the fact that the internal CE resistance of Q1 (when the transistor is on) is much smaller than R3. Thus, Q1 can charge the C2 very fast, while the discharging rate of C2 is made at a much slower rate (dictated by the C2 R3 time constant). Q1+C2 makes a peak detector for the 1MHz carrier, then C2 is slowly discharged by R3. Same principle (fast charging vs slow discharging) can be applied to other than BJTs. For example it can work with a FET (source-follower), or with a tube (cathode-follower), or even with an open collector opamp.More generic, any device with a non-linear I-V transfer curve can act as an AM detector.Even in the normal diode, it's the non-linearity of its transfer function that does the detection.As for the linearity and distortions of the emitter-detector (distortions of the recovered AM envelope), the emitter-detector has less distortions (better linearity) than a normal detection diode. The response of emitter-detector at small signal is linear (small signal as in less than Vf diode), while the response of a normal diode-detector is quadratic (not linear, which translates into distortions in the AM detected signal).In practice, the distortions can go less than 1% for the emitter-detector, see an example in Fig.8 at page 26 of 100 of this pdf (page 24 of the document):https://www.worldradiohistory.com/Archive-DX/Ham%20Radio/80s/Ham-Radio-198004.pdf

author: paul cotter
author_rank: Regular Contributor
timestamp: July 09, 2025, 12:23:53 pm
content: I fully get what you are saying but we are talking at cross purposes. What I am querying is that if the transistor is cutting off at some point in the waveform then it is not operating under class A conditions and if it is not class A it cannot reproduce the original modulated RF- the case where no emitter cap is present. The detection scenario is fine by me, no problems. What I find hard to understand is the correct modulated RF signal on the emitter with no cap in place. I hope I am now being clear and thanks again for your input.

author: Njk
author_rank: Frequent Contributor
timestamp: July 09, 2025, 09:25:16 pm
content: Quote from: RoGeorge on July 06, 2025, 09:57:27 amI do remember something like that was published in Russian radio-amateur journals in 70-x and 80-x.Another one:https://fancydev.ru/pru/detam1.htm

author: RoGeorge
author_rank: Super Contributor
timestamp: July 10, 2025, 08:15:09 am
content: Interesting, thank you!Luckily, the original magazine/article is available athttps://archive.org/details/1991-04_202304/%D0%A0%D0%B0%D0%B4%D0%B8%D0%BE%201991-01%20%D1%8F%D0%BD%D0%B2%D0%B0%D1%80%D1%8C/page/53/mode/1upQuoteLow Distortion Amplitude Detector - page 53 from Russian magazine Radio, Jan 1991The translation was made with Google, and if I understood correctly, the transistor is not powered.Is that so? Then, how does it work?Another question, what is the translation for the magenta text, at the input and output of the circuit from Fig.1?

author: Njk
author_rank: Frequent Contributor
timestamp: July 10, 2025, 09:18:21 pm
content: Quote from: RoGeorge on July 10, 2025, 08:15:09 amYes, it's not powered from external source. I presume the power voltage is provided by C2 and is then stored in C3. During the negative half-period of the IF, the B-E junction of the transistor becomes forward-biased so the upper terminal of R1 becomes negative (related to GND) and C2 charges. During the positive half-wave, the B-E diode is backward-biased so the R1 voltage drops to zero. Which means the right terminal of C2 becomes positive.The left magenta reads IF input, the right magenta reads LF (i.e. demodulated signal) output

author: RoGeorge
author_rank: Super Contributor
timestamp: July 11, 2025, 03:06:45 pm
content: Did some LTspice simulations. Strange how most of the time the transistor is polarized with voltages of opposite sign than normal, only during the short current peaks that are charging the output capacitor it is properly polarized.The plots are for the last few carrier oscillations from the entire simulation (corresponding to the 455kHz IF, not the 1kHz audio envelope), to observe the transient behavior after all the voltages have reached their stable working regime.

author: Njk
author_rank: Frequent Contributor
timestamp: July 11, 2025, 07:29:44 pm
content: Quote from: RoGeorge on July 11, 2025, 03:06:45 pmA nice work, thanks. BTW, taking into account where's this circuit came from, the IF is supposed to be of 465 kHz, not 455. Though I don't think that'll make a noticeable difference. But why the magenta trace for V(c,b), actually for the voltage across C1 on your diagram, is a straight line?

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2025, 08:06:06 pm
content: Standard US frequency for AM IF stages is 455 kHz, which the FCC kept as a clear channel.Before that was standardized here, there was also use of 465 kHz.Did Russia standardize on 465 kHz?

author: Njk
author_rank: Frequent Contributor
timestamp: July 11, 2025, 09:05:17 pm
content: Quote from: TimFox on July 11, 2025, 08:06:06 pmIt was always so, to my memory

author: RoGeorge
author_rank: Super Contributor
timestamp: July 11, 2025, 09:07:56 pm
content: Quote from: Njk on July 11, 2025, 07:29:44 pmVcb wiggles very little in the rhythm of the carrier frequency (those were the last 10 microseconds from the first 15 milliseconds), the wiggling amplitude is not not visible at all because of the voltage scale.I've moved the Vc trace one panel up, and re-scaled the voltage axis in the 3rd panel, to make visible the variations in Vcb (first attachment in this post).When zooming out to view the entire 15ms (second attachment here), Vcb is wiggling more, in the rhythm of the 1kHz envelope (the modulating signal), then after about 10ms, Vcb settles to about -0.6V.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2025, 11:00:39 pm
content: Quote from: tom66 on June 09, 2025, 11:00:26 amWhen I was driving cross country a couple years ago in my 2002 pickup, I found that AM was pretty much useless everywhere because of high noise levels. At home here in New Hampshire, I can barely hear one AM station.

author: Njk
author_rank: Frequent Contributor
timestamp: July 12, 2025, 07:20:22 pm
content: Quote from: RoGeorge on July 11, 2025, 09:07:56 pmConcerning the Vcb, I think this is similar to what happens when you connected a capacitor to a DC PS output. The very purpose of the capacitor is to reduce the ripple, etc., to make the V(t) a straight line. Perhaps the capacitance may be reduced because in this case, the effect from that is not clear.As with every proposal, it's the producer's job to propose something new while the consumer's job is to reject it because the disadvantages overweight the benefits. Everything comes with a price. While the bright side is easy to notice (because it's advertised), sometimes, a lot of work is required to identify the dark side which is always there. In this case, while the circuit is simple, the analysis is not because as you'd correctly noted, it does not seem intuitive clear. It needs to be investigated how sensitive the key advantage (improved linearity) is to the component values, to the transistor type, to the variation of its parameters in the temperature range, the I/O impedance requirements, etc. And it's just a detector. Perhaps the designer having a lot of other work to do. The article is of 1991 year. A decade earlier, I did other receiver modding, an active detector, which was easier to understand, actually a double half-wave version of what you'd mentioned before. Already forgot about that, thanks for reminding.I have Blaupunkt Bremen CD72 car radio. A good source of heat but also, it's of rather unique design, not only because it's HF-capable (although only 49 m band). Most interesting, it integrates the Twinceiver(TM) feature. It uses two antennas and two tuners, which can be configured either for directional reception on FM or to search for new stations in the background. Antenna diversity. It's based on Motorola DCR3 chipset, which, in turn, is based on the 56300 DSP. Digital car radio. It was initially developed for Bosch/Blaupunkt and later was marketed to others. But it didn't gained traction on the market. Perhaps because the innovation was not perceived by the customers necessary.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 13, 2025, 10:04:01 pm
content: Quote from: Njk on July 12, 2025, 07:20:22 pmThe easiest to do would probably be to just add power to the transistor, and turn it into an emitter-detector. The following is a performance comparison between the self-powered BJT (out1), the BJT as passive diode (out2), the powered BJT as emitter-detector (out3), and a Germanium diode last (out4).The modulation factor is from 0.2 to 1.0 (changed in 5 steps). There is a ".fourier" in the simulation, to measure THD (total harmonic distortions at 1kHz, the THD can be found in the simulation log). The powered BJT used as emitter-detector, V(out3), has the minimum distortions:

author: RoGeorge
author_rank: Super Contributor
timestamp: July 19, 2025, 04:41:46 pm
content: #003- Take a Linux with KDE Plasma, press CTR+Alt+T and type:Code:qalc '1/( 2*pi* (1Gohm || 30Mohm) * (0.29 + 0.06 + 0.08)pF )'Or, start 'qalc' from the command line, and use it in interactive mode. Arrows up/down will retrieve history calculations.Note how 'qalc' recognizes the type of EE input units and prefixes, and the result is automatically converted to the proper physics unit, and written using engineering prefixes (here as a meaningful number of kHz).Plus, 'qalc' knows gimmicks like||for parallel resistors (or series capacitors), or direct units conversion, for exampleN/m^2returnsnewton/(meter²) = 1 Pa.Plus, in KDE 'qalc' can be called by touching the 'Meta' key (AKA the Windows key), then directly typing some calculation,=3.3V/2.2kohmand the result will be displayed live, updated as you type each character!

author: RoGeorge
author_rank: Super Contributor
timestamp: July 25, 2025, 01:17:35 pm
content: Tough crowd.Also has a GUI, can plot functions, or import and plot your data, do fancy transformations, and dunno', alchemy and stuff.Qalculate! - the ultimate desktop calculatorhttps://qalculate.github.io/https://qalculate.github.io/manual/

author: RoGeorge
author_rank: Super Contributor
timestamp: July 25, 2025, 01:24:12 pm
content: #004- Self powered transistor as AM detector, brought to attention byNjka few messages ago:https://www.eevblog.com/forum/chat/secrets-from-the-ee-scrolls/msg5972637/#msg5972637

author: RJSV
author_rank: Super Contributor
timestamp: July 26, 2025, 05:24:39 am
content: Like jpanhalt sez, there are 2 ways to use/interpret the word'Emerge' could have an image of an already developed snake, or a political philosophy or idea can emerge, unchanged.But, in addition to those examples, could also IMPLY that a transformation and subsequent emergence can happen. Like, for example when a beautiful butterfly emerges, changed now.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 28, 2025, 04:00:23 pm
content: #005- take an opamp, make with it a non-inverting voltage amplifier with amplification A=2, like this:A voltage source Vs placed at the input will see the amplifier as a very high impedance (in theory infinite). And, of course, at the output V(out) will be exactly double of V(in). All good.Now, add a resistor Rx between the output and the input.Still works, but what does the signal source sees?(Rx connected between out and in will see at its terminals a voltage Vout-Vin. Vout is always twice Vin, because A=2. So Rx will see twice Vin minus Vin, so it will see Vin. The current through Rx will flow from the highest voltage Vout, to the lowest voltage, Vin. Because the opamp's in+ does not take any current, all the current passing through Rx will sink into Vs, into the signal source. So instead of sourcing current, Vs will have to sink the current coming from Rx. Since Rx happens to have the same Vin voltage as the Vs generates, overall Vs will see the opamp circuit as a resistor of value (-Rx). Vs will see the opamp circuit asnegativeresistor).Now what if instead of Rx, you use a Cx, or an Lx?You are now into the non-Foster networks territory!Almost trivially simple so far, isn't it? Yes, but it is more to it. If interested, you may want to read the links posted bymawyattinhttps://www.eevblog.com/forum/rf-microwave/examples-of-varactor-diode-or-gyrator-based-antenna-tuners-atus/msg5989853/#msg5989853

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2025, 10:26:37 am
content: The very small text from the above post says the circuit will become a negative resistor:Now, a negative resistor is a funny thing to fool around with, because such a negative resistor can cancel a normal (positive) resistor.That's a way to get a perfect 0\$\Omega\$ short-circuit, if a +Rx and a -Rx are in series, or to get infinite resistance, if Rx and -Rx are in parallel.Notice how a +Rx will dissipate energy, while a -Rx will deliver energy. That delivered energy is coming from the power supply of the opamp, so no overunity yet. Also, a +Rx will produce heat, while a -Rx ... still produces heat.Another example, a -Rx of just the right size can be used to "cancel out" the energy loses in an LC tank, or any other resonator, so to make it oscillate forever. Well, in fact a -Rx doesn't stop the losses, but it delivers the same amount of extra energy as it was lost, such that the oscillations do not decay. The condition is -Rx to be equal with Q*X. For an LC tank, -Rx should be Q*sqrt(L/C).Luckily, LTspice allows to set a negative value to a resistor, so one can draw and simulate an oscillator apparently without any power supply.Notice how the value of the -Rx has to be just the right size. If the -Rx adds too little or too much energy (compared with what is lost at each oscillation), then the amplitude either decays or goes to infinite.The next simulation runs a longer simulation, in order to observe the trend of the oscillations, decay or grow, for 5 different values of -Rx, a different color for each -Rx value tried.The voltage source there is only to indicate the colors for each -Rx value. Notice how for -1.950k\$\Omega\$ (green), the oscillations Vout (shown in the lower panel) are keep growing, while for -1.970k\$\Omega\$ (red), the oscillations clearly decay.Similar with the examples about a -R, a lot of other tricks can be done with a -L, or -C, or with a combination of these. That's what the non-Foster networks theory is about. Such a mouthful name.Should've been called "Tricks with Negative Components".

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 05:12:57 pm
content: Note that negative resistance circuits always require external power.Truly passive circuits always have positive resistance components.

author: paul cotter
author_rank: Regular Contributor
timestamp: July 30, 2025, 05:36:56 pm
content: That is seriously dangerous talk, ie negative resistance/resistors. If the free energy/overunity crowd hear of this the forum will be swamped with bullshit.

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 08:41:04 pm
content: Negative resistance dates back to Poulsen arcs and dynatron oscillators a century ago, both of which consumed power from an external source.No free energy was produced and physics survived.The dynatron (tetrode) was replaced by the transitron (q.v.) after the invention of the pentode.The transitron does not depend on the uncertain magnitude of secondary electron emission from the plate.The tunnel diode was the wave of the future when I was a Boy Scout.

author: paul cotter
author_rank: Regular Contributor
timestamp: July 30, 2025, 08:54:47 pm
content: Yep, I know all that. The free energy/overunity believers are of the opinion that Gabriel Kron actually developed a real world negative resistor which was subsequently suppressed. Bullshit of course as Kron's focus was network analysis and not magical energy sources. I have tried arguing with some of them, to no avail. PS: correction, the transitron is not familiar offhand.

author: TimFox
author_rank: Super Contributor
timestamp: July 30, 2025, 08:56:52 pm
content: Yeah, too many people confuse the ratioV/Iwith the derivative dV/dI, both of which are measured in Ohms.The former may not be constant, and the latter may have regions on a curve where the derivative is negative.I have played with transitron operation of 6AS6 pentodes, without destroying the universe.

author: paul cotter
author_rank: Regular Contributor
timestamp: July 30, 2025, 09:00:54 pm
content: For sure, a simple unijunction will give a transitory negative resistance. Living on this side of the pond it is past my bedtime so I say goodnight, Tim.

author: tggzzz
author_rank: Super Contributor
timestamp: July 30, 2025, 09:58:30 pm
content: Quote from: TimFox on July 30, 2025, 08:56:52 pmThe VI curve of a battery is useful for forcing such people to confront their misapprehensions. Get them to state whether the resistance is positive, zero, negative or infinite.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 03, 2025, 08:46:10 am
content: Quote from: paul cotter on July 30, 2025, 09:00:54 pmSame will do a BJT with swapped CE:https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4422982/#msg4422982and that negative resistance region can be used to make an oscillator, by adding a parallel capacitor:(pics not mine, stolen frommawyatt's topichttps://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/)\[ \star \ \star \ \star \]Now, about using a non-inverting voltage amplifier + a resistor in order to make a negative resistor, thought that maybe an application might be fun. From that simulation made before:it can be seen how the circuit oscillates only when the negative resistance is small enough (such that the negative resistance can compensate the losses inside the LC tank, or else said -Rx cancels the Rq of the LC tank). So, if instead the Rx we use a potentiometer, we can rotate the potentiometer until the oscillations start (or stop). Then we can disconnect the potentiometer, and measure its resistance with an ohm-meter.That resistance will tell the losses resistance in the LC tank. From this, the quality factor can be deduced, Q = Rx/X(where X is the reactance at the resonant frequency. Resonant frequency f can be found with a frequency-meter, then by knowing C, the reactance X can be calculated. Knowing X together with the measured Rx, the quality factor Q of the LC tank can be calculated)Of course, there are other ways to measure Q, and those might be more practical than the one proposed here.Main problem I can see would be a potentiometer used at RF frequency (which potentiometer is usually a large mechanical component not meant for high frequencies). But there might be a way to not use a potentiometer at all.The next, might be a way to get rid of the potentiometer.In the #005 example, the amplifier was chosen to have A = +2, but this was only to ease the understanding (to make Rx to see the exactly same Vin value, but with a reversed sign).But instead of keeping A fixed at 2 and varying Rx, it should be possible to use a fixed Rx and vary the amplification A (in order to get different values of -Rx from a fixed Rx).The immediate way to vary A would be to replace R1/R2 with a potentiometer, but those are also exposed to RF oscillations, so that's not the way of doing it. However, we can give up entirely to the idea of using an opamp (any voltage amplifier will work just as well to obtain negative resistance), and use an analog multiplier in order to get a VCA (Voltage Controlled Amplifier).With a VCA and a fixed Rx, the resulting value of the -Rx can be changed by changing the amplification of the VCA, so the value of -Rx can be controlled with a DC voltage. A microcontroller can sweep that DC voltage, find the point between oscillating/non oscillating, then deduce the Q of the LC tank.I didn't try to build such a Q-meter, but it looks like it should work.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 03, 2025, 10:45:50 am
content: Drafted a block schematic for the above Q-meter idea:B1, B2, B3 have no contribution to the circuit. The B1, B2, and B3 behavioral voltage sources are there only to be able to plot on the chart at the left the calculated values of k, Rp and f.- E1 plays the role of a VCA (the Voltage Controlled Amplifier can be, for example, an analog multiplier)- k is the variable gain (supposed to be swiped in value by sweeping a DC voltage controlled by a microcontroller)- Rq is the series resistance of L, Rp the equivalent parallel resistance of L- C is considered known and ideal- Rx here is fixed and known, the negative resistance seen by the LC tank is varied by changing the gain kA microcontroller (MCU not shown in this draft schematic) is supposed to produce an adjustable DC, and that DC controls the gain k of the VCA. Overall, by changing the DC, the value of the resulting negative resistor is changed. So, a voltage controlled negative resistor.The MCU is supposed to browse the DC while checking at the output of E1 if there are oscillations or not. The MCU is supposed to measure, too, the frequency f of the oscillating LC tank.The DC value for which the oscillations become sustained will be when the negative resistance cancels Rp. Starting from that DC value, together with the measured value of f, and the known (fixed) C and Rx, the values for L and Q can be calculated.