Diode envelope detection is a real thing, and it works! - Page 1
Forum section: electronics / general

author: Analog Kid
author_rank: Super Contributor
timestamp: May 29, 2025, 08:41:58 pm
content: [This thread is an offshoot of a divergent discussion that arose in another thread here in the "Dodgy Technology" sub-forum]Regarding all the brouhaha and indignation over the matter of whether an AM demodulator using a diode does so through the classic method we've all been exposed to--half-wave rectification followed by filtering to remove the carrier--or whether there's something more mysterious going on, having to do with diode power-law behavior and other arcane mechanisms--I offer this explanation from my textbook,Electronic Principlesby Albert Paul Malvino (1979):All I'm going to say at this point is that this explanation suffices for me; I believe it. I really don't see the objections to it: what about the operation described here does anyone not believe? It seems obvious enough that one need not look for quantum or subatomic effects to explain how an AM radio works.

author: TimFox
author_rank: Super Contributor
timestamp: May 29, 2025, 09:05:32 pm
content: That describes a normal "peak envelope detector" nicely.Note that there will be distortion at negative modulation, where the envelope is a low voltage compared with diode voltages.For such envelope levels, the diode will operate in the square-law region, rather than as a switch.Malvino discusses the important "failure to follow" distortion with improperly-chosen time constant in the rectifier capacitance.Diode detectors with high-level inputs can also be operated as "average envelope detector", where the rectified signal (only positive input sensed) is averaged (rather than peak-detected).It's maybe easier to implement the latter with a BJT, rather than a diode, where the rectification is done in the BE diode of a grounded-base transistor.A detailed explanation of AM detectors is in Chapter 10 of K Clarke & D HessCommunication Circuits: Analysis and Design, Addison Wesley (1971), reprinted Krieger (1994).I can recommend that book highly to anyone who wants non-handwaving mathematically-correct description of such topics.A discussion on pp 499-500 discusses square-law operation with an input signal below approximately 50 mV (either Ge or Si diode).The same circuit at such low signal levels will produce square-law results, producing distortion of the original audio.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 29, 2025, 09:15:48 pm
content: Quote from: TimFox on May 29, 2025, 09:05:32 pm"Negative modulation"? Never hoid of that.All I could find online in a quick search was this definition from Merriam-Webster:QuoteIs that what you meant? That sounds like only an inversion of the meanings of min/max signals.I though modulation was always limited to the range 0-100%.

author: TimFox
author_rank: Super Contributor
timestamp: May 29, 2025, 09:25:31 pm
content: Quote from: Analog Kid on May 29, 2025, 09:15:48 pmAmplitude modulation:With audio input at zero, the output is a carrier, say 10 V peak amplitude.The modulating signal increases and decreases that high-frequency level by multiplication by [1 + (audio/max audio)], but the output magnitude can never go negative.The maximum positive modulation (increase with positive audio voltage) is not limited by mathematics, but regulations frown on >100% for normal audio transmission.The maximum negative modulation (decrease for negative audio voltage) is limited by zero at -100% modulation.Positive overmodulation has no important problem, but negative overmodulation gives horrible distortion as the output stays at zero for part of the audio cycle.AM is somewhat wasteful of transmitter power, but allows a very simple detector in each of many receivers.At our college AM station, we monitored modulation by a simple CRT with arbitrary horizontal axis and modulated RF fed to the vertical deflection.With no audio, there was a band on the screen; increasing the audio level resulted in a series of "dots" along the axis when overmodulated.For "DSB", double-sideband suppressed carrier, the multiplier omits the "1", so negative modulation inverts the instantaneous amplitude.It requires a very different detector to recover the audio. Efficiency (especially for voice) is improved, since there is no RF output during silent periods of the audio."SSB", which deletes one of the two sidebands before amplification, uses less bandwidth for a given audio bandwidth, and is popular for communication, but requires yet another complicated detector.

author: Xena E
author_rank: Frequent Contributor
timestamp: May 29, 2025, 10:16:12 pm
content: Quote from: Analog Kid on May 29, 2025, 08:41:58 pmThe text book you have is a good one, the controversy was that the original assertions were made from the back of comments about low level signals being detected directly from the tank circuit of a crystal set, those signals most often are too low an amplitude to be envelope detected and are therefore detected in the region of the diode square law not from rectification.Unfortunately the detractors then pounce as the envelope detection method where a simple diode is used as a rectifier is put forwards as the only functional method because it was perhaps as far as their education has taken them.Both models are correct but have to be applied to the relevant task, it's not about taking sides.If you have a traditional AM superhet radio there will be a mixer, converting the, say, 500 to 1700kHz signals from the broadcast band to a fixed inermediate frequency, that will be amplified through a couple of stages then when at a level of perhaps a few volts passed through the detector circuit as in the Malvino book, that as Tim states quite eloquently is peak envelope detection and thats an end to it... where the people in the other thread got their panties in a bunch is because they were also trying to assert the idea that envelope detection would apply in all cases including to the TS mention of passive radio in the form of a "crystal" set receiver, which unless the incoming signal is particularly large, it can't.Square law detection is a deal more complex than purely envelope detection, but both modes work in the same circuits, and are only proportioned or selected due to the level of the applied signals. Its not a war between two schools of thought as a lot of people and internet pundits try to make out.Envelope mode has the least distortion.Tim has the handle on this and I won't go any further over what he has taken the trouble to explain.RegardsX

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 29, 2025, 11:18:56 pm
content: Also note that the old AM receivers with very simple front-ends usually used germanium diodes for this purpose, which have a much lower threshold than silicon diodes.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 30, 2025, 01:53:33 am
content: Let's consider a crystal set that instead of the ancient point-contact diode uses a modren 1N34 germanium diode, as many did back in the day (and they worked).The 1N34 has a Vf of 1 volt. Obviously it's going to be operating far below that, as there's no way any amount of antenna is going to yield that much voltage (unless one is right next to a radio transmitter). And yet it works.But does that mean it's operating in "square-law" territory and not still functioning as a simple rectifier? I'm not convinced that that's the case.It could be that it is, in fact, functioning as a rectifier but as a highly imperfect one, basically lopping off the peaks of the signal and generating lots of distortion in the process.Can someone explain what actually happens in a device like a 1N34 when the signal going through it is far below its Vf?

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2025, 02:57:44 am
content: AboutVf= 1 V for the Ge diode 1N34: the data sheet shows a max value of 1 V for a large current. That isolated max spec is misleading.If we approximate the turn-on for a Si diode to be 0.7 V (as I keep saying, not a hard threshold), then the same condition for Ge is 0.3 VThis poster has a host of measured current vs. voltage curves for different diodes:https://www.reddit.com/r/diypedals/comments/hi0ak1/about_diode_vi_curves_series_resistance_and/Some of his curves are for a diode with series resistor: read the legend carefully. Guitar pedal effects guys like to use germanium.The book (Clarke and Hess) that I cited has a detailed description of the diode detector from low to high input levels: below 50 mV, the diode detects, but with a square-law response.With a good crystal set, the input resonant circuit can boost the voltage into the diode.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 30, 2025, 03:01:24 am
content: Aaaarrgh; sorry for the brain fart. Of course I know that typical Vf for Si is 0.7V, and that for Ge is 0.3V.My bad for just unthinkingly copying datasheet values ...Quote from: TimFox on May 30, 2025, 02:57:44 amBookmarked.

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2025, 03:25:42 am
content: Have a play with a LTspice sim of a simple biased 1N34A diode detector driven by a low impedance mV amplitude 1MHz modulated carrier.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 30, 2025, 03:43:28 am
content: Quote from: Ian.M on May 30, 2025, 03:25:42 amSo tell me, what am I looking at here? I run this and I see a whole bunch of horizontal traces at different voltages, from 0 to 1V. What means all this?

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2025, 07:40:07 am
content: That's because you are probing in the wrong place, and the varying {bias} DC offset +LTspice plot autoranging is swamping the signal. Points worth probing are 'mod' (the signal before AM modulation), 'AM' (the AM modulated feed to the detector) and 'det', the detected signal after low pass filtering, and with residual DC stripped by the AC coupling. It may also be worth adding another plot pane and examining voltage across and current through the diode. See attached images.The multiple colored traces V(det) show the effect of {bias} on the detected signal, However they are tiny (max. 25uV peak-to-peak), so due to the varying DC offset very difficult to see on the same plot if you probe before the AC coupling capacitor. Select a single step if you want to probe elsewhere, and as the autoranging will otherwise ***** you over, only plot a single node per plot pane.

author: iMo
author_rank: Super Contributor
timestamp: May 30, 2025, 09:14:14 am
content: .. also mind the tube based circuits were higher voltage and higher impedancies compared to today's transistorized one..So the detector's voltages were much higher and impact of the diode's Vf much lower therefore..

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2025, 10:40:28 am
content: Yep. In the sim, try increasing V1 for 120mV offset and 100mV peak to peak, and increase the 10K resistors to 100K, decreasing the 1nF capacitors to 100pF to keep the same time constant, and you will see that at 100x the previous signal level, and 10x the impedance, its far far less sensitive to bias, and far less lossy.

author: madires
author_rank: Super Contributor
timestamp: May 30, 2025, 11:01:45 am
content: Quote from: TimFox on May 30, 2025, 02:57:44 amVf of a Germanium pn junction can be quite low for low currents. The OSHW Transistortester checks diodes with two different series resistors (680 and 470k). The 470k resistor limits current to about 10 µA and at that current Vf (or V_be for a BJT) can be as low as 30 mV.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 30, 2025, 04:01:37 pm
content: As Tim said, the max Vfd given in the datasheet of the 1N34 is at best misleading. Unfortunately, the datasheets we can usually find lack a lot of useful information. You're supposed to know how a germanium diode really works. In practice, its Vfd is about half that of a silicon diode for a current of a few mA.And yes, the actual Vfd drops for lower currents. That's true for silicon diodes as well.A 1N34 typically has a Vfd ~ 0.3 V at a few mA and down to ~ 0.1 V or lower at a few µA. And it doesn't cut off abruptly either. There is a "knee", so it becomes non-linear as voltage goes down but not in a symmetric way, from which you get a "rectifying" effect. Far from perfect, but enough to demodulate a AM signal and get something audible. Not Hi-Fi.

author: Ian.M
author_rank: Super Contributor
timestamp: May 30, 2025, 04:21:59 pm
content: This is illustrated by the Vf curves from the SPICE model I used (from LTwiki). Of course in real life its more complex and less predictable, but it at least gives you some idea and shows why biassing in the knee region (of the linear plot) helps.

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2025, 04:50:22 pm
content: An interesting variation on diode envelope detection (for relatively high signal voltages):The usual vacuum-tube detector circuit in AM receivers used a single cathode with three elements: two diodes and one triode, with a common cathode.Examples: 6Q7, 6SQ7, 6AV6, and 12 volt heater variations.Since the cathode of the triode audio amplifier was usually biased a few volts positive (with bypass capacitor), one diode could be used as a normal AM detector, feeding the triode's grid, while the other could use the cathode bias so that the AGC voltage did not become negative (to reduce gain of the RF and IF stages) until the signal level exceeded a threshold.This was called "delayed AGC", although the delay is not a time delay.This article describes variations on this circuit:https://schematicsforfree.com/files/Vintage%20Electronics/Theory%2C%20Reference%20and%20Education/Vintage%20Radio%20-%20Automatic%20Gain%20Control%20-%20whai%20is%20it%20and%20why%20is%20it%20necessary%20Part%202%20SC%202010-06.pdf

author: EPAIII
author_rank: Super Contributor
timestamp: June 01, 2025, 12:29:03 am
content: OK, lets have some real fun.FM! Yes FM, which was and still is touted as so much superior to AM. There are FM receivers that, after clipping to ensure a constant level, convert that FM signal to to AM with what is effectively a filter (uses the slope of the envelope) and then an AM detector.I am sure there are other methods of FM detection, but I am also sure that somewhere in the circuit of those detectors there is a good old diode or something functioning as a diode.AM, FM, and probably other forms of modulation, I don't think you can get away from some kind of diode.

author: Ian.M
author_rank: Super Contributor
timestamp: June 01, 2025, 12:39:17 am
content: https://en.wikipedia.org/wiki/Foster%E2%80%93Seeley_discriminator

author: TimFox
author_rank: Super Contributor
timestamp: June 01, 2025, 02:44:43 am
content: Yes, the Foster-Seeley discriminator and the ratio detector use clever tuned circuits to convert FM to AM, and then diode detectors demodulate the resulting AM.A clever discriminator (invented by McIntosh) uses two 1/8-wavelength coaxes, one open and the other shorted, instead of the LC tuned circuits.Another method, sometimes called pulse counting, generates one pulse per period of the FM waveform, and then averages the pulses to extract the frequency and thus the change in frequency.If a phase-locked loop uses a linear voltage-to-frequency converter inside the loop, the VCO control voltage (under locked conditions) represents the frequency.

author: fourfathom
author_rank: Super Contributor
timestamp: June 01, 2025, 03:39:00 am
content: And there are still other non-diode AM demodulators. Direct-conversion and homodyne receivers use mixers or analog multipliers to convert AM to baseband audio (or other modulating signal). SDR receivers do much the same, just using DSP instead of analog circuitry.

author: tggzzz
author_rank: Super Contributor
timestamp: June 01, 2025, 10:50:51 am
content: Yes, diode envelope detection is a real thing, and it does work.Here are a couple of products from reputable manufacturers...An HP oscilloscope probe:http://hparchive.com/Manuals/HP-11096A-Manual-HF-Probe.pdfA Genrad voltage monitor in a 50ohm coax system:https://www.grwiki.org/grwiki/images/6/67/GR-0874-0213-D_1963-11_874-VR-Manual.pdf

author: paul cotter
author_rank: Regular Contributor
timestamp: June 03, 2025, 04:08:12 pm
content: Ssb is easily demodulated with a diode and a beat frequency oscillator(bfo).

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 04:18:34 pm
content: SSB is easier to demodulate than DSB, since the distortion with an incoherent BFO in the detector is acceptable, but tuning the carrier oscillator for DSB is critical (hence the need for a pilot tone).

author: Xena E
author_rank: Frequent Contributor
timestamp: June 03, 2025, 05:46:10 pm
content: Quote from: TimFox on June 03, 2025, 04:18:34 pmIs DSB (suppressed carrier ?) with a pilot, Similar to using a sample of the carrier from full AM for homodyne reception?X

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 05:50:53 pm
content: I believe so: if too much carrier is inserted during homodyne reception, it only increases the DC bias of the recovered output.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 03, 2025, 05:54:36 pm
content: Yep, I would think dsb could be difficult. I never even thought about it prior to today, does anyone actually use dsb?

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 06:03:23 pm
content: Quote from: paul cotter on June 03, 2025, 05:54:36 pmSee the literature about analog FM multiplex stereo broadcast standards: the "L-R" channel is DSB as a subchannel of the broadcast signal.There is a lower-level 19 kHz pilot tone included to allow regeneration of the 38 kHz suppressed carrier.From 1961:https://www.worldradiohistory.com/Archive-Modern-Era-Miscellaneous/FM-Stereo-Multiplex-NRI-1961-09.pdfInstead of the product detector (analog multiplier) shown in that older article, the demodulation of the stereo is normally done now with switching at 38 kHz rate.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 03, 2025, 06:38:32 pm
content: I know about the stereo fm system, what I was wondering about does a simple dsb transmission find any application like ssb does- I can't see any advantages over ssb and only disadvantages.

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 06:53:52 pm
content: I don't know of any commercial use, but I have seen articles about ham use of DSB, where the bandwidth of the receiver rejects one of the two sidebands before the product detector and BFO.DSB is much simpler to generate at the transmitter than proper SSB.A more complicated question, for which I refer you to the literature, is "vestigial sideband" modulation, common for video transmission.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 03, 2025, 07:11:40 pm
content: Yes, I know about the vestigial video system too. Dsb would be easier to generate without the necessary filtering or the phasing system for ssb. Receiving it might be where the difficulties would lie. In my teens I was all up for ham radio but I became disillusioned with the increasing use of commercial equipment and by the time I had become reasonably proficient at morse I gave up and never went for my license. I wanted everything to be home brew and I guess I got more fun from building receivers and transmitters than I would have ever got from using them.

author: TimFox
author_rank: Super Contributor
timestamp: June 03, 2025, 08:43:22 pm
content: I believe the original AT&T "long lines", for multiple channels on one long-distance pair, were DSB with the suppressed carriers integer harmonics of a pilot tone transmitted along with the modulated signals.This was later replaced by twice as many channels using SSB.https://en.wikipedia.org/wiki/12-channel_carrier_system

author: Analog Kid
author_rank: Super Contributor
timestamp: June 05, 2025, 05:38:00 pm
content: I notice this discussion is still going on over in that other thread in Dodgy Technology, the one about Chet's over-unity fantasy.What bugs me about that (the ongoing discussion) is that people are making distinctions without a difference.OK, so for extremely low-level RF signals, the detection becomes square-law rather than (more or less) linear.So what?It's still detection via rectification. The diode is stilll functioning as a rectifier. It's just in a different operational, um, domain for lack of a better term. So why all the controversy? It's not as if the diode suddenly starts exhibiting weird quantum-level behavior when the signal falls below a certain level ...

author: paul cotter
author_rank: Regular Contributor
timestamp: June 05, 2025, 05:51:41 pm
content: That would be my view too. The transfer function could be square, cubic or any power of the input but it is still rectifying, albeit poorly. The deviation from linearity is what is responsible for the distortion. Xena will probably put out a contract on me for saying this!!

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2025, 06:15:36 pm
content: Again, rectification is a result of even-order non-linearity (which includes square law)Odd-order distortion will not result in a shift of DC level.This can be worked out from algebra of polynomials.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 05, 2025, 06:44:18 pm
content: Quote from: TimFox on June 05, 2025, 06:15:36 pmSo for the somewhat math-challenged here, a couple questions:Even- and odd-order: does that refer to harmonics of the signal? how does that affect rectification?And how is rectification (which is removal of half of the signal wave, negative or positive) a result of even-order non-linearity?And what's this about DC level: doesn't rectification result in a DC level of zero?

author: paul cotter
author_rank: Regular Contributor
timestamp: June 05, 2025, 07:07:15 pm
content: I stand corrected having spoken too soon. Okay, any even order of transfer function will work, though with increased distortion at higher orders. Analog Kid, Tim is referring to the transfer function of the diode, whether the output is linear mapping of the input, or a square of the input, or any other multiple.

author: Xena E
author_rank: Frequent Contributor
timestamp: June 05, 2025, 07:10:19 pm
content: Quote from: paul cotter on June 05, 2025, 05:51:41 pmWell I'm doing some tinkering at the moment... its looking as if square law with one of my Russian Ge detector diodes falls over completely at around 10mV P/P... thats pretty good for a component with a Vf of around 0.3Volt at 1mA.Mind, it doesn't have a transfer knee, its more of a transfer bosom 😁If you want to think of it as an increase of resistance as the current falls along the transfer curve, that model could work for you if you don't want to think about the full mathematical approach.I don't think there's going to be any contracts put out on anyone, Paul, but I could murder an Irish stout ATM

author: tggzzz
author_rank: Super Contributor
timestamp: June 05, 2025, 07:19:36 pm
content: Quote from: Analog Kid on June 05, 2025, 06:44:18 pmHave a look at third order distortion of a sine wavehttps://www.wolframalpha.com/input?i=simplify+sin%28x%29*sin%28x%29*sin%28x%29Vs second order distortionhttps://www.wolframalpha.com/input?i=simplify+sin%28x%29*sin%28x%29/In particular the "Reduced trigonometric form" and graphs thereof

author: paul cotter
author_rank: Regular Contributor
timestamp: June 05, 2025, 08:18:35 pm
content: Hi Xena, as far as I can see the only powerful am transmission left in the UK is the BBC on 198khz(500kw from Droitwich) and even this is on life support and could cease soon. The alleged reason is the output devices being close to being unavailable- I find this to be a bit suspect as I used to get valves(tubes) rebuilt when emission began to fail and I'm sure there must be some rebuilders still going, though not as many as previously. 198khz will require a lot of turns on your coil. Hurry up while there is still a signal! PS I could do with a pint myself.

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2025, 08:32:51 pm
content: Quote from: Analog Kid on June 05, 2025, 06:44:18 pmOne way to state a non-linear function is to use a polynomial. Even and odd order refers to the exponent in each term of the polynomial.A linear functiony(x) =mx+b, wherexis the input,yis the output, andm(slope) andb(offset) are constants.A polynomial isy=a0+a1x1+a2x2+a3x3+ .... , which is non-linear.Now, the termbin the linear expression is a constant DC offset in the output, unaffected by the input signal.Looking only at the first two non-linear terms (quadratic and cubic) in the polynomial for a sinusoidal input:The quadratic terma2x2is positive-definite (assuming the constant is positive), and therefore gives a positive DC shift of the output that depends on the input amplitude.The cubic terma3x3, however, gives a positive output for positive input, and negative output for negative input, and does not give a DC shift, as discussed above by tggzzz above.Now, for the perfect rectifier, since the output is always positive, the DC level (average level) of the output must be positive, as can be seen in your drawing, where the output is always above the zero level.

author: Xena E
author_rank: Frequent Contributor
timestamp: June 05, 2025, 09:39:25 pm
content: Quote from: paul cotter on June 05, 2025, 08:18:35 pm'I'm having to feed this experiment with a signal myself, but It is picking something up as it is, so I'll have to lay it so the plane of the coil is flat for the low level stuff.It would need a longwire, and earth to get enough signal off air for headphones. Im not arranging that myself at the moment as I dont want to "bust my stitches."if I was going to pick one station, surprisingly the little 4kW radio Caroline TX at Orfordness just across one county from here is the best at the moment.I had one of the student group make the main coil for me, I gave her the dimensions and the inductance (400μH) as I wanted it to tune around the bottom of the MW broadcast band with 220pF cap, (its so it can have a variable cap if needs be)... it actually tunes around 560kHz, but thats OK.It just looks a dogs breakfast!I'd need nearly 3 mH for the Droitwich transmitter with a 220pF cap, that's alot of wire, I may try it but will stick a loading coil in series with this one.And use a bigger capacitor.The Droitwich TX site is being looked after by a private company, Arquiva and not the BBC technical section any more.Its expensive to power and run, so sad as it is, it doesn't make any sense to carry on supporting it. Theres very few people even listen to analog radio anymore.Those stories about the valves is an old urban myth and yes there's no doubt the valves were rebuilt when needed, theyre certainly just a bit bigger than a pair of 6BW6s.X

author: TimFox
author_rank: Super Contributor
timestamp: June 05, 2025, 11:03:23 pm
content: AM broadcast is still popular in the US, mainly for car radios.There are 50 kW "clear-channel" stations in many cities, which are not literally clear:http://www.csgnetwork.com/amclearradiotable.htmlI don't believe any US AM stations are licensed for more than 50 kW, but there were stronger Mexican stations across the border that now are apparently long gone.Efforts to have only FM reception in new cars have run into opposition, due to the need for emergency and weather reception in wide open spaces between cities in, for example, Montana.As far as I know, the Federal bill has not yet been passed:https://www.congress.gov/bill/119th-congress/senate-bill/315Of course, present-day power electronics, as used in EVs, pollutes the aether with electromagnetic interference in the 1 MHz region, and therefore many EVs don't have AM.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 05, 2025, 11:34:57 pm
content: Quote from: TimFox on June 05, 2025, 11:03:23 pmWhen I lived in Flagstaff, Arizona, at night I would tune in to KNX (1070) in Los Angeles, listed in that table as a "clear channel" station. (I was able to do so because of "skip".) Sometimes it would fade in and out, other times it came through strongly and steadily. Distance ≈ 390 mi.

author: Xena E
author_rank: Frequent Contributor
timestamp: June 06, 2025, 12:13:27 am
content: IIRC My experience of the use of AM in the US is for sport and talkradio, community, and religious, (in the southern states anyhow).That seemed to be the way it was going in Europe, specifically music broadcasting stations were mostly dead on AM long ago, not helped by the low B/W, it did seem it would struggle on in hill areas where FM and digital were impractical to service, but that is shifting to internet for domestic radio.There has been talk of the remaining AM music stations being allowed to expand the bandwidths, but with spiralling costs the broadcasters are not seeming too keen to increase their outgoings for no advantage.Commercial Broadcasters are even now moving away from FM .Things are even further along on the continental Europe.I've just tuned the MW BCB on a sensitive analog radio and there are only a total of 8 useable signals at my location. I'm listening to Radio Caroline on 648kHz at the moment, (a music album station ironically), there's a request program on and the people messaging are mostly from the US, and at distance within Europe so obviously the larger proportion their listeners are using internet mostly now.X

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2025, 01:35:46 am
content: Quote from: TimFox on June 03, 2025, 04:18:34 pmIt was not very practical in the past when DSBSC might have been desirable, but today a Costas loop could be used for demodulation without a pilot tone. It also works for SSB.I think a pilot tone would still be desirable for analog audio to prevent the loop from unlocking during silence. I have seen SSB modulators, and old DSB designs, where the modulator was deliberately unbalanced slightly to provide a low level carrier aiding in carrier recovery. I wonder why this was not preferred over a pilot tone. DC balance issues?

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2025, 01:39:01 am
content: Quote from: Xena E on June 06, 2025, 12:13:27 amIn the US, AM was finally used where fidelity did not matter and because the FM broadcast band was completely saturated with stations.

author: themadhippy
author_rank: Super Contributor
timestamp: June 06, 2025, 01:54:14 am
content: QuoteAny reason for not winding the coil on ferrite rod?

author: fourfathom
author_rank: Super Contributor
timestamp: June 06, 2025, 03:38:20 am
content: Quote from: David Hess on June 06, 2025, 01:39:01 amIn the USA, AM broadcast became popular starting in the 1920's. FM broadcast as we know it (88-108 MHz) didn't take off until the 1950's.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 06, 2025, 04:22:35 am
content: Quote from: fourfathom on June 06, 2025, 03:38:20 amWhen I lived in the Bay Area I listened to KPFA, the Pacifica public radio station in Berkeley. Radical as all hell. And it wasn't in the public radio "ghetto" down at 88-89 MHz; it was in the middle of the fat commercial part of the band at 94.1. (Which made the station a bone of contention in the late 1990s when some of the Pacifica board had their greedy eyes set on selling that slot which had become quite valuable.)It was actually the first ever public radio station in the US and among the first FM stations. In fact, when the station opened in 1949, they actually had to provide their listener-subscribers with FM radios, the technology was that new.

author: Xena E
author_rank: Frequent Contributor
timestamp: June 06, 2025, 06:07:21 am
content: Quote from: David Hess on June 06, 2025, 01:39:01 amThe UK historically is a skewed example as only the BBC was allowed to (legally) broadcast. VHF FM was late (1955) and restricted compared to the US because of that, only three of the national stations were broadcast on a restricted section of band II, the situation didn't change until the 1970s.Germany developed FM early because it had been limited by AM frequency allocation.X

author: Xena E
author_rank: Frequent Contributor
timestamp: June 06, 2025, 06:30:47 am
content: Quote from: themadhippy on June 06, 2025, 01:54:14 amA few reasons, main one being not having the ferrite rod to hand.I asked for an air core coil of 400μH and approximate dimension on a cardboard former to be made. I left it to the student to research... that planar coil was not what I'd envisioned, but was what she sent, and fitted the request!As an eventual finished demonstrator for the students I've ordered some rods and a mechanical variable capacitor.X

author: RoGeorge
author_rank: Super Contributor
timestamp: June 06, 2025, 07:36:10 am
content: For LW without ferrite, spiderweb coils are possible, not very much wire:https://www.eevblog.com/forum/rf-microwave/antenna-winding-questions/msg5929490/#msg5929490

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2025, 07:47:57 am
content: Quote from: fourfathom on June 06, 2025, 03:38:20 amI am referring to why AM is predominantly "sport and talkradio, community, and religious" now (finally) in the US, hence the quote I included for context.In the 1980s and 1990s, I used to hear music stations on AM, but they were rare even then compared to sports, talk-radio, community, and religious stations. I even had a Walkman style AM stereo receiver, but even ignoring the limited fidelity, it was increasingly too noisy. Now AM is completely washed out by noise and FM content is very limited.

author: EPAIII
author_rank: Super Contributor
timestamp: June 09, 2025, 03:51:38 pm
content: There is one thing that is being forgotten in all this discussion about square law vs. linear. I have operated a lot of TV transmitters that used vestigial sideband as well as the occasional AM transmitter and in both cases the first thing that is to be avoided at all costs is carrier cut-off due to over modulation. If the modulation level reaches a point where the carrier is cut off at the negative half of the base band signal, then an awful amount of spurious emissions are produced, causing interference far out of the licensed band. This is a big no-no.So, as a precaution against this ever happening, the modulation level is never deliberately set at 100%. The RF carrier level is at least at about 10% of the maximum level when the envelope reaches it's lowest level.Not only does this prevent those spurious emissions, but, depending on the level of the received signal, it also raises the level of that signal at the detector circuit in the receiver either completely or at least partially out of that dreaded square law region of operation and up into the more linear region. Even a crystal AM receiver can operate in a mostly linear region when tuned to a strong, local station.A much greater limitation to AM radio reception is the limited bandwidth of each AM channel. This limits the AM audio to about 8 or 10 KHz at the upper end. You just can not get any more frequency response out of AM radio due to this fundamental design limitation at the transmitter. It is simply not there to begin with. Unless you are tuned to a distant and therefore weak AM station and are employing amplification after the detector, that bandwidth limitation will produce far more audible problems than any problems in the detector itself. It may, in some cases be better to add more amplification in the RF or IF sections of the receiver before the detector. But that will wind up amplifying the noise that the antenna will certainly pick up. You can't win and there IS a penalty for trying.FM radio avoids this by amplifying the RF above a certain level and then running it through an AGC and/or clipper circuit to bring it at a standard level. Then it goes to a FM detector with any AM component removed.All the rest of this is just rearranging the deck chairs on the Titanic.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 09, 2025, 10:32:25 pm
content: Quote from: EPAIII on June 09, 2025, 03:51:38 pmI believe the international standard is 9kHz spacing, which was introduced a long time ago but definitely within my adult lifetime because I remember when BBC Radio 4 was on 200kHz, and they had to change it to 198kHz.Anyway, am I right in thinking that 9kHz spacing implies a maximum audio frequency of 4.5kHz?

author: TimFox
author_rank: Super Contributor
timestamp: June 09, 2025, 11:16:48 pm
content: Quote from: SteveThackery on June 09, 2025, 10:32:25 pmUS spacing is 10 kHz on the AM band, allowing 5.0 kHz. Adjacent channels are avoided in the same area, but alternate channels (20 kHz apart) are licensed here.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 09, 2025, 11:54:26 pm
content: Back when I was closely involved with AM radio in Australia, we, for many years, tested the frequency response of our transmitters out to 10kHz. When the channel spacing was standardised to 9kHz, filters were fitted to roll off the audio input after 9kHz.The frequency allocations normally left at least one channel width spacing above & below an allocation & we tested to 9KHz.Programme lines all went out to 10kHz or better.The idea of "just a little better than 'comms' standard" was not entertained.The problem was that when the "bean counters" stopped Australian manufacture of receivers, all we could buy were the narrow band things from other countries.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 09, 2025, 11:57:34 pm
content: Quote from: TimFox on June 09, 2025, 11:16:48 pmYes. Don't know where that previous poster got a 10kHzbandwidthfor AM; that would be practically high fidelity compared to what it actually is (apart from the noise compared to FM, of course).

author: Jr460
author_rank: Regular Contributor
timestamp: June 10, 2025, 12:03:41 am
content: Just ran across this thread and thought I might help with a little history. DSB surpassed carrier with a different program in each sideband. Yes it was used. Not for commercial. This had to be the mid 1970s, got a tour VOA transmitter site near me in southwest Ohio as part of the 2 meter club I was in. Of course most the guys that worked at the VOA transmitter site where also had dario guys, so we got the grand tour. They had a few transmitters that put put 500kW, yes half a million watts.Some were directly on broadcast band for some place across the globe, but many where at very odd frequencies, or this odd DSB modulation. The idea was to get the program across the Atlantic, maybe the middle of Africa, solid, no matter the conditions and make it very hard to jam. If it would be picked up by a remote VOA site, maybe even mobile, and decoded and retransmitted in he local broadcast band.Of course as satellite transponders became cheaper to rent and a C band dish was getting cheap and easy, the need to pound this signal from the middle of the US went away.The site was started I want to say around the WWII time and program to be transmitted came on records/discs, or phone lines. Later, dedicated lines from VOA HQ in Washington. At the time I toured the site they had recently put up a C band dish out back to get the feeds from DC rather than phone lines.The massive antennas are long gone, the site was decommissioned in 1994. They lasted a few more years, as they where brought down. All the land except for the transmitter building is now VOA Park and a concern a shopping mall. The building is now a museum.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 11, 2025, 06:49:04 am
content: Quote from: Analog Kid on June 09, 2025, 11:57:34 pmThe 5kHz audio bandwidth is not used everywhere.10kHz, & latterly, 9kHz audio bandwidth was the Australian standard.Most of the Transmitter service areas with adjacent allocations were far from geographically adjacent.Even our domestic Shortwave broadcasters went out to 7.5 KHz audio bandwidth.I spent enough time testing the things to know this!The difference between AM at 10kHz & 15kHz on the FM band is quite a serious one.---- I tested them, too!

author: vk6zgo
author_rank: Super Contributor
timestamp: June 11, 2025, 06:56:24 am
content: Quote from: Jr460 on June 10, 2025, 12:03:41 amThe term for that kind of modulation is Independent Side Band.ISB was used extensively for HF communications equipment.The comms ones would shoehorn two telephone bandwidth channels into each sideband, plus a couple of telegraph channels.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 11, 2025, 09:06:24 am
content: That ISB is new to me, thank you.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 11, 2025, 11:08:05 am
content: Quote from: vk6zgo on June 11, 2025, 06:49:04 amThis is interesting because I don't understand how it works and I would really appreciate you explaining it more fully.I thought this: a normal AM signal has two sidebands. Each sideband is as wide as the modulating signal. So, for example, if the modulating signal goes from 0 to 4.5kHz, then each sideband is 4.5kHz wide, thus the total RF bandwidth would be twice that, ie 9kHz.I thought the 9kHz spacing of AM bands in the short- and medium wave transmissions used in Europe and elsewhere meant that the audio bandwidth when demodulated would be 4.5kHz. But you talk about a 9kHz audio bandwidth.Where am I going wrong?

author: Circlotron
author_rank: Super Contributor
timestamp: June 11, 2025, 11:37:26 am
content: Here's an active envelope detector. The opamp forces the diodes to become ideal diodes.

author: Circlotron
author_rank: Super Contributor
timestamp: June 11, 2025, 01:14:38 pm
content: Another source of distortion in envelope detectors is when it is loaded too much. Referring to the diagram, say the unmodulated RF input voltage is 20V p/p. At 100% modulation, neglecting diode forward voltage drop, the voltage across R2 at the positive peak of the audio signal will be +10 volts. But at the negative peak of modulation when the carrier amplitude hits zero the audio signal across R2 will only be -5 volts. This is because the diode output can pull the audio signal as high as the RF peak, but R1 is responsible for pulling down for negative peaks, and 100K pulling on 100K won't swing all the way down to -10V. R1 needs to be lower value and R2 higher value to minimise this kind of distortion.

author: mikerj
author_rank: Super Contributor
timestamp: June 11, 2025, 01:34:03 pm
content: Quote from: SteveThackery on June 11, 2025, 11:08:05 amI read something about this some years ago, due to the large geographic separation of transmitters some Australian stations supposedly broadcast with a higher audio bandwidth (and hence higher RF bandwidth). The part that doesn't make much sense to me is that very few commercial radios could have taken advantage of this.

author: Circlotron
author_rank: Super Contributor
timestamp: June 11, 2025, 02:02:00 pm
content: Quote from: mikerj on June 11, 2025, 01:34:03 pmMaybe engineers have the last word in radio station construction whereas bean counters have the last word in radio receiver design?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 12, 2025, 12:34:05 am
content: Quote from: SteveThackery on June 11, 2025, 11:08:05 amEasy enough!If you AM modulate a carrier with an audio signal which is band limited to 4.5 kHz, you will have RF sidebands out to 4.5kHz above & below the the carrier freq, for a total occupied RF spectrum of 9kHz.If you AM modulate a carrier with an audio signal which is band limited to 9kHz, you will have RF sidebands out to 9kHz above & below the the carrier freq, for a total occupied RF spectrum of 18kHz.It is perhaps easier to imagine a single modulating tone, so for a real life example:-6WF where I used to work, has a carrier frequency of 720kHz.Modulating (which is simply a mixing function) with a single tone of 4.5kHz will produce a lower sideband at 715.5 kHz & an upper sideband at 724.5 kHz.The upper & lower sidebands will appear on a spectrum analyser as being 9kHz apart.Modulating the carrier with a single tone of 9kHz will produce a lower sideband at 711kHz & an upper sideband at 729kHz.The upper & lower sidebands will appear on a spectrum analyser as being 18 kHz apart, with the carrier equidistant between them.But wait! there's more!If your spectrum analyser will work down to audio, & you examine a typical programme, it is evident that it doesn't contain a lot of components at the high end of the audio spectrum, so the problem with interference between frequency adjacent allocations isn't as great as it may seem.And more, yet!In the UK, & Europe, AM Broadcasting was originally set up on the "Longwave Bands" in the Frequency Range 148.5 kHz to 283.5 kHz.There is very much less spectrum space in those bands, so it made sense to have lower bandwidth transmissions, especially with the limited frequency response of audio sources of the day.When they went to MF broadcasting, the old 4.5KHz limit was "grandfathered" onto that band.Australia only had very limited Long Wave Broadcasting, with all the LW stations moving to MW almost immediately.MW standards were strongly influenced by those of the USA at the time.Interestingly, at least as at 2024, the USA specifies 10 KHz audio bandwidth, although, according to NAB many stations are using lesser bandwidths.https://www.nrscstandards.org/standards-and-guidelines/documents/guidelines/NRSC-G100-B-2024.pdf

author: vk6zgo
author_rank: Super Contributor
timestamp: June 12, 2025, 12:48:01 am
content: Quote from: Circlotron on June 11, 2025, 02:02:00 pmBack in the day, Australian made AM receivers made a fairly good attempt to accommodate the wider bandwidth signal, but with the advent of pocket "transistor radios" in the late 1950s, followed over the next few decades by the effective collapse of Australian domestic radio production, receivers were almost invariably imported, many with the major quality emphasis being on the FM band, with AM being pretty much an afterthought.That was a "bit of a bummer" as with FM broadcasting only commencing in Oz in the 1970s, people had to put up with crap AM bandwidth or find an old radio.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 12, 2025, 08:05:18 am
content: Quote from: vk6zgo on June 12, 2025, 12:34:05 amSo I'm not going wrong, then, because that's exactly what I said. 😁👍Quote from: vk6zgo on June 12, 2025, 12:34:05 amSo a minimum of 20kHz spacing at RF, yes?

author: Analog Kid
author_rank: Super Contributor
timestamp: June 12, 2025, 08:22:50 am
content: Quote from: SteveThackery on June 12, 2025, 08:05:18 amNope, wrong.In the US, AM stations (in the 540 to 1600 kHz band) are spaced 10kHz apart, making the audio bandwidth 5kHz, not 10.There seems to be a lot of misunderstanding about this even in publications of official agencies like the FCC:QuoteNo, the bandwidth isn't double the sideband width: it's the sideband bandwidth alone, 5kHz.Although technically this is true, that according to this scheme (10kHz "slots" for AM MW broadcasting) the audio bandwidthshouldbe limited to 5kHz in order for adjacent stations not to stomp on each other's signals, apparently it's not always the case.Interesting article on the subject:https://www.radioworld.com/tech-and-gear/5-khz-audio-bandwidth-on-amOne thing is that in any "market" there won't be stations every 10kHz; they'll be more spaced out. However, what about "clear channel" stations at night interfering with each other?

author: mikerj
author_rank: Super Contributor
timestamp: June 12, 2025, 11:28:59 am
content: Quote from: Analog Kid on June 12, 2025, 08:22:50 amContext is everything, are they referring to audio bandwidth or the RF channel bandwidth?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 12, 2025, 01:27:57 pm
content: Quote from: Analog Kid on June 12, 2025, 08:22:50 amLooking at the RF channel, the "bandwidth" is the "occupied spectrum" of the station, including both sidebands."Audio bandwidth" sometimes called "baseband bandwidth" is the occupied spectrum in the audio region occupied by the programme material.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 12, 2025, 06:01:37 pm
content: Quote from: Analog Kid on June 12, 2025, 08:22:50 amOh dear, I've got experts disagreeing with each other.Guys, can we clarify? My understanding of a normal AM broadcast is that both sidebands are transmitted, and that the RF bandwidth of each sideband is equal to the AF bandwidth of the modulating signal. Therefore the total RF bandwidth must be twice the AF bandwidth.Is that correct or not?I also believe that the sidebands "mirror" each other about the carrier frequency, so they are both the same RF bandwidth as each other. Essentially the two sidebands carry the same information.Is that correct?

author: Analog Kid
author_rank: Super Contributor
timestamp: June 12, 2025, 07:02:16 pm
content: My understanding (and I believe the estimable @TimFox agrees with me here) is that the total signal (audio) bandwidth is equal to the width of each of the sidebands,nottheir sum. In other words, half of the AM "slot" (10kHz).Think about it: both sidebands are identical, with the same information, just shifted in frequency, so they cannot be added to give a wider bandwidth.Anyhow, I now leave the stage to the RF/modulation, etc., experts here.

author: paul cotter
author_rank: Regular Contributor
timestamp: June 12, 2025, 08:17:28 pm
content: It has to be the sum of the sidebands- although they contain identical info they are spaced above and below the carrier frequency so the total composite bandwidth will be twice the modulating signal.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 12, 2025, 08:33:28 pm
content: Quote from: paul cotter on June 12, 2025, 08:17:28 pmBut how can that be true when the sidebands contain identical info?One of my textbooks which I regard as authoritative (Electronic Principlesby Albert Paul Malvino) states:Quote

author: TimFox
author_rank: Super Contributor
timestamp: June 12, 2025, 08:45:38 pm
content: Yes, AM produces two sidebands anti-symmmetric about the carrier.Each sideband has a width equal to the width of the "baseband": the audio to be transmitted.Therefore, the total bandwidth occupied by the signal is twice the width of the baseband, assuming the spectrum allowed for the baseband is fully occupied.The channel spacing is established by regulation (e.g., 9 kHz or 10 kHz depending on the regs), which determines the allowed baseband width to avoid "coloring outside of the lines" given by the regulation.Note that the bandwidth for an FM signal is more flexible: the "modulation index" is the ratio of (maximum shift in frequency for max baseband voltage) / (maximum frequency of baseband).Regulations define both the baseband bandwidth and the frequency deviation for licensed broadcast stations.One can choose this index, from "NBFM" used in radiotelephony to conserve bandwidth to the higher index values used for audio broadcast, and the total occupied bandwidth of the signal depends in a somewhat messy way on that index.Higher index values give larger improvements in SNR, compared with AM.Looking up these details is left as an exercise for the reader.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 12, 2025, 08:58:17 pm
content: Seems as if some of us are confusing the audio bandwidth with the bandwidth of the transmitted signal, which is indeed twice the audio bandwidth.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 12, 2025, 10:08:20 pm
content: Quote from: Analog Kid on June 12, 2025, 08:58:17 pmI'm not sure which of us was doing that, but now we all agree. 😊👍

author: Analog Kid
author_rank: Super Contributor
timestamp: June 12, 2025, 10:11:11 pm
content: And peace reigns over the kingdom ...

author: vk6zgo
author_rank: Super Contributor
timestamp: June 13, 2025, 12:10:58 am
content: Quote from: Analog Kid on June 12, 2025, 08:58:17 pmThat is correct, but some people are so wedded to the idea of a 5KHz audio bandwidth "baseband" signal, that imagining a country where the baseband signal is 9KHz or even 10KHz wide, with each station occupying a RF spectrum 18kHz or 20KHz wide sends them into a spin.There are ways that you can get away with this:-(1)Real life Audio signals don't contain a lot of energy at the highest frequencies, so the signal strength of the resulting sidebands are less likely to interfere with a stronger local station,(2)Stations on adjacent frequencies are usually geographically separated.(3) Both very low & very high power AM MF stations are rare in Australia, with the highest power ones being around 50-55 kW, & the lowest power around 2kW.I did say "rare"---there are a few lower power ones, but to my best knowledge, none higher than the high figure I quoted.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 13, 2025, 12:26:44 am
content: Quote from: Analog Kid on June 12, 2025, 08:33:28 pmpaul cotteris referring to the RF bandwidth of the modulating signal.As to Malvino, he was a very fine engineer, but he wasn't a Broadcast Engineer.Historically, there was a lot of material on AM Broadcasting, which can probably be found by research.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 13, 2025, 12:31:30 am
content: Quote from: SteveThackery on June 12, 2025, 06:01:37 pmThe detected audio from the two sidebands is of the same bandwidth & phase so adds to increase the demodulated audio signalamplitude.

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2025, 07:42:17 pm
content: Quote from: vk6zgo on June 13, 2025, 12:31:30 amThat is consistent with what I remember. I thought I remembered AM stations transmitting only one sideband, with the carrier, and envelope detectors still working. I tried searching for this but found nothing.

author: TimFox
author_rank: Super Contributor
timestamp: June 13, 2025, 07:48:17 pm
content: Quote from: David Hess on June 13, 2025, 07:42:17 pmNormal AM transmitters work by varying the amplitude of the RF power up and down from the quiescent level (carrier), and therefore produce both sidebands.It is more complicated to remove one sideband in the transmitter, and requires a linear amplifier after the modulation, which is less efficient than a good modulated class-C power amplifier.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 14, 2025, 12:12:36 am
content: Since we're on the subject of modulation, a question:When exactly in the transmitter chain is the carrier actually modulated? Is it close to the input of the RF power amplifier (class C), in which case all the power amp stages are amplifying an image of the final transmitted signal, or is it near the output of the power amp? Is modulation ever done at the final power amp stage?

author: vk6zgo
author_rank: Super Contributor
timestamp: June 14, 2025, 02:57:23 am
content: Quote from: Analog Kid on June 14, 2025, 12:12:36 amIn the old days, in MF Broadcast service, it was almost invariably done at the final RF stage, but required very powerful modulation stages, often using the same type of tubes as the PA.Modern AM transmitters may do the modulation at a lower level & different frequency IF, then up or down convert the already modulated signal (this may be derived in either an analog or digital way) to the transmitted frequency & feed that to a linear amplifier, or alternatively, use PWM modulation applied to the PA or earlier stagesWikipedia has a brief discussion of modulation methods.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 14, 2025, 03:37:28 am
content: Quote from: TimFox on June 13, 2025, 07:48:17 pmSending one sideband, plus the carrier is called Compatible Single Side Band (CSSB).It had a brief vogue when various HF comms services like the Australian Flying Doctor Network were converting from AM to SSB, as it allowed interaction between the two modes.Interestingly, in Analog TV transmission, the whole upper sideband (USB) is transmitted, but only a "vestige"of the lower sideband (LSB). The mode is called "Vestigial Side Band" (VSB)This makes receiving easier, but the presence of around 1.25 MHz of the LSBdoescause both amplitude & phase problems in the portion of the detected signal which is derived from the "DSB" part of the signal.These are corrected by various measures, both at the receiver & the transmitter.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 14, 2025, 03:40:50 am
content: Quote from: vk6zgo on June 14, 2025, 02:57:23 amRight. My bad in writing "class C" amplifier; of course it would have to be a linear amp, which class C is definitely not.

author: Andy Chee
author_rank: Super Contributor
timestamp: June 14, 2025, 05:01:06 am
content: Quote from: Analog Kid on June 14, 2025, 12:12:36 amThis video is handy;

author: paul cotter
author_rank: Regular Contributor
timestamp: June 14, 2025, 11:45:53 am
content: The Doherty system of producing AM, as used on Continental transmitters, was a great improvement over the old anode(plate) modulation using a transformer.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 14, 2025, 11:33:43 pm
content: Quote from: Andy Chee on June 14, 2025, 05:01:06 amInterestingly, back when my old employer started buying AM MF transmitters from them, the company was called "Gates" then it suddenly became "Harris"---now, after so many years, it is called "Gates" again.I said in an earlier posting that lower than 2kW was rare in Australia, but we did have a few Harris 1KW ones with multiple FET PAs.The FETS were notably fragile, with the joke being that they would go "fft!", "fft!", "fft!" one after another.I think they didn't like lightning induced transients on the "vertical radiator", whereas a tube PA would just grunt, recycle & be back into service.One selling point was that the newer transmitters were smaller than the old 1950s style ones, but the 50kW shown isn't notably smaller compared to the 55KW STC 6WF used for many years, whereas the replacements from another manufacturer are.