IMD measurements on radios? - Page 1
Forum section: electronics / rf-microwave

author: mw2000
author_rank: Newbie
timestamp: March 13, 2020, 12:56:02 pm
content: Hi everyone.Wondering once again if you experts can help a radio enthusiast with a question!I was reading that in order to test an amplifier you needed to use 2 signals or tones to do that?How is this guy doing it in this video?Everything I read tells me this guy is wrong. It doesn't seem to make sense to me?Is this video just BS or is he right?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 13, 2020, 02:07:06 pm
content: I've tried to watch the video but my god this guy is just unbearable. I have no clue what he is trying to do. Maybe I'm just an idiot though..He never really gives a clear and concise description of what he is doing.But indeed, IMD tests would usually be done with two generators. It is important to note that often you can't just connect them together directly, you need good isolation on many generators because (at least this is how I understand it, I'm not a PA guy myself) the two generators will interfere with eachother (and/or the ALC loop) and that will make them generate intermodulation products themselves. I believe Minicircuits had a nice appnote on the topic.

author: k8byp
author_rank: Newbie
timestamp: March 13, 2020, 02:35:00 pm
content: "Two-tone test" and "IMD test" are somewhat different animals.Yes, one tone or nine tones are "modulation" BUT... IMD rears its ugly head when using solid state circuits with complex modulating signals (mainly voice or data)A Two Tone test in radio hastraditionallybeen used to inspect for percent-modulation (over-drive that causes flat-topping or clipping) and amplifier bias (non linear region operation). That's much simpler than IMD testing, and doesn't require nearly as expensive equipment as IMD testing, and can be done with a cheap oscilloscope and a trigger signal. Part of that "traditional use"' is the fact that in decades past, especially in Amateur radio, test equipment was either not available or extremely expensive.An early text describes two tone modulation as mostly related to amplitude distortion. Two tones in, two out, compare them. The text does mention frequency, but in the 1940s, again, who had spectrum analysis?(Radio Engineers Handbook, Terman 1943, pp 966, 967, "Amplitude distortion in amplifiers")The difference between one and multiple tone testing is that multiple tones can mix together and thru the modulation process, produce other signals that aren't supposed to be there. This is also a factor in amplifier 'overhead' or how much information (bandwidth) an amplifier can process.Amplifiers are also mixers, tubes are the best, transistors the worst.From Electronic Design:https://www.electronicdesign.com/technologies/communications/article/21798494/understanding-intermodulation-distortion-measurementsDistortion products consume part of the amplifiers overhead- they waste power and cause interference. Since that's in the voltage (power) vs. frequency domain, spectrum analysis is required.That's not possible with a $50 'scope.Notice the article mentions "harmonics." Those are mixer-products very close to the carrier frequency and require a good spectrum analyzer with very good RBW (or mod analyzer) to accurately measure, unlike a second harmonic of a carrier (2f, 3f...) which is far enough removed from the carrier frequency that most anything can detect it.IMD and such complex parameters are often used in the context of device design and Regulatory Agency (US-FCC) certification and requires high end equipment for reliable measurements.Oddly enough, a good radio receiver makes a fine 'seat of the pants' test- just listen to the received signal while tuning up and down in frequency.To hear severe cases of distortion, listen around 7170 KHz (40M amateur band) to the operators grossly over-driving transmitters with audio processing equipment and notice the extreme distortion produced, in some cases, roughly half the transmitted power wasted in spurious products (and highly against FCC regulations, BTW).Attached: a 'scope display of a two tone test I recently did on an old Drake TR3 Amateur transmitter, using an AWG and scope. It "looks" clean and regular, but that's deceptive, as it only shows amplitude versus time, not frequency. There must be distortions there which are not shown in that trace.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: March 13, 2020, 02:38:29 pm
content: I was under the impression that a two-tone test here referred to testing with two tones in close proximity and seeing how the IM3 tones increase with increasing input amplitude of said tones on a spectrum analyzer. Perhaps I'm just confused as to what the 'other' two-tone test is.

author: mw2000
author_rank: Newbie
timestamp: March 13, 2020, 03:11:55 pm
content: Alright guys so a little background.The guy in the video is supposed to be an expert that redesigned the receiver in that radio in the video and is the only one that can fix the "IMD problem" that these radios are supposed to have.Several others out there say that it's a bunch of nonsense. The ones he attacks in his video descriptions and video comments especially say that it's smoke and mirrors to charge people money by saying that you need these new parts and adjustments to fix the "IMD" problem they have from the factory.What I am getting from the conversation is the same thing the people he attacks are saying which is that he lacks the equipment to do it and that he is not actually doing the testing for this "IMD" thing to start with.He says he can see it in his radio test box that has the spectrum analyzer but he never points it out.Is it really there or is he just saying this "IMD" is there but it's not?He mentions it being in that spectrum analyzer in the pyramid somewhere on several of his videos but how do you look in the pyramid for it?I found one video on youtube where the person performing the test had a completely different set of equipment and showed something completely different on his equipment than this other guy did. They used an amplifier and some things that looked like splitters to join the signals together in that video but this is a whole radio not separate parts. Is a test for this "IMD" possible in a fully assembled radio? How would that work?I guess, the main questions I have are..Is this "IMD" there?If it is there, what does it look like on the spectrum analyzer and on the oscilloscope?The other guys never used an oscilloscope to do this "IMD" test so which one does it show up on?

author: vk6zgo
author_rank: Super Contributor
timestamp: March 13, 2020, 03:35:17 pm
content: Quote from: TheUnnamedNewbie on March 13, 2020, 02:38:29 pmYeah, they are two different tests.The one for looking at a modulation envelope uses two non harmonically related baseband tones (commonly audio).It only gives an indirect idea of linearity.This is only used for systems using reduced/suppressed carrier, such as DSB, SSB, or ISB.With full carrier AM, only one tone is necessary.In the other type of test, two carriers are generated, & a Spectrum Analyser, or formerly, a Selective Voltmeter is used to search for intermodulation products generated due to non-linearities in the device under test.Many years ago, it was a regular test we did upon our Independent Sideband (ISB) transmitters, using specially provided signal generators & Selective Voltmeters built into their exciter racks.PS: Measurement of intermodulation distortion in a receiver is much less commonly done than on transmittes/ power RF ampliers..I have not personally done it on a receiver, nor have I watched the video.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 13, 2020, 04:41:02 pm
content: Having watched most of the video, he isn't looking at a receiver, but at the RF output of a plain old full carrier AM CB radio.He has a single tone input, which shows up as the classic "AM" envelope "in the time domain".His Spec An is showing the output "in the frequency domain".in other words, it shows the carrier & upper & lower sidebands, as well as some noise artifacts inherent to the carrier generator.(An ideal carrier would appear as a line of zero width, but we can't make ideal carriers).I don't know why he was rabbiting on about "intermodulation distortion"-- what he is seeing on the SA with the second radio is plain old audio harmonic distortion probably in the audio stages prior to, but possibly in the modulator itself.If you look at the modulation envelope, you will notice a fair bit of "flattening" as it approaches carrier cutoff.He should have given up while he was ahead!

author: StillTrying
author_rank: Super Contributor
timestamp: March 13, 2020, 05:49:54 pm
content: Quote from: TheUnnamedNewbie on March 13, 2020, 02:07:06 pmI wonder why.Perhaps he thinks IMD stands for imaginary distortion. The vid was so bad I had a look at some of his others.Can you imagine him soldering a load of 0603's.0603 SMT Resistor And What The # Means - Toleranceyoutu.be/2myWOeNpWyA?t=264Be Careful With 18650 BatteriesReal World CB Radio - Amplitude Modulation Down The Drain

author: mw2000
author_rank: Newbie
timestamp: March 13, 2020, 08:39:27 pm
content: Well I guess I understand what you guys are saying.It's not there because he isn't testing for it properly or at all.He has these little explanations that if you cannot see what he sees that you suffer from "dunning-kruger effects" and that you have "no integrity"I guess at the end of the day this must be some marketing stuff and not actual science or engineering.I will keep reading and learning what to look for.This is another one of his "IMD" videos:Is there anything in this one that's about "IMD"?

author: vk6zgo
author_rank: Super Contributor
timestamp: March 13, 2020, 11:25:04 pm
content: A bit on intermodulation that has just surfaced in my memory--- I'm retired, so I spend my time pontificating on the 'Net, insead of "doing".In areceiver, intermodulation is primarily a problem in the RF stage, or if there isn't one, the mixer.Several strong signals which the radio is not tuned to, may be able, due to non-linearities in the RF amplifier, or mixer, to produce a resultant signal which is either within the desired passband of the receiver itself, or that of the following IF stages.Re the video guy using "Dunning-Kruger".I always have a chuckle when people use this term, as unless they are Psychologists or similar, they are exhibiting just the behaviour thay are accusing others of.By the way, I hope all you EEs have got your "badges" on you at all times---you never know when you might need one."OK, Freeze!""Who sez?""Electrical Engineer"Lawyer:- "On what grounds are you charging my client?"EE:- "The current charges are that he didn't obey Kirchoff's Law!"Lawyer:- "Ah, but which one?"

author: StillTrying
author_rank: Super Contributor
timestamp: March 14, 2020, 07:14:27 am
content: Quote from: mw2000 on March 13, 2020, 03:11:55 pmI can't see much evidence that he knows much about electronics at all. In the first link I posted he seems to be thinking that the 1% tolerance of a 0603 SMD is how accurate the physical size is.I'm not surprised he doesn't do drop-ins or repairs, just new radio tune ups!Why does he think accurate calibrations are needed to see if audio sine waves are flattened or clipped, he prefers clipped!https://youtu.be/by98x4eu2lM?t=1He wants any communications with him to be concise,and he wont answer any questions.https://youtu.be/P0W_Ltzeyyc?t=1

author: vk4ffab
author_rank: Frequent Contributor
timestamp: March 14, 2020, 10:46:35 pm
content: You are not a REAL cb repairman unless you have 10 bird meters. Bird watts are the real watts in your 2 pill, 4 pill and more pill kicker. Most of these guys are as bad as audiofools with the amount of rubbish they think is real.

author: Mark19960
author_rank: Regular Contributor
timestamp: March 16, 2020, 05:56:12 am
content: I know what is going on with this cheeky bugger that the OP has not articulated. I can elaborate and fill in some missing bits and bobs.Part of the IMD discussion is that the aforementioned bloke uses IMD to market devices that he claims eliminate any nonlinearities.Now, he doesn't say exactly that but that is the message - that IMD is eliminated because they are perfectly linear and matched.The devices in question are an ERF-2030+ of which there is conveniently no datasheet.They are marketed as replacements for IRF520s to the CB crowd.Claims include...Elimination of IMD due to their supposed perfect linearity.Run coolerWill increase output powerWon't cause your lorry to burn to the ground like the IRF520 or FQP13N10 will...yes, that is a claim made by this chap.They are indestructable.Hard to type this with a straight face. I find myself in a chuckle at times. It's a miracle! I would love components that have unlimited blue smoke!Especially when they refer to the IRF520 or FQP13N10 as a miniature stick of dynamite or miniature road flare that will burn your home or lorry down.Repairs? What repairs? We just design things and build them! They last forever!(it's a fear based sales tactic, nothing more.)But, back to the planet earth.... big red flags here. If these devices are so wonderful why only market to CB radio chaps?Why no datasheet? I would think that companies would trip over themselves wanting data on such a wonderful device!Who needs the IRF520 or FQP13N10 when you can have the indestructable ERF2030+ ! No blue smoke!!If you think these claims are dubious, you suffer from "The Dunning-Kruger effect" that these cheeky buggers sometimes refer to improperly as a syndrome.Oh, and there is one man on the planet that knows the special gate voltage to make it work. It will not work properly for anyone else without the voltage.Yes, that's right. It has a special secret gate voltage.Idle on these is not set by adjusting the gate voltage for a specific idle current, but a super secret voltage.Oh, and maths don't matter. Maths are not "real world".I bet they feel at home with the audiophools.

author: StillTrying
author_rank: Super Contributor
timestamp: March 17, 2020, 09:02:31 am
content: I'm not an expurt in AM modulation,but why is he always looking at near 100% modulation at >3kHz, surely for a voice channel he should be concentrating on 400Hz to 1kHz.

author: Mark19960
author_rank: Regular Contributor
timestamp: March 17, 2020, 11:04:31 am
content: SHOULD be. But it's not what they care about.But in America they want to be LOUD on the radio. That usually means exceeding 100% modulation.The smarter chaps have designed/changed the circuit to increase positive peaks whilst supressing the negative peaks.It's done in commercial AM broadcasting - not being an RF expert on this I am certain someone else could weigh in on that.I think they stick to 125% modulation max (I think it's 125%?) as ordered by the regulatory body in the US (FCC)Now, with CB radios the regs don't matter anymore. It's a larely abandoned band in the states with little oversight.In order to sell a radio it has to conform to standards set forth by FCC. That's about where the oversight ends.Once it's sold they open it up and remove any form of limiting they have, or turn the modulation up to 11.Another thing that's done is the import of the so-called "10 meter" radio. It's a CB radio in discuise.Remove a jumper or change a menu setting and bang, it's a CB radio as intended.They call them "exports" but they are not exported, but imported into the states.They also want to be WIDE. They are looking for AM broadcast quality. That means altering the BPF in the audio path.As this is beyond the abilities of most, they do "direct injection" in chich the BPF/limiting is bypassed altogether.The radio god in the states has a video up where this is demonstrated.It's not the best demonstration but it does suffice. Fortunately it's short.That's done here:Using a spectrum analyser and sweeping would yield a much cleaner and easier to understand picture for the end user but either he doesn't know how or the instrument doesn't have max or min.It's for testing cellphones so probably not. I have not looked in the doc to see.Now, they worship the results of this as if it's impossible for anyone else to achieve them but this bloke.First thing you see is the sweep of the radio. He turns on an analog signal generator and starts from pretty much zero until > 5KHz.You see it start to roll off around 5Khz.This is what the "hifi" CB crowd likes and since they think this is the only bloke that can change a couple of parts in a bandpass filter he is worshipped by a few cheeky buggers.It will and does bleed into the adjacent channel but that's not shown.At the end of the video he shows you spans of 300KHz and 100KHz and such and proclaims it "clean".(concealing the incursion into the adjacent channel, since CB channels are on 10KHz centers.)What the casual observer doesn't know is that you cannot see much about a narrowband AM signal at a 100KHz span.We are talking about an AM signal that (should) be consuming less than 6KHz of bandwidth but in this case closer to 10KHz.In the beginning of the use of the cellphone test set you see center frequency around 27Mhz and the span is probably 300MHz or so.He reduces the span to various points declaring it "clean" which is apparently his typical showing.According to the manual for the 8924 the resolution bandwidth and span are locked, so the RBW @ 100KHz is 1KHz.You won't see much. the span is too wide to have a decent look at a narrowband AM signal.This is likely done by omission. Can't show people the two sidebands and any resultant harmonics, now can we?Or that it's now in the space occupied by an adjacent channel.As to the audio response....It took me longer to copy the bandpass filter from the drawings of the radio and input them into LTSpice than it did for me to compute new values.One is "stock" and one is after the changes.But this is how things are done on American CB.At least when I am in England I can use the radio without hearing this rubbish all of the time about who has the loudest radio.I think Ofcom is just as lax as FCC when it comes to CB in the UK. Still have our "tech wars" but not like the Americans.You are not missing much mate. It's a bloody joke mate.