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- Опубликовано: 3 ноя 2024
- This short video discusses 3rd order IMD (intermodulation distortion) products, what they are, and how they can adversely affect radio reception under crowded band conditions. Finally, I show how the proper use of input attenuation can help mitigate the negative effects of IMD.
Notes: www.qsl.net/w2a...
Spectrum display courtesy of the Spectrum View feature on a Tektronix MSO64 oscilloscope - gotta love that fast update rate, even with 50Hz RBW!
W2AEW is back! Lockdown suddenly seems more bearable 🤓
Yup
The Outlook "message received" sound scared me for a second. Thought I was at work.
That's the first explanation I've read or heard that really makes it clear. Many years ago, a friend taught me to run the audio nearly all the way up and control level with the rf gain control. All kinds of crap on the band fell into the background noise and weak signals I couldn't hear before seemed to pop out. It always seemed counter-intuitive. This was during Solar Cycle 19 with a well-used 75A2 when hams were shoulder to shoulder on every band and lots of signals were strong.
Thanks for this and the hundreds of other clear, interesting videos.
one of my final classes for my degree is a communication links course, so this helped explain this clearly to me, thank you very much!
I'm coming back to this video after 2 years, on the night before I have a test on this topic. Finally getting formal education in the RF field
Hi Alan,
Good to see you're back making videos. You make it easy to understand IMD with the visuals. Hope you and the family are healthy. Stay safe. 73 WB3BJU
You've explained not only 3 order intercept but the reason one might use attenuation better then any other source I've ever come across, which is MANY. Thank you.
Glad you're back, miss your face. Back to paper!
Thanks Alan!
73
Thank you so much for the video. Very informative especially while illustrating factor by which 3rd order IM is affected.
Perfect! Just what my Ham license students need to refresh IMD before their exam next week! Thanks!
Thanks. This short video explains well why do we need to set attenuators at the receiver. During measurement in pna, we have to set attenuators for that.
Hi Alan, what a clear explanation! I am so glad you are around
to teach us all this kind of stuff. Please do more of this if time allows! Greatings from The Netherlands.
Glad to see you back Alan, coming back here, what a nice surprise.
As always you can make things as simple can be and easy to understand. that needs a lot of knowledge, congratulations, thumps up.
Thanks, take care...
I have watched different video on this topic. Really very nice. Precise, to the point & very informative. Keep it up.
Great explanation of IMD. I now really understand why the IPO button on my rig is called what it is. Thanks Alan
Clear, concise, easy to understand, and very informative, as always! Thanks for posting this, your videos have been missed. I hope you and yours are happy and healthy.
Great explanation of IMD, a term I new nothing about but often encountered when looking at specifications. Thank you for sharing. Your presentations are at just the right level for me.
Great to have you back Alan
Definitely needed this to cement ideas in my RF class! Helps greatly👌🏿
Thanks, Alan! Nice to see you back. I hope that you and yours are staying safe.
Thank You Alan - excellent and informative as usual. Hope you and your family and friends are all safe and healthy.
This is a great video, with clear and concise explanation of IMD. Good work as usual!
Alan, great video, thank you. Your simple explanation of the increase in the 3nd product is the best 6hat i have ever seen. It explains the difference in slope between 2nd and 3nd order.
73,
Danny, KI6VBC
Yey you're back! Cant get enough of your videos!
Excellent explanation. I always use low level signals whenever experimenting with filters, combiners etc Not sure where I got that idea from, perhaps a fear of over driving inputs. Nice to be able to give my colleagues another rational explanation for keeping input signals low as possible.
No, no just the contrary! IMD could be an issue in ACTIVE NON LINEAR devices as stated by Alan. When you have to measure a PASSIVE device, such as a filter, a combiner or a divider it is sensible to use all the dynamic range of your instrument, let's say a VNA, decreasing the IF bandwidth (increasing the measurement time, obviously) and using a decent input power level, say 0dBm. Doing so, you can use all the dynamic range to measure rejection, slope of your filter and so on. Remember that accuracy means a lot of time.
Again, a great video and explanation. Very easy to understand and follow.
One really tiny labelling error (very easy to overlook) is at 1:20 into the video where you introduce 3rd order IMD products and both representations on the right hand side of the display are labelled 3f1 (I think it shroud be 3f1 and 3f2).
Looking forward to the next video.
Yeah, I caught that after I completed the video. The downloadable notes has this corrected.
Good explanation! Wes Hayward would congratulate you! A detail: for receivers with no input passband, only a lowpass, 2nd order IMD can be an issue too, if your station of interest lies near f1+f2, 2*f1 or 2*f2. Keep up the good work. 73s from Argentina.
Dang you're good at this! Even I understand it now.
I'm very glad to see you back.
Glad to have you back!
excellent explanation!! I had read this information in the ARRL book called Amateur Radio Perfomance testing (read a couple of times), which kinda made sense but didn't clearly "ring the bell". Your explanation on the other hand "rang the bell" clearly. Thank You!!
So pleased you’re back. Best IMD discussion I’ve ever seen. Thanks, as always.
I learn so much from your videos! Please keep posting such amazing content.
Clean and relevant. Greetings from switzerland.
Agree with many other of your viewers - good to see you back Alan, and hope we can see more good videos like this to come!
Ah...Finally! So glad that you are back with another great video. Can you please also make one on 1dB compression point and then show both of these (IP3 and P1dB) in an actual RF amplifier (not a mixer).
I’ll add that to my list.
Good to see you back!
Also rule of thumb roughly 9.6dB above 1dB compression point is the OIP3 point
thisoldman99 yes
Glad your back Alan!
Great to see another Video! I enjoyed your video #170 basics of IQ signals. If and when you have the time could you please do a video on phase modulation versus frequency modulation. I think FM is more easy to wrap your head around but PM for me seems to be a different animal. Thank you for all your work and hope to see more videos. Please stay safe in these trying times! John
Great info, as usual. Thanks for starting up posting again. I know life gets in the way of RUclips, but your efforts are much appreciated.
Welcome back! Hope you and the Mrs. are doing well! Better days are coming soon! 73's de KC2RDU
Great video! I've seen some discussion about not using LMR400 type coax cable in duplex repeater applications, supposedly due to the 2-layered shielding (braid over aluminum foil) potentially causing PIM. Now, in a UHF repeater situation with 5MHz offset the closest 3rd order products would fall either 5MHz above or below the RX frequency, depending on whether the offset is positive or negative. Depending on the magnitude, this should be easily filtered out by the RX radio's front end filters. What I can't find anything about is how big the amplitude of the 3rd order products would be if the let's say -93dBm (S9) RX signal is intermodulated with a +40dBm TX signal 5MHz apart. I would guess that in a passive system the magnitude of the intermod product would be smaller than the smaller of the two fundamentals but I can't find anything to support or disprove this. What happens in your experiment when you turn one of the two fundamental signals down by 50% for instance?
С возвращением!
Thank you!
Спасибо!
This was another great video.
I can't imagine why anybody would hit the dislike button unless they happen to be a con artist. It would be cool for you to do a video showing nonlinearities in a transmitter.
Very instructional. Didn't think about the attenuation. Thanks for the video
Thanks for this. It’s a very clear and concise explanation and demonstration along with why it matters.
Been waiting for a new video!!
Beautiful! Tgif ! Hope to see more videos again soon ! Great to have you back !
RF == Black Magic. Thank you Alan for another great explanation.
Excellent, as always. I always learn something here.
Excellent detail on regarding IMD. Thanks for the clear presentation with examples and sharing!
You are my favorite teacher....
Thank God he's fine!
miss u ...
any video from w2aew hugely appreciated !!
tnx and please keep going
Glad to see your back Alan! keep it up sir!
Excellent explanation! Thank you very much! This has all the information necessary to get started exploring this in our own receivers. I suppose there are even some occasions where you'd want to add attenuation _before_ reducing the RF pre-amplification (in those designs that have the attenuator before the pre-amp). I would certainly love to watch a follow-up video going into more depth on this topic, but as I said this video does give a good starting point for us all to explore this more on our own. 73 de LA1WUA Magne
Spectacular explanation and presentation OM!
Like always, clear explanation and practical example. Thanks! ON9SG
You are the best youtuber in my book! Thanks man! Very useful and very much appreciated!
just two minor remarks imho: I am not sure if you changed your microphone? I kind of prefer the voice in previous videos. Also, I generally like more to see you interacting/touching physical buttons of test-equipment not just software.
This said, the content is just as amazing as usual!
The thing is the mixer is a non-linear device. But we need this nonlinearity so we can get the down conversion to IF.
Thank you Alan. Great explanation on a subject I didn't fully understand, much appreciated!
Hi Alan,
Glad to see you're back making videos. As always best explanation. Hope you and your family are healthy. Stay safe. Thanks for the Video. 73 DB1IAT
This is why many older FM stereo receivers from the 1960s through the 1980s had "Local/DX" switches in them. People would switch it to DX, thinking they wanted to hear the distant signals, and in fact the opposite would occur. The distant stations would disappear in to the noise and interference.
Many times I'd hear someone say "that danged switch is backwards." No, it wasn't backwards. It was intended for those who lived next to broadcast stations. Often that nearby broadcast station would create all sorts of intermodulation products across the band. Placing the switch in "Local" would attenuate that nearby station and thus reduce the IMD products, making many more stations audible.
So if you see a radio with a Local/DX switch, switch it to Local if you're in a city. Switch it to DX if you live in the outer suburbs and you know there are no significant radio transmitters nearby.
Nice description and video. Well done.
He's alive !
Hi Alan, thank you - this an excellent topic to cover. I enjoy thoroughly your videos and have an interesting request for future topic, which I hope you could cover in your future videos…
You elaborated on amplitude of IMD products, but didn’t mention anything about examining the phase angle with respect to fundamental. In Many evaluations, situations, both in RF and AF, phase angle information of a IM product with respect to fundamental frequency tells a lot of information about the nature of IMD.
I wanted to ask you… can you do a video on a practical way of Finding both amplitude and angle of a say second order or 3rd order harmonic? I know finding the amplitude is easy with a spectrum analyser (Nyquist plot, Fast furrier FF). BUT how can we find/isolate phase angle of 2nd or 3rd order IMD with respect to fundamental frequency in output of a mixer, buffer or amplifier??? I appreciate if you can do a video suggesting demonstrating a practical solution. It may be fun just to investigate and experiment … all the best.
Always great subjects and best explanation. 5 starts Alan. :)
Thank you. We had real problems with intermod interference when I was in Omaha and Lincoln, Nebraska. It was not fun. N0QFT
Yeeeeeeeeeeah. Glad to hear from you again.
Clear and simple - well done.
Very well explained
You got a way with making this make sense thanks much!!
👍👍👍👍👍Your explanations are clear, simple and very straightforward. Wish had a teacher like you when @EE college back in early eighties.
Do you people in USA, also have this issue of clumsy professors? Or is it limited to developing world only?
There are both good and clumsy professors here too.
Thanks for yet another great video.
Your videos are top notch. 💪
Thanks
OY1R
Excellent video. But I've one question - if I want to understand why 3rd order product amplitude increases by 3xtimes? How exactly this happens?
The theory can be found here: en.wikipedia.org/wiki/Third-order_intercept_point
You are legendary. wonderful video as expected! thanks.
Thank you! Stay safe.
That makes sense! I've struggled with this for a long time. So in your example, if you put in 10 dB of attenuation followed by 10 dB of relatively good amplification, would you end up with the original IMD component at -20 dBc where the signal of interest is essentially unchanged?
Assuming your amplifier has very good linearity...
Thanks Alan. Great explanation. Helps me understand why RF colleagues always concerned about 3rd order inter mods.
On slide page 1, should the left hand yellow harmonic be f2-f1 ?
Also, can the 3rd order IMD land on the signals of interest? e.g. (f2-f1) + f1 => f2, which I imagine would be really a problem.
Shouldn't it be f2-f1? Otherwise the freq would be negative? Assuming that f increases from left to right. Anyhow, a great video, as all of yours!
Very good explanation! 73s
Thank you! Great explanation.
4:45 kind of confuses me. I thought the IM3 would decrease by 3 times the attenuation only if the attenuation was applied to the fundamental signal that "created" it (in this case if the attenuation was applied before the fundamentals entered the mixer) as shown at 3:50. It's interesting that the IM3 product continues to experience this 3x relationship even after the IM3 leaves the mixer.
w2aew is back and he worked so hard during his hiatus he's got himself a Tek 5 series! Can't wait to get back to great little snacky lessons from one of our favorites.
It’s a 6-Series actually (belongs to my employer, of course)
@@w2aew
I figured as much even with the 5 series (my original comment was tongue in cheek)! But with the 6 series is appropriately more expensive. What an impressive instrument.
I thought you had a 4-channel version of the 5 series scope, but I suppose they must have designed a new ASIC which does an additional time-interleaving step to get the bandwidth and sample rate up that much higher?
I can't wait to see what fun things you can show with it!
I wish I could have kept my nerd cred intact, but I see there was precisely one way for me to notice this couldn't have been a 5 series scope: the measurement at the top that says "3.99 GHz".
Actually, the data acquisition ASIC can natively do 25GS/s, hence the 8GHz BW on the 6-Series. It also has a low noise preamp, giving it about 10dB lower noise floor than the 5-Series. Both have the digital down converter in each channel, hence the fast update on the spectrum even with the 50Hz RBW used here.
Thanks :)
good to see you
In this video the harmonics are treated as IMD. I think this makes sense because they are generated in the same fashion as IMD. However, I think by strict definition ("inter") they are distinct. Does their amplitude increase in the same way as the normal IMD products? For example, is IP2 the same for the 2nd-order harmonics as it is for f1 + f2?
Yes, I should have made the distinction - the harmonics are of the same "order", but are not INTER-mod products per se. Yes, the harmonic distortion products, in theory, should vary in amplitude similar to the same-order IMD products.
So for audio, what frequencies should I cut more?
An excellent demonstration, though it still pains me to have to use nearly 20dB of attenuation on 40m!
Counter-intuitive, I know, but under those conditions, it's the way to go.
@@w2aew next I will find that a -135dBm MDS is not useful to me on 40m! An excellent video as always, thank you.
Thanks for the video, great help to help understand those often quoted parameters seen in rx test data. So, my question is this: if the two primary signals causing this in ones receiver are sideband, what does tuning to that third order signal sound like? Is it just a beat note, or does it just sound like garbage, like splatter? Thanks, always enjoy learning from you.
It would sound like garbage, and would occupy about twice the spectrum of a normal SSB signal, due to the 2x term.
Excellent video on 3rd order intermodulation distortion. Now I have a better understanding of IMD. I also have a video suggestion, but I am not sure how to send my suggestion to you.
Video Suggestion:
I have two wattmeter’s one seems to be correct on VHF and off on UHF. The other wattmeter seems to be correct on UHF but off on VHF.
Both wattmeters’ seem to be off by 25% to 35%. Is there a way to confirm what the true output wattages would be by using a voltmeter or some other method?
Have you made sure that you're measuring power when transmitting into a dummy load? If you have any reflected power at all, the measured power readings will not be accurate and will vary with frequency and line length.
@@w2aew Hi Alan, Yes, I am using a dry dummy load and old Bird 43 Wattmeter with the appropriate plug-in elements and an old Diamond SX-1000 Wattmeter.
I also own a Tektronix, 305 DMM Oscilloscope and a few new Digital Oscilloscope as well as a various VOM’s.
Is there a way to use this equipment to determine the correct output wattage from the radios to determine which wattmeter is accurate?
How could I use V^2/R to determine actual wattage of the radio? Then I will be able to calibrate the two wattmeter’s.
I am glad see you are back.
Thanks for all the great videos and your help! I hope you and your family stay safe.
@@Rich-fr5td I'm sorry, but there really isn't a good way to accurately measure power with the 305 scope or a VOM. And, unless your digital oscilloscopes have a rated bandwidth in excess of 500MHz or 1GHz, then they can't be used to check the UHF signal either. The only practical way for you to do this would be with a known good wattmeter, and compare with your suspect meters.
@@w2aewAlan, thank you for your time. Please keep up the excellent videos. I wish you and your family well.
NOOOOOOOO... Please bring back paper and pencil! Along with the bench background, they added to the atmosphere.
Actually, this video was an offshoot of some things I was going for work, hence the more "professional" look.
Use whatever format you like if it will help you produce more great content.
and the audio used to better in the traditional formats but if that is basically recycled content thats a non issue
@@urugulu1656 Yeah, just leveraging some other work I was doing, so it wasn't my normal workflow for my channel...
What is the equation of DQPSK modulation? please
Can IMD be present in vintage tube amps? Thanks for the great videos Alan!
Sure it can.
@@w2aew Thanks! Would you say that IDM is always a bad thing or can it fall into the "desirable clipping" catagory regarding guitar tube amps?
@@fullwaverecked Certainly for audio applications, distortion can be a desirable effect. For RF transmissions, it is definitely NOT! For RF receivers, it can result in reception problems.
@@w2aew Thank you!
@@w2aew absolutely agree. You are a trusted source or I wouldn't bother you so much, but you are the the trusted source for us beginners. Question: I am building a dim bulb tester/current limiter w/3 40w vintage 12" x 1" bulbs, (120 watts =1 amp thank you Mr. C,) but it takes up too much space on my bench. So instead, one 40w bulb (for visuals) and two 40w power resistors in siries. Could one use this setup as a dummy load for testing a rudimentary dummy load indicator? For testing output for an amp for example.... Thoughts?
TY ALAN!!!
Thanks for the information ,there is no imd on this channel.,
thank you
So how much attenuation for mixers is best to defeat IP3 ? 3db,6db,10db attenuator ?
There is no single answer for this because it depends on many factors including the magnitude of the signals involved and the design of the mixer and associated circuitry. The best answer is - keep increasing the attenuation until the negative effects of IMD are reduced sufficiently for your needs, remembering that the IMD level will ideally drop by 3x your attenuation setting.
Thanks for the Video And 73 from K4TEP look forward to your next video