#310 

The Outlook "message received" sound scared me for a second. Thought I was at work.

W2AEW is back! Lockdown suddenly seems more bearable 🤓

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.

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

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!

Glad you're back, miss your face. Back to paper! Thanks Alan! 73

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

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.

Great video Alan. My club was fortunate enough to have radio engineers in it. They did a cool demo with two radios and an old IFR to show the mixing products on 2m. It is very interesting that attenuation can bring out weak signals affected by IMD. It does seem counterintuitive. n3kca, 73

Perfect! Just what my Ham license students need to refresh IMD before their exam next week! Thanks!

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.

Definitely needed this to cement ideas in my RF class! Helps greatly👌🏿

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.

Dang you're good at this! Even I understand it now.

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.

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

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.

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.

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.

Great explanation of IMD. I now really understand why the IPO button on my rig is called what it is. Thanks Alan

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.

Thanks, Alan! Nice to see you back. I hope that you and yours are staying safe.

Great to have you back Alan

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.

Thank You Alan - excellent and informative as usual. Hope you and your family and friends are all safe and healthy.

Thank you so much for the video. Very informative especially while illustrating factor by which 3rd order IM is affected.

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).

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.

Glad to have you back!

I'm very glad to see you back.

Also rule of thumb roughly 9.6dB above 1dB compression point is the OIP3 point

I learn so much from your videos! Please keep posting such amazing content.

Yey you're back! Cant get enough of your videos!

Welcome back! Hope you and the Mrs. are doing well! Better days are coming soon! 73's de KC2RDU

Good to see you back!

Great info, as usual. Thanks for starting up posting again. I know life gets in the way of RU-vid, but your efforts are much appreciated.

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

Very instructional. Didn't think about the attenuation. Thanks for the video

С возвращением!

Always great subjects and best explanation. 5 starts Alan. :)

Glad your back Alan!

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!

Been waiting for a new video!!

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.

The thing is the mixer is a non-linear device. But we need this nonlinearity so we can get the down conversion to IF.

Excellent, as always. I always learn something here.

Beautiful! Tgif ! Hope to see more videos again soon ! Great to have you back !

Clean and relevant. Greetings from switzerland.

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?

Excellent detail on regarding IMD. Thanks for the clear presentation with examples and sharing!

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

miss u ... any video from w2aew hugely appreciated !! tnx and please keep going

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

Thanks for this. It’s a very clear and concise explanation and demonstration along with why it matters.

You are the best youtuber in my book! Thanks man! Very useful and very much appreciated!

Spectacular explanation and presentation OM!

RF == Black Magic. Thank you Alan for another great explanation.

Thank you Alan. Great explanation on a subject I didn't fully understand, much appreciated!

Like always, clear explanation and practical example. Thanks! ON9SG

Nice description and video. Well done.

Very well explained

Yeeeeeeeeeeah. Glad to hear from you again.

You got a way with making this make sense thanks much!!

Glad to see your back Alan! keep it up sir!

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.

You are my favorite teacher....

Thank God he's fine!

👍👍👍👍👍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?

Thank you. We had real problems with intermod interference when I was in Omaha and Lincoln, Nebraska. It was not fun. N0QFT

Thank you! Great explanation.

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.

Very good explanation! 73s

Thanks for yet another great video. Your videos are top notch. 💪 Thanks OY1R

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.

Clear and simple - well done.

You are legendary. wonderful video as expected! thanks.

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.

good to see you

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?

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.

He's alive !

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?

Thank you! Stay safe.

Excellent video. But I've one question - if I want to understand why 3rd order product amplitude increases by 3xtimes? How exactly this happens?

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.

Thanks for the Video And 73 from K4TEP look forward to your next video

Thanks :)

So for audio, what frequencies should I cut more?

NOOOOOOOO... Please bring back paper and pencil! Along with the bench background, they added to the atmosphere.

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!

When you say an attenuation reduces intermod by 3x, are you attenuating the transmitting signals, or the receiving antenna?

Hi Alan. Thank you so much for this video! I learned a lot. One question though, in the picture for the Third Order IMD you show 3f1 and 3f1. Should that be 3f1 and 3f2?

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?

Still a novice but are there methods of effectively "filtering" the intermodulation frequencies that are undesirable?

Can IMD be present in vintage tube amps? Thanks for the great videos Alan!

How can I characterize a Spectrum Analizer for non-linearity testing of IP3 and IP2 in an amplifier?

So how much attenuation for mixers is best to defeat IP3 ? 3db,6db,10db attenuator ?