Measuring RF Power With an Oscilloscope & Other Instruments
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- Опубліковано 1 сер 2024
- In this video I wanted to see if an oscilloscope could be used to accurately measure the RF power from a typical HF transceiver. It also shows how you can verify the accuracy of your existing watt meters and even your Bird slugs. I apologize for it being a bit long but hope in the end you find it useful & entertaining. 73
- Наука та технологія
I enjoyed your clear discussion on power measurement techniques and am glad that you brought out the need for accuracy. I too have a
Kenwood TS-2000X transceiver and wish to use it to operate a microwave transverter that I designed. My concern is power transients
and their duration upon transmission, preceding the steady state condition. Have you observed such behavior? Can you recommend
a method of performing such a measurement? I was thinking of triggering my oscilloscope using the PTT pin, but am uncertain of the
procedure. 73's, AA0BR/VE1GR.
Hi Phillip. Glad you enjoyed the video. Yes using the PTT as the main trigger source would be how I would start. You can always use delayed triggering to ensure you capture the start of RF output.
I would use a directional coupler to sample the RF and then attenuate that RF as needed not to damage the scope. Try it out. If you run into trouble let me know and I will see if I can set that up and make a video. 73
No, you taught us more than your power measurement process. Learned about your instruments, language of RF electronics, and a few more things. Pace was good, it takes time to absorb the information.
Thank you for taking the time to comment and glad the video was of value to you.
Thanks for showing different equipment.
One of the best training videos I've ever seen. It is timely that I found and viewed it as I need to calibrate a power sensor in the most accurate way possible. That you gave us equipment model numbers is icing on the cake. Very interesting and easy to follow from start to finish. Thank you for giving us this lesson.
Hi Ted and thank you so much for taking the time to comment. I am very glad to have been able to help out. Good luck and let us know how you made out in the end. 73
There are several tidbits in your video that are a must watch. Thanks for sharing!
Great and perfectly job ! 73 from Brazil.
Thank you so very much for this extremely informative video. I watched it a couple months back, but was overwhelmed with so much gear. This time I understood much more, and even bought an ebay hp35b. It should be a big help. Again, thank you for your un-hurried interesting discussion/explanation.
Hello there and thank you so very much for taking the time to watch and to comment. I am very happy that you found it useful and of help.
I climb cell phone towers and this is so educational
Thanks for the feedback,,,,,stay safe !
That was a most interesting video!
I enjoyed it immensely Gedas! I got a lot more out of the latter part, where you were explaining the Math and proving the theory out. I love ❤️ Math, but so difficult for me! As discussed, remember that once you set the Element to 4/5 scale, that you perform Scale Shaping at 2. 10. And 30 MHz. If the reading anywhere is greater than +|-2% of FS defection, then readjust the Element until you have achieved scale shaping on 2, 10 and 30 MHz at 1/5, 2/5, 3/5, 4/5 and Full Scale. It is possible and normal that due to the diode used, the Element can be too sensitive or insensitive which messes with Scale Shaping and FS Deflection. It's always something!
Thanks so very much for taking your time to produce this video, not an easy chore by anyone's book! 73's David de K8KEM l
Hi David. Thank you very much for taking a look at the video and leaving some feedback. I look forward to re-calibrating several of my slugs and feel much more confident with all your guidance & advice ! TU agn es 73
Loved it
TNX for taking the time to make this video! oh, I didnt fall asleep at any time !
Awesome....there may be some hope for me yet ! Thanks for the feedback.
The math part almost put me to sleep but you did it in a way that that wasn't drowsy lol ! Be interesting if u fired up the ol TEK scope and see what it sais !
I still have the one that I got from you and picked up a dead one from where i use to work at. should be an easy fix, has short on one of the supplies, probably tantalum cap shorted, very common problem with these scopes 73
Thanks for the video. I'll be running this test tomorrow using my Tek MDO3012 scope, then I'll try to work out the ratio of my w2aew RF sampler I built.
Sounds great & do let us know how things turn out. As mentioned in the video pay real close attn to the BW (and sampling capabilities) of your O-scope. The amplitude accuracy starts to quickly drop off long before you hit the "rated BW" of most scopes. For example a 100 MHz scope is usually already down 3 dB (or even 6 dB sometimes) by the time you hit 100 MHz. You will see the signal and be able to sync to it but it's amplitude will read low. 73
Thanks w8bya. My scope has 500MHz bandwidth, and I'm only testing on HF. The testing went well. Although the attenuation of my homemade w2aew resistive RF sampler varies across the bands. It works great as an RF sampler, just not good for power measurement. More work required.
Hey that sounds great. That scope looks like a real beauty. One thing that you could get for less than $40 many times on e-bay is a nice directional coupler rated for 100W in the HF range. Narda makes many for example and the net is always full of used ones.
This tutorial was simply SENSATIONAL 👌..
I think you are definately the person to do a similar video on SSB 2 tone procedure and generating an S9 signal with the cheaper signal generators out there , eg Feeltech FY 6800 etc, as there minimal voltage out is .0001v.
Much appreciated
✌️73oz😊
Thank you for taking the time to comment and making suggestions. So many projects, so little time :-) Let me see what we can do. 73
@w8bya that's OK mate I have nearly everything sorted, understanding dBm took me ages..
HANGING to find a FLUKE 8920A , not having to worry about a matching power sensor is a win win.
✌️from 🇦🇺 🪃
@@gixerags750 Awesome ! Just remember that the instrument you end up with has enough bandwidth to accurately measure the RF at the frequency of interest. For example the 8920A would not be a good choice to measure power at say 10m or 6m etc. since it only has a 20 MHz BW. 73
@w8bya DRAT ! , I'm an 11m ChookBand fan boy...oh well ,back to the options.
Peace ✌️
@@gixerags750 No worries. You know here is another possibility for you to consider. During the last 1-2 years there has been an explosion of really cool, small, & portable VNA and SA units for super cheap prices. If you wanted to get a pretty reasonable power reading you could try padding down the RF output (by a known measured amount) and feed that into one of these new portable SA's. You would be within a dB I imagine of the "real" power level.
Nice video, very informative.
TU for the feedback and glad you found it useful.
Fascinating and I enjoyed every minute. Thanks for such a detailed video. 73, Jamie VK2YCJ
Hello Jamie. So glad you enjoyed the video, thank you for taking the time to comment. 73
I'm saving all this for reference. By the way. After converting P-P for the Ohms Law measurement, you should have compared it to the actual ERMS measurement, that your scope also makes and skip all the calculations.
Skip all the calculations? I don't think so ! There are so many folks out there that do not have "fancy" scopes that I felt it important to show how the calculations can be made with virtually any scope. I did not want to leave them out in the cold.
Good topic you have covered, interesting and useful for me. Once clarification, as i can see when the measurement is being done in FLUKE where the max.frequency limit is 2mhz. in this video are we measuring a CW signal or demodulated CW signal. OR a CW signal below 2mhz ?
This Fluke meter has a BW of 20 MHz. The HP-3400 has a BW of 10 MHz. The test signal I used here was an unmodulated CW carrier.
Very nice video - it's always beautiful when all adds up right :) I'm just a little courious about the formula part at the end: Is it common to use the letter 'E' as formula symbol for voltage? I'd think it could be mistaken for electric field strength. We mostly use 'U' here in Germany, I've seen 'V' is used often in the US. 'R' for resistance and 'I' for current seems quite everywhere the same..
73 from DL
Hello and thank you for your FB comments. Depending on how old you are and which school you went to the standard definitions in Ohms law used the letter "E" to signify the voltage. I suspect it goes back to many, many years ago when voltage was referred to the Electromotive Force (EMF) and for very short just E was used. Thus it was/is common in Ohms law to use E like in I = E/R, or R = E/I, or P = E x I, etc even today. So to be consistent with Ohms law I like to use "E" to designate voltage but I also find myself using "V" for times when reading the voltage on meters & scopes etc. So you are right.....it can lead to confusion when using different letters for the same parameter, esp if one starts to think of E & H fields hi-hi. TU for taking the time to look at the video & for taking the time to comment. 73
Just discovered your videos, absolutely fascinating, quite amazing consistency between the results. Was partially hoping you might try the diode probe approach to get Vpk. Just curiosity really how well it works when signal swamps the forward voltage drop region of the diode. Regarding the Bird 43, it might be interesting to perhaps test at 2Mhz intervals across the range of the slug, not sure they will be so optimistic at 4Mhz or so. Great video, well done
Hi David and thank you taking the time to comment. I am glad you found the video of value. I have not modified the TS-2000 to XMT broadband and don't have a way to easily generate high level RF across a wide bandwidth. But yes I suspect the slugs are not completely flat and do have ripple.
w8bya, thank you for the swift reply. I must admit that I had just read the Bird 43 or 4304a manuals a few hours before finding your video. One of the test/cal procedures did involve taking measurements at multiple frequencies and doing some "averaging". (I suspect this was the 4304a as it has the wideband slug). I found the video excellent and it was amazing to see such correlation between measurements, of course you have data on various parts of the set up. Regarding the Kenwood not being general coverage transmit I was thinking more along the lines of seeing how the 43 behaved across the HF bands but I expressed it badly. 73s
@@g0fvt No problems David. When I saw 2 MHz intervals I had assumed taking something like the HF slug and testing it every 2 MHz. That would actually be a neat test. As a point of interest one of my viewers here was a line technician at Bird and performed the calibration on the various slugs. He and I talked about it over the phone many times. As you say it is not all cut-n-dry and involves a bit of talent to tweak the coupling and make it meet specification. He said some slugs were very problematic and when other techs could not get them to pass they were given to him because of his many years of expertise. He has long retired but it was fascinating to hear the story of these components. 73
@@w8byathanks for the reply once again, I think many of us that have used them for a while have a great fondness for the Bird 43 and it's relatives. It is unfortunate though that a useful set of HF slugs works out expensive, so despite having 7 Thrulines and a few Termalines I need other power meters. One of my personal favourites is the MFJ-826B, sadly one of the negative reviews on Eham cited that their 100w radio measured 130w PEP on one of these meters, so they concluded it was the meter's fault! Thank you for taking the considerabe time to make these videos, I have been a subscriber to W2AEW's channel for years but only discovered yours yesterday. IMHO the amateur radio community needs more intelligent discussion, for a science based hobby it seems to have been permeated by a lot of witchcraft.
73s
@@g0fvt Yes I agree with you 100%. There are WAY too many "self proclaimed experts" out there who will put out information or a video and have no clue as to what they are talking about. And unfortunately those who may be ignorant about the subject take it in as fact. I always tried to teach the engineers & technicians who I taught & mentored when I worked that is it absolutely perfectly ok to tell some "I do not know" or "I am not sure" rather then to spit out any answer to save face in front of a senior engineer or management......Sometimes the pressure is so great that an answer or opinion is provided with a great deal of uncertainty. Also, yes I completely agree, W2AEW is an awesome gentleman who makes terrific videos. I am not one of those people. I have found it is VERY hard to make a clear, intelligent and succinct video that does it's job. My hat off to all those, like him, who have that talent. 73
Meant respectfully, your video confirms the old aphorism that a man with one watch knows the time of day but a man with two watches is never quite sure!
I found the fact that the Bird 43 slugs and the meter itself can have as much at a +/-5% error intolerable for the price they charge. I keep my bench scope calibrated down to a gnat's nut and go with that. The Diamond SX-100 and SX-200 meters are surprisingly accurate for a nearly FS reading. I had a Mirage that was dead balls on but I would not touch an MFJ with a barge pole.
That said, my pastime is QRP and QRPp and the need to measure in milliwatts critical.
But excellent video as it shares your thoughts and ideas in a complete and informative manner.
71/72 de k6whp
dit dit
Hi Bill and TU for your comments. Yes I love the 2 watch saying as well hi-hi. Maybe we can amend it....and a man with 30 watches has the best chance of knowing the time statistically speaking . But it was exactly that point, the fact that my different slugs measured so differently that made me want to experiment with different measuring techniques to see who was telling the truth. The nice thing about the bird slugs is they can be easily recalibrated.
Another thing many do not realize is that depending on the "topology" of the PM design it may become quite useless as an absolute power reading meter if the load is not very close to 50-ohms resistive. I will keep not only the Bird's around but even the MFJ's as they both have their purposes in my shack. The MFJ's are used only for peaking the PO of my HF amplifiers as I QSY bands & must retune them. With my bad eye's the large meter works great. The Birds I love because they are so robust and durable & could be mil-spec ready for field use. In any event, glad you enjoyed the video & appreciate your comments. 73
Gedas, I happened to stumble by and read your reply. Last time I neglected to mention that my lab scope is ALSO a venerable 7704A! You are 100% in to your assertion re the different wattmeters' worth -- so long as the user is aware of the relative drawbacks. For examples, one cannot drag a 7704A out to the field with a generator, so a Bird or MFJ that has been suitably tested and calibrated is just the ticket.
I also belatedly want to to thank you got your algebraic derivation of the Vpp to power formula. I was with you my long-ago dissipated math skills!
Oh, and as to 30 watches? I have a small collection (~12) of Casio timepieces. And I am never quite sure. I rotate their use of weekly and hack them to WWV!
I AM CERTIFIABLE!
VY 73 DE K6WHP
regarding using different instruments ie two watches anology, in first year physics class they teach uncertanty principles and you can calculate them and end up with a +- figure. I had forgotten the gist of it.
Bird forever ! 👍👽👍
I do love my Birds !
Appreciate these videos, I bought that Siglent Spectrum Analyzer at Dayton this year, as well as a Siglent 200Mhz scope. I am just an average guy no engineering education, I just like to play engineer in my spare time! Your videos are helpful in showing me practical ham shack applications for my investment in Siglent test gear! There are many educational UA-cam videos, but not many using the same spec an I have, and I totally need help to better understand what it can do for me! 73 de Seth - W8FG
Hi there Seth and thank you for taking the time to comment. I am really glad you appreciate & find the videos of value. If I can help out or illustrate anything in particular in the future do not hesitate to ask. 73
WOW! what an excellent video and tutorial. I have been wanting to learn/do this exact exercise in measuring RF power. Hats off to you sir. Thank you for sharing!! 73, n7kbc
My pleasure. 73
Excelente. Thanks.
Your very welcome.
Besides the fact that i fell asleep on my keyboard, I did learn alot here! thank you
You are welcome. Sorry for knocking you out ! Any suggestions for me on how I can minimize torturing others?
w8bya I've been starting with UA-cam videos recently and I also have a hard time. But everyone says to bring the energy level up. Even in your voice. Idk how but gotta bring excitement into your voice
That's gotta be it......just noticed this video got it's 1st thumbs down today :-(
that stinks!! I wouldnt let a thumbs down get ya down though! I wish that when they leave a thumbs down they need to leave a reason! then we could narrow down what we gotta do to keep the videos strong! Its tough because Im doing the same thing, explaining what is going on while im thinking and building and its not easy to do those 2 at once, Maybe try narrating your video during video editing (if youve got the time), that way you already know what you are going onto next as youre explaining it all. My advice may be bad, im not sure, your channel is doing much better than mine is! I have been just watching and reading on how to attract more people and keep them hooked. Your content itself hooks the viewers so I'd say just work on getting it a little upbeat, toss in some dad jokes and such!!! i dunno lol!
Thank you very much for all the great suggestions. I make an effort to go slow because I notice when I watch videos and trying to learn something new or see something critical for the 1st time they normally whip past it so fast that I have to pause the video, rewind it, and play it again. So I thought if I go slow & steady I would be helping others out best. Oh, I also wish when people left a thumbs down we at least knew who it was. Tnx agn es 73
Nice video...would have liked a bit more calculation and explanantion on how you came to the 30db and 60db ateenuation for the Kenwoods 90watts...
Cheers
Glad you enjoyed the video. So the 1st attenuator, the big bird 30 dB was used because it was the only pad I had that could handle 100W. Remember if you put in 100W into that 30 dB attn you will only get out ~0.1W. That means that 1st attn must be able to dissipate 99.9W ! If that Bird was a 40 dB I would have used it then adjusted the value of the 2nd attn accordingly. Likewise if it was only a 20 dB pad I still would have used it because that first attn is the one that will have to dissipate the most power.
Ok so then we move onto the 60 dB pad.....As seen in the video I mention that 100W equates to +50 dBm. You must add enough attenuation to that +50 dBm signal so you do not damage your SA or power meter sensors etc. The SA for example can safely input a +10 dBm signal and since I already knocked down the +50 dBm signal by 30 dB all I would need is an additional 10 dB pad and I would be good to go. BUT, because I was going to also measure the power with various sensors that could not handle so much RF input I used that 60 dB attn which was handy. The very last thing I would want to do is blow up my 8484 sensor etc ! So you can see you have some flexibility in what the value of that 2nd attn can be as it depends on the instrument you plan to use and how much your 1st attn was able to knock the signal down to. I picked 60 dB because it allowed me to get reasonable levels on all my instruments. Hope that made sense. 73
@@w8bya thanks very much for the detailed answer...i need to study this to understand dbm more.
Much appreciated....love your setup especially the Boonton and HP power meters...seen a few for sale but none with the the special input adapters..
Great channel.
@@gixerags750 Feel free to e-mail me directly at w8bya at mchsi dot com and I will be happy to help you get set up with some neat basic test equipment.
Your my lil slug!
What brand model spectrum analyzer are you using?
Siglent SSA 3021X for these measurements. I also have an Agilent E4408B that I use for uW measurements.
@@w8bya thank you
Can I make a dummy load? When testing?
Sure. Just make sure it does change it's impedance as it warms up and is very close to 50 ohms to keep the math easy.
You can actually calibrate your Bird slugs (not sure if you were going to simply characterize or physically calibrate, that may be where you were headed) with some minor surgery and an accurate source. I had to do that to one of my VHF slugs that was way out of calibration...little scary at first opening one up but once you get the hang of it not too bad. 73 - Dino KL0S
Yup, QSL Dino. That was the plan here, give the slug a lobotomy and carefully adjust the trimmer and align a bunch of them. 73
Wow Dino!
Hi GEDAS, thanks for the video, excellent material! Not sure if you noticed but when you totaled the Siglent Spectrum Analyzer power you added incorrectly. You wrote down a sum of 49.62 dBm when it should have been 49.64 dBm which results in 92.04 Watts and leans a be more in the middle of the others. 73 AI4Y
Hi Richard. Yes sir you are absolutely right. Thank you for catching that .02 dB. A lot of people would consider it pointless to look at such small differences but it all makes a difference. Tnx again. 73
@@w8bya As a learning exercise for myself I spent a couple of days trying to correctly report the output power from my IC-9700 for 145MHz and 435MHz for RF Power % 0-100% in 5 percent increments. I really wanted to know if RF Power % on the Icom transceiver meant watts or a percentage of gain so I set out to answer that question. I also wanted to know just how much power I am sending out of the transciver when transmitting to Satellites. Is RF% 10 really 10 watts for example on each band.
My testing was done for both 145MHz and 435Mhz frequencies used by the majority of the LEO's uplink and downlink. I produced a CW tone into a Bird 8323 -30dBm attenuator followed by an HP8495B DC-18GHz 70dB step attenuator set to -30dBm and my Siglent SSA3021X. I used two pieces of 30" StormFlex 141 (rated to 18GHz) with N-SP connectors and a new 10' LMR-400 cable. For the life of me my output calculations were no where close. Eventually it occurred to me that I needed to account for all attenuation and insertion losses not just the attenuators. So I characterize every cable and both attenuators and I was able achieve my goal. Largely with the aid of your video so Thank you!
I also tried using my Bird 4431 meter which I had installed a PEP kit in and it was in the ball park but an analog meter is a bit crude by comparison to the Siglent using MAX Hold. I also tried using a Bird 8251N dummy load fed with an N-Tee much like you did and using a probe tip to bnc adapter to connect a Tektronix 2465B for Vpp readings and calculate the wattage. I used the 1M input impedance and various probes such as P6131, P6139A, P6063B but it seems my readings were way high or low on 2 meter. The readings were closer to being correct on 70cm but stray capacitance came into play above 400Mhz relative to my body coming in contact with the cord and/or moving the cord around. I tried 6 probes in total including a brand new P6139A rated to 500MHz with the same varied results. While I've been a fan of Tektronix for years, I love my Siglent Analyzer and may have to invest in the SDS2202 as well. I find I don't require a 4 channel scope anymore, so 2 channels will be fine for my use. The SDS2202 can be opened to ~350MHz w/o to much trouble in my opinion so I'm considering it; especially since it has a real-time sampling rate up to 2 GSa/s. At 350MHz when using a single channel it can still capture enough data points to be useful at a great price.
I agree the with your comments on the Siglent Analyzer you made in some of your videos. It is a remarkable analyzer and when you have all the options enabled you can do amazing things without too much effort. I decided not to buy the Reflection Bridge as I have a RigExprt AA-1400 which covers my needs and their AntScope software is extremely useful as well.
73 de AI4Y
@@richardcreasey401 A couple of comments: I was thinking back when I did the max-hold on the Siglent. To be fair I should have been taking average readings to be consistent with the way my eye/brain was gathering readings off the analog meters. I should have set averages to say 30 and go with that number not a max-hold number. Also, and this could be a topic for a future video, are the amplitude accuracy (or lack of) modern o-scopes. There once was a time when you could use an o-scope to make accurate RF voltage levels but I found that almost every scope I have tried becomes just a relative indicator of RF amplitude at even modest frequencies. The rated BW is at least already -3dB down.
I would also characterize your scope & probes and know their frequency limits wrt amplitude accuracy. You would be shocked at how quick the amplitude starts to roll off at even 1/4 the rated BW of a modern scope. Personally I am a bit disappointed in their frequency response. And then, as you have observed, different probes will result in different readings. Grab a calibrated sig gen and feed it into the scope and start climbing in frequency and note where the amplitude starts to roll off.
Also, I am with you.....years ago I used more than 2-channels but nowadays 2-channels meets my needs 99% of the time. I do have several 4-channel scopes (most modern is a Tek TDS 744A) with lots of BW & sampling capability but I will take my Siglent over it 10:1. The user interface and capabilities of the old Tek are dated but if I ever need more than 2-channels at least I have something (other then my ole 7000 series stuff).
Oh 1 final comment about the setup.....this method of converting the scope probe tip into a coaxial interface is a bit of a kludge and not really suited for frequencies about HF. If I were to try this at 2m I think I would devise a more direct and fully shielded method to sample the RF......as far as 432 MHz.....ur a very brave sole for trying this way up there . The stray RF must be significant and I think I would use at least 5-6 type 61 ferrite cores and make sure all of the cables have a CM choke at each end to help dampen the RF floating around.
Hello ..
congratulations for the videos and I did not fall asleep at any time. The truth is that at some point I still had that concern about precision when measuring power. configure a device to 100 watts then an attenuator of 30 db plus another of 30 db and apply it to a tektronixs oscilloscope and the port at 50 ohm that seemed more direct and less lost by cables and connectors and eureka! measurements were similar with an ifr 5970.
I would like to see a video of you with the bird tuner and directly to the oscilloscope and see your results.
Thank you very much for your video. !!
Enrique Vidal
CE5LBM
CHILE
Hello Enrique I am glad you did not fall a sleep :-) I do not understand the term "Bird tuner"......can you please tell me what this device is?
I get asked this all the time and people should be very clear on this point. When your using any instrumentation like this where the power terminates in the instrument the instrument is your dummy load. And being the dummy load you need to be very careful so you don't blow the front end out of your SA or sensor head. Making sure you add enough attenuation is critically important.
For sure Mike.....good advice !
Is that what happened to your old one?
@@rabbibacongrease88 no old scope still around and get used here and there. I have been lucky and can say I have yet to fry a scope. 73
Ok. I think I got it.
ZzZzZzzz eh no i was watching honest, the bit about "using radio waves to fry peoples brains " erm that didn't happen did it ?, darn it was only a dream,Doh , I still have a CRT scope i built when i was training back in 81 ,!(1981 that is not 1881 lol) ,you can never have enough test equipment. Many Thanks. take care all, Mac
Hello there, thanks for this video. It was very very interesting and exactly what I was looking for.
I was running some tests yesterday cause i bought two used bird43 and I suspect they’re not very calibrated. Also taking the same slug from one meter to the other shows 10% difference. I’m not happy with that. . I have an AC millivoltmeter (Racal DANA 9300F) plus a Boonton 93E is on its way. Bought a sig gen not long ago, my very first, and it’s a Marconi 2022E. The problem is I don’t have a reference to start from. All instruments are used.
Furthermore I have very little experience but I’m getting more interest in electronics now after been ham for 20 years or so.
So I’m buying some equipments and trying to understand theory behind.
Here’s my question: at some point you said that a lot of people don’t realize you need a feed through to connect your voltmeter, in your case the Fluke 8920A. So why is that? My AC millivoltmeter has 1meg input impedance and I have connected it with a T across a line going from my rig to a 50ohm dummy load. It seems to work. Is that connection correct? What’s the purpose of that feedthrough that you mentioned?
Thank you so much for your kind answer 🙏
Looking forward to it.
73
Hi Diego I am very happy that you found the video useful. If you want to make accurate (absolute) measurements then the entire system must be 50 ohms (source, transmission lines, terminations). You can use a T-adapter and dummy load at the voltmeter as long as the dummy load is very accurate and 50 ohms. If your load is not exactly 50 ohms then your readings will be off. Most good quality 50 ohm feedthroughs will be accurate for these readings. If you did not have a 50 ohm load at the T adapter your readings would be off.
@@w8byaThank you for your replay!
I’ll play around a bit and try to reach a solution for doing some calibration but I’ll need to buy some new equipment I guess to have at least a reference for all the rest. Any suggestion? How could I calibrate the birds 43? I mean as I said the same slug on two different instruments show 10% difference. Let’s say I test the meter with a circuit for 30uA FS and is not accurate, can I calibrate it in some way or I’ll have to buy a new instrument?
@@IZ0MTW That is a basic problem every lab faces. That is obtaining a known standard. This is why their are calibration labs. The Bird 43 series power meters are rated for +/- 2% accuracy and the elements rated at +/- 5% (full scale). As you can see you can have quite a range and still be in spec. You can either buy a more accurate power meter or have your signal generator or power meter calibrated by a trusted lab. If you needed a quick and dirty power level that you could "most likely" trust w/o sending off your TE to a lab I would suggest purchasing a HP 435 or 436 or 437 etc. The 0 dBm 50 MHz standards on these meters are usually very accurate even when they have not been calibrated in a while. I am not saying you may not get a lemon but of all the power meters I have owned those 0 dBm standards were VERY close to being dead on.
@@w8bya Hey ok thanks for all the nice tips! I loved to watch your long videos, it was like a full immersion in your lab. Today I’m gonna buy some wood to make a table for the instruments. It’s exciting to start playing with these things, especially now with this covid situation where we have to spend most of our time at home. Cheers!
@@IZ0MTW Awesome and thank you for sharing with me your thoughts and opinions on my videos. I get some negative criticism from some people for talking too slow and not getting exactly to the point. I respect their opinions as well but I make my videos that way I like to see other peoples videos as I like to learn from them and do not like to be rushed. One very important tip if you are going to make your own work bench....MAKE SURE YOU MAKE IT DEEP ENOUGH ! As you can see in some of my videos the TE can take up a lot of room (front-to-back) and this does not leave you much room in the front of the TE for your experiments and gadgets being measured. I suggest a bench at least 4' deep front-to-back.
At first, you used the Bird 30 db attenuator with the 60 db attenuator added in series. Why did you switch out that system to a straight dummy load when measuring with the oscilloscope? I like the idea of keeping ~ 90 db of attenuation ahead of the scope for protection.
By the way, the video was absolutely great. Not too long at all. I learned a ton from it. Thanks.
Finally, it's funny how everyone bad mouths the MFJ equipment but in this test it outperformed the Bird. I don't understand why MFJ gets the bad rap, unless it's because the Bird is $1000 plus with the plugs and the lowly MFJ is maybe $150 and no plugs required. - status symbol maybe? Unless willed to me, I'll never own a Bird 43 on my income.
Hello and thank you very much for taking the time to comment. Anytime you have a question do not hesitate to ask.
The reason the 90 dB of attenuation was removed is because it is not required. In the case of the oscilloscope measurements we are measuring the voltage ACROSS a 50-ohm load. The scope can safely handle this amount of RF voltage so no other attenuation is required. Just be mindful about the 1x/10x settings on the probe and esp the 50-ohm/1-meg-ohm setting on the scope itself when making this kind of measurement. Very easy to wipe out your scope if you leave it in a 50-ohm setting. That is why we use the 50-ohm load for a termination ! It can handle the power, the scope cannot.
In the case of spectrum analyzers and "series" type power instruments (unless it is designed for high-power input) we must insert extra attenuation to protect the sensitive detectors in the power head. Remember some of the test equipment are meant to measure very low levels of power only so extra attenuation is required and be accounted for.
So in short, to answer your question, the extra attenuation is simply not required and we are at no risk to damage the scope.
Regarding the MFJ.....I have owned a dozen of their items. Here is my take: The ideas they have for products are brilliant and they offer so many neat gadgets for an affordable price which is really awesome. Where they do "fall down" is in their build quality and quality of components. If you open up many of their items you may find that many components are marginally soldered in and many cases parts are even loose etc. In the case of this power meter the SWR "set" pot is 100% useless and it is all but impossible to line it up where it needs to be in order to make a measurement. It MUST be a 10-turn pot. That is a mod that I will be doing for sure.
Now, as far as the bird goes.....well, one thing they have going for them is they are very rugged. I hate to admit so but I have dropped one of mine at least 4-5 times in the last 20 years on the carpet floor and it has not affected it's looks or performance in any way. I suspect if I were to drop the MFJ, like I did the bird, esp from 5' high, well....I fear I might have destroyed the plastic meter face or messed up it's calibration. Accuracy on the birds is usually better PROVIDED you have a known calibrated slug. If I had to pick only one meter it would be my birds. It is esp handy and more accurate when measuring power into loads that are not exactly 50 ohms. Given a slight SWR the MFJ (and other meters like it) can quickly provide inaccurate power measurements while a bird will provide more reliable data. That actually gives me an idea for another video. So there are pros and cons to both, not to mention price as you mention.
I love having them both ! I always leave one MFJ on my AL-1200 when I tune it up and want to make sure it is putting out a kW and want to quickly check SWR. Likewise I always keep a bird in line after my 2m kW for the same reasons.
Thank you again for your kind comments and question. I hope I have addressed them properly. 73
@@w8bya Great response. Keep up the youtubing...
I did not understand at all how you got -20 dBm on the 435. No number corresponds to what you were reading.
I think you made another post where you said things finally made sense about where the -20 dBm signal came from. Let me know if you need any more clarification. In short it was just an arbitrary level I picked and set my sig gen to and had each of my "traditional" power meters verify as in fact -20 dBm. I could have picked -30 dBm or -10 dBm, it would not have made any difference. Again, only used to give us some confidence that all the meters read the same level. 73
Thanks for responding. I was having difficulty seeing a 20 on the meter scale. I didn't realize that, once you picked the range, the scale reading just shows a deviation value from the 20. I am used to scales where the range value is part of the scale.
Wow, the take away was that the Bird was the least accurate and good ol MFJ was the closest, who would have predicted that
Hi Mark. Yes it was a fun series to run and to see how different meters & techniques varied the results. Please keep in mind that these results were with a very small test sample so we need to be careful making generalizations. Also, if I dropped the MFJ off the desk onto the carpet floor the calibration pots might quickly alter these results while the bird would probably fare much better (don't ask me how I know this). Another neat test would be to see how the two compare at 4-5 different power levels and on 3-4 different ham bands ! My gut tells me the Bird would be more accurate, overall. I would not go out on a limb & say the MFJ is always going to beat out a Bird meter. I also suspect if I were to measure the power into a load with, say, a 2:1 SWR things would get very interesting very quickly.
Also, I suspect if I calibrated each of my bird slugs & meter (or purchased them brand spanking new or calibrated by a reputable cal house) the results may have been quite different. And, someone else's slugs will with 100% certainty read differently then mine if not new or calibrated.
But like you I was a bit surprised how well the big faced meter did. I am really glad I have them as they are easy to read from a distance esp as I tune up an HF amp etc. Thanks for taking the time to look & comment Mark ! 73
Nice vid, but you made the same mistake i did and by luck did not fry my front end.The inputs of the scope are rated 400 volt pp but at DC (!), at rf frequencys there is a downfall the derating graph, the probe has a derating spec and the scope also has one, look them up and do the math. My setup can handle 200 volt pp at DC but >100KHz it derates down to 30 volt for the probe and 5 volt for the scope, i stuffed 100 volt pp in it and it survifed ;D
Hi there. hmmmm....Not sure what mistake you are talking about. During my experiment using the O-scope the scope was never used in the DC mode and no DC voltages were ever applied to the probe or scope input. The scope was always in the AC mode and always in the 1 meg-ohm input impedance setting. The 50-ohm reference was being established by the external Bird 50-ohm load not the scopes internal input. The scope probe used was well within it's specified operating range before any derating had to be considered. The scope probe has a -3 dB BW of over 300 MHz and we were measuring only a 14 MHz pure RF signal.
I was half way a long post to explain when i searched the manual for your scope i found thiss and it explains it all : www.eevblog.com/forum/beginners/scope-maximum-input-voltage-(dc-an-ac)/
Your probe is exactly the same as mine, here’s the manual and derating curve of my own probe: cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_manuals/gb_1/r/rt_zp/RT-ZP03_UserManual_de_en_02.pdf
Look at the derating curve and the notice next to it, the imput impedance of your probe derates to 50 volt rms max at 14 MHz, if i understand your setup correctly you have put 90 watt/67 volt in the probe and 6.7 volt in your scope at 14 MHz.
The imput impedance of the probe and scope drops from 1 Mohm at low frequencys to maybe a few Kohm at 14 MHz and so you fry your probe, or scope :-€
Ahhhh, I totally understand what you are saying now. Sincerely thank you for pointing that out. I can honestly say I was not aware of such a quick fall-off of the probe & scopes specs as far as Z(in) and max V(in) was concerned. Thanks !
Youre welcome,saved another scope :D, i just discovered this myself and am now contemplating a solution as the scope is my means of measuring rf power, i have a r&s hmo1202 and it specificly mentions in the specsheet that it derates to 5volt rms >100 KHz at the 1Mohm input and also 5volt rms at 50ohm which aparently does not derate this means basicaly i guess for most scopes
Looking at other scopes which have a 50ohm input usually says 5volt rms max@50ohm so i guess it is safe to say that 5 volt rms at frequencys above several MHz is the safe limit for most scopes and so if you have a x/10 probe its 50 volt rms or 50 watt in a dummyload.
An extra attenuator or high voltage-rf probe is the solution i guess, or a little attenuator circuit in my dummyload as i more and more understand that sticking a probe in a dummyload is not the best way to measure, as i understand it is better to have the scope terminated at a 50 ohms and use a voltage divider with dedicated bnc terminal at the source
I also have a couple of old Welz and Diamond vswr/power meters and I find them to be absolutely great . 73 Ian G7HFS/PA3IKH
Yes I love this old meter. I remember I was fresh out of college and it was my first SWR/PM that could do HF or VHF. I still need to calibrate it hi-hi. 73
@@w8bya they are so easy to recalibrate and built to last👍
@@ianharling9569 The only thing I did not care about the meter was I found the little ring nuts used to screw onto the SO-239 connectors were always coming loose and hard to tighten so I grabbed a 400W soldering iron and hard soldered the SO-239 and ring nut right to the back metal chassis. Never regretted doing that.
@@w8bya that never happened to my Welz yet but I now know what to do if that ever happens.Thank for the advice.👍
Video abit long for the info im looking for.
Brian, can you share with me exactly what information you were after? Knowing that will help me plan accordingly for future videos and where I went wrong with this one. Last thing I want to do is frustrate folks. Thanks
@@w8bya Trying to work out what the simplest way to measure RF Power With an Oscilloscope. If you could lay that out in the first 2 1/2 to 5 mins would be great. The length of the video is not so important. Just the opening of the Video lays out the theory and logic of what we are about to learn. FIY 10 to 25 min videos in my experience are key.A red box around your thumbnail and maybe a arrow pointing and something random will bring in view to, but if not relevant some would call that click bait. My 2 cents :-)
Also thx for that quick response. P.S. I'm trying to measure a QRP rig's output power. A HackRF One with a 3W Wideband LNA PA to be specific.
@@brianlink391 Thanks Brian. I found out just how hard it is to make a really good video. I take my hat off to those who have mastered it. I however have a LONG way to go LOL. The engineer in me wants to be thorough and explain from the ground up but I need to learn to be more succinct. TU for the feedback, wish more were like you.
@@w8bya As an engineer, we know that there are two equal opposites to everything(sine and cosine for example). You could record a 1 hour long video. From that video, you can cherry-pick the points you want to make in a more succinct way. Or you can create a bullet-point layout sheet and touch on each subject while your recording. While editing, use your bullet points to cherry pick them points. I think it would be a great way of condensing a long video down and ultimately, making it more succinct. :-).(voltage doubler) I explained it better the first time around but my word processor crashed and I had to rewrite this post over again. It never goes as well as it does the first time.
A lot of waffling for the first 20 minutes, a brief description would do, I just kept skipping through until the measurements started.
I try the best I can and simply want to be thorough. I am not a seasoned video producer like many on YT & sri it bored you. Did my waffling earn me a thumbs down?
I don't mind hearing some history about what I'm seeing.
I'm 5 minutes in and I have to say you don't have to justify your mfj meter with me. I love big meter faces for one, but I think exact wattage readings are over rated anyway. Plus the only one that has to like your equipment is you anyway.
Right on !!!!
Excellent & Thank You so much for the very Informative Video ⚡🙏⚡
You are very welcome. 73
False.
Scopes indicate voltage,not power.
An Oscope cannot indicate power.
Well of course they do not measure power directly but we can calculate the power based on the voltage which the video shows us how to do. Just like some "power" meters do. Hard to fit all that in a brief video title.
If the Resistance / Impedance is known, then the voltage can determine the power as was explained very well in this video.