Brilliant, simply brilliant. No more rules of thumb, no more wet fingers in the air. Now we amateurs can do what scientists and engineers do...MEASURE!
I watched some of your intro videos on the NanoVNA last evening and was laying in bed wondering if the NanoVNA could test some preamps and then thought I'll likely need some attenuators. Then I stumble across this video doing just that. I really wish the NanoVNA was around 20 years ago when I was in college. I might have done much better in my microwave circuits class. I'm just realizing that I did a 3 year telecom course and never got hands on with a VNA or a spectrum analyzer. Now 20 years later I feel like I'm finally getting it. Had a similar experience the first time I seen AT&T's similarities of wave behavior like 15 years ago. I feel like college gave me a really good foundation in physics and math, but the radio electronics end of things was pretty weak. I've also never really worked in the field. Spent the first 10 years(starting from 17 before college) of my career working in AV doing sound, lighting and video. These days I do science center exhibit design, fabrication and repair. I've been a ham since I was 16(25 years ago). I keep coming back to the hobby and having new breakthroughs in understanding. Like the first time I seen SSB on the SDR waterfall, modulation suddenly made so much more sense.
Glad to see this precise and systematic tutorial although I know it from my last job. As a VNA metrology engineer, I can say everything is correct and done with respect to NanoVNA limitations.
My father was an Environmental Test Engineer at Tektronix when he died in 1974. Your speech reminds me of how he use to try and explain questions I might have. Great video and great podcast on the Amphour
Thank you Alan. This helps so much. Armed with your info we can all now use the NanoVNA to accurately measure circuit impedances. This will be an enormous help for those of us who routinely take bits and pieces of circuitry from multiple sources and try to put them together into working rigs. Your video and the NanoVNA will remove the Z guess work. Thanks again and 73 Bill
I am gonna download all your vids as a reference, i was fumbling around with an active probe trying to caracterise it and failed for just about all the reasons you mentioned here 😄 Thank you Alan 👍🏻
There is so much to learn from your attentiveness to every small detail, I will be the first to admit I am always in hurry and missing a lot in the process. Well, maybe in time, who knows Thanks for a awesome video again, Alan
Great Video Alan! (as always!) I normally would have caught that because any time I do an analysis on something, I'm always cognizant of how much signal to put into beforehand. This really true for direct-coupled amplifiers where you can pin-one to the 'rails' and blow things up! In saying that, I can see where you would intuitively-assume that it's output would not be real high (i.e. tiny-box = tiny-signal!). For the RF, I'm at the point of always first figuring out how much attenuation I can insert without compromising my measurements--good practice...
You’re a freaking genius, how the hell did you figure it was the VNA out amplitude. And how did you figure the amps input limit. I’m 59 yrs old I wish I had your brain. Great video 👍. I have a nanoVNA and I don’t remember why I bought it hahaha.
Great example of using different instrument to validate amp functioning correctly and rapidly moving on to VNA . Looks so easy but requires rock solid understanding of fundamentals , a broad base of knowledge and years of experience to know how to join the dots . Thanks again
Thank you Alan for an excellent video. Also for giving me a D'OH moment... a while ago I bought a Nano to help with designing a front end for a 433MHz remote control IC which has a wierd-ass input Z. After a lot of calculations and prototype it didn't work out. Now I realise the back to back input ESD protection diodes might have had something to do with it! 😖
I ran this same test on my H4 for a HB 2M preamp. I had to severely attenuate the vna output. Really like your videos. I also ran the preamp through the Siglent SSA3021X, and the nano compared very favorably...keep up the good work. 73 Glen K4KV
An OUTSTANDING video!! I had not considered the need for an attenuator on my nano VNA in some apps. Though, I use them all of the time on my spectrum analyzers. One of those Ah-Ha moments for me ! Thanks es 73 Glenn SC
Thanks for the video tutorial, Alan. So many would have been tripped up by this lesser known fact of NanoVNAs when trying to measure amp or similar parameters. God bless!
Hello Alan, Your videos take me back to that sense of wonder I has with my first arrl handbooks when I was a kid. I hope you are considering publishing a workbook or series of reference workbooks at some point as your style is effective and enjoyable.
Great video. An attenuator reduces the dynamic range of the VNA, so I've used a directional coupler to get a little less noise. You're at the upper end of dynamic range, so not a concern in this case.
That was great! Thank-you! I'm trying to teach myself about some of these RF radio frequency and IF intermediate frequency concepts. I'd like to be able to easily and accurately determine stage gain in radios and televisions as well as test instruments and AF audio frequency as well. I'm a hobbyist at middle age trying to learnand master many things I failed to when I was younger. Thanks again and God Bless.
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This video comes at an opportune moment, as I'm playing with one of these little BGA2866 MMIC amplifiers. The S21 gain measurement on the NanoVNA is also invalid when the signal distorts. Adding attenuation changed the gain figure from 11dB to 18dB on this test board. Thanks!
Yep, good workaround for this situation. I have played around with 2N3904's with some negative feedback also and they are amazing performers for such an ordinary transistor you can get cheaply anywhere!
Excellent video. A check for overdriving the input will be something I’ll always consider in the future. But the star of the show was the HP15C calculator! I wonder what the kids are using in school today?
Any video by you explaining the function of every component used in the 433 MHz transmitter and receiver RF circuit, explaining working of the both the RF transmitter and receiver circuit
Great video to learn from. One (maybe stupid) question: is the tested amplifier under power when you test the impedance and gain, I.e. during the S11 and S21 tests?
I NEVER HAD THE TIME TO SPEC THE OUTPUT LEVEL, AND NOTICE THE EFFECT IT HAD ON INPUT LEVEL SIGNALS. THE NE602 MIXERS NEED TO BE LOOKED AT IN MORE DETAIL, MAYBE THERE IS A WAY TO ADD A CIRCUIT TO DO THIS VERY SAME TASK, BUT ALLOWING IT TO BE SWITCHABLE AS WELL. WHEN YOUR BOX OF TOYS DOES NOT HAVE SUCH ATTENUATORS, YOU ARE FORCED TO ADD EXTRAS, BUT THAT REALLY CREATES A MESSY TEST BED. 3dB, and 10 dB PADS I HAVE, NOT THE INTERMEDIATES. GUESS I WILL BEGIN ACQUIRING THESE AS WELL. THANKS!
You would simply terminate the input of the amplifier, and then connect the output to CH0 and run a S11. But, also you can see this basically from inspection. An emitter follower has a very low output impedance (basically 0.026/Ic), in this case just a few ohms. Then, there is a 47ohm in series - so it's basically very close to 50 ohms.
Hi Alan, I think this is your most recent video, so I'd like to reach out and ask you to set up an experiment that's right up your alley but I haven't seen you do it. It applies largely to PCB design and I think it was Rick Hartley that mentioned it but it's pretty spectacular and very simple to to. If I have this right (and I may not!) the idea is you get a loop of coax - maybe a few metres/yards long (I'm European so I use metric) and insert an ammeter in the ground shield some where at the top of the loop. Then you "short" the shield at each end with a short piece of wire - and you've probably guessed - put another ammeter in that short run. Rick points out (and I believe him but I'd love to see this done) that at DC, the current all flows in the short return path *but* as the frequency increases, and he says the effect starts at audio frequencies, the current flowing in the "direct" path to ground begins to diminish and increases in the much longer loop. Rick explains this is because as the frequency increases the energy, which is travelling along the dielectric as a magnetic and electrical field (again, I might be wrong here) starts to "hug" - my word - the shields on the coaxial line. I can do this at home with my own paltry "free shipping from China with a counterfeit XR2206" equipment ROFLOL but you're a great teacher and with your skills and superb equipment you could demonstrate this to a far wider audience than Rick or Eric Bogatin are able to reach!
You've heard the addage that "current always takes the path of least resistance". We'll that's not the most accurate way to state that... It would be more accurate to say that current always takes the path of least *impedance*. For return currents flowing in a ground path, the "least impedance" translates mainly to "least inductance" as the frequency increases. Inductance is minimized when the "loop area" between the forward path of the current (center of the coax in your example) and the ground/shield return path is minimized. Thus, the image/return currents "hug" the region closest to the forward current path. I talk a bit about this in my video on bypass capacitors (ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-9EaTdc2mr34.html)
@@w2aew Well blow me over with a whisper! :) Somehow I missed that one - probably (cough) bypassed (cough) it because I already knew about placing small caps near to the VCC/VDD rails on my ICs. I've been doing this like a trained monkey since I was a kid with no real appreciation of why. I've been working my way through your stuff and I wish I'd had teachers with your skill at explaining stuff when I was learning back in the 1970s. It's hard to explain the debt of gratitude I feel for all your work! I'm still a little vague on this transmission line stuff since I tend to associate it with RF stuff that I don't personally work with. Of course, as you've pointed out here, the fast edges we're creating with modern chips (heck, I can recall near-chip bypasses back on early CMOS chips) produce masses of RF harmonics off into the multiple Ghz just by the switch time. I forget if it was Rick Hartley or Eric Bogatin who described the experiment but (if you do care to recreate it), I believe the idea was to put an ammeter in the "short" side and another into the long loop. As the fundamental sine wave is swept up into the high audio frequencies and beyond (if I've grasped what they were saying) the current - continues to follow the path of least impedance, so the current in the dead short drops away while the current in the (measurably) longer DC path starts to increase. Presumably this would require the sort of bench gear that many of us don't have as DVMs won't be able to measure AC currents accurately. I think it was Paul at EEBlog who traced around a ground plane with a fairly high end current probe attached to a Tec scope and demonstrated how signals propagated through the copper. I know this is pretty basic stuff to some but my poor brain opened the lid of my noggin and a white flag popped out. ;) Apologies for the humour, it's the shortest path to describe my ignorance and astonishment at these wonderful demonstrations. Thank you again.
Incredible video as always. I am using a Nanovna in order to check the input impedance of a transceiver IC used for an RF-433 Mhz module. The tranceiver IC has an LNA stage this is why I need to use a 30dB attenuator in the CH0 so that the IC's input stage amplifier will not get saturated(exactly as your video). The problem is that when I try to calibrate the nanoVNA using the 30dB attenuator the results I get are very very noisy. It seems that the impedance values in the smith chart are all over the place and I cannot understand why this happens. I have tried a second nanoVNA with no better results.
This is to be expected, unfortunately. Even high performance lab grade VNAs will be noisy with a 30dB attenuator inline, expecially in CH0. Think of it this way - the signal generated out of CH0 is attenuated by 30dB (1000x) before going to the LNA input. Any reflected signal from the LNA will return through the same 30dB attenuator, making it even smaller yet. So, for a device with a good return loss, the reflected signal will very small, and will be reduced by another 30dB before being measured by the VNA. The only things that may help are the use of averaging and a lower RBW (if your unit has the ability to adjust the RBW or measurement BW).
Can you consider doing an episode on amplifier stability and oscillations? Things like k factor, u1, and viewing oscillations on a spectrum analyzer. Thanks. I appreciate your videos a lot
Hello Sir, can you make a video about how to safely measure the output impedance of a RF Amplifier circuit with attenuator, there are already many videos covering how to measure input impedance of a RF Amplifier circuit using a Nano VNA. Especially for non-50ohm amps, how do I measure the output impedance on the VNA?
I know that Bob K he doesn’t do amateur radio but is knee deep in the free flight and rc airplane arena still. He is in the same rc club I belong too Keystone rc Club
This is awesome, Alan. I have been fretting over this very thing for the past few days and was slowly working out a process to do it. I'd even bought, a couple months ago, a TO-72 HP 11858A & 11600B transistor fixture for my vintage HP8753 which allows you to insert and bias bjt's in the 3 different amplifier configurations. How would you measure output impedance? Specifically, how would you do this for tube amps? or high power LDMOS amps? I'm not the sort of person that just accepts that you do xyz (i.e use a 1:9 output impedance transformer as rote for an LDMOS part) because everyone else does it that way :)
Greetings.... Is there not a way to set output levels using Dislords software during calibration ? Was that added later, after this video ? I looked at menu, have not done any measurements, Dislord version 1.2.20 - NanoVNA-H 4.
Thanks for your really outstanding videos! I've got a NanoVNA SAA2. There's no isolation or through nenu during calibration. Guess it mean it's taken care of? Thanks. 73
Can you do an additional video and drive this amp into compression observing S21 and the phase in particular. I would love to see what phase compression looks like.
Hi Alan. Are these (budget) VNA's, that probably all of them use a square wave for the stimulus, in general suited to validate active circuits like amplifiers (of course making sure in- and output levels are correct, as we see in your video). As you say in the video, you are not concerned with this fact? Maybe the input of the amp will filter out some sharp edges? Or other way around, in what use cases these devices would be generally unsuitable? Other issue would be sometimes the devices that are using harmonics, whereby the strong fundamental is also present? But that would be a sub-question 🙂 This is contrast with a "real" VNA, that probably uses a sinus stimulus.
It is really highly dependent upon the device being tested, whether its performance would affected or different due to the squarewave stimulus vs sinusoidal drive. It requires applying knowledge of the DUT and the effects of the harmonic-rich drive signal.
Hello Alan, thank you for the video. I have a question about adding an attenuator at at the nanovna output. Wouldn't that cause an accuracy problem with the S11 measurement?
The affect of the attenuator can be calibrated out. However, since the signal injected to DUT is attenuate, and the reflected signal is also attenuated, there will be some additional uncertainty in the measurement - especially for DUTs with a good S11 (low reflections).
Good morning sir, How can i measure the output impedance? And when you measure the input impedance you put a 50ohm load at the output?and when you measure the imput impedance The amplifier is powered? F4JMY
When I first saw it, I was bit surprised that this design does not start to self-resonate, given that the bandwidth is quite high for a 2N3904 based design. What is the phase shift of one amplifier path at unity gain?
Thanks for sharing w2aew! Does this mean that I have to use a 13 db just to check the input tune of my Mini-Ball TL60 60 watt one transistor amplifier? The problem is that the SWR is 3.1 to 1 with the amp on. It is putting out 50 watts with 1 watt in. I suspect that it is tuned to the center of the 10 meter band. In fact my Nano VNA actually shows that it resonates at 28.500 MHz if I sweep the input of the amp and has an SWR of 2.82. I'd like to tune it to 11 meter and get the SWR down as low as possible.
I didn't calibrate at the end of the adapter because I don't have a set of female open/short/load standards. Not worried about it though, since the phase shift through that short adapter was not significant at the operating frequency of the amp.
In 12:12 when you connect amplifier's output to ch1 , the amplifier is connect to a DC source, is it correct? are there not problem with nanovna? Thx for the video. 73 from LU8ALI
Hello Sir...I just run the same experiment, and it works using the 20dB attenuator, the TIA gain was around 23dB. BUT the nanoVNA graph show wierd pattern when calibrated using the 30dB or 40dB attenuator. I dont know why ?
I have watched at least 3/4 of your videos and love your content and explanations. Not quite the place to ask, but I don't know how else to get a message to you; Is there any way to do simple transformer/inductor ring testing using an oscilloscope? I know you can buy specific testers, but for occasional use I wondered if some simple oscillator could be used to see if a coil was shorted on an oscilloscope? All internet searching has failed to get me any ideas. Any info would be really great. Seems like something others may also want. Please keep up your great content.
Have you watched these two videos? ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-74fz9iwZ_sM.html ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Ff5xOENID7w.html
Amazing video. Was thinking of trying this same thing two months ago. Great tip about the nonlinear effects. The amplifier has to be turned on (biased) to make this measurement? Do you need a DC block to protect the NanoVNA?
Inquiring minds need to know! Is there anything significant about the 13dB attenuator? It seems very "random"? Great video showing one of the many "gotcha"s in electronics!
Very enlightening, sir. I'm building that exact W7ZOI amplifier. If you would be so kind as to offer me some advice, I would appreciate it. I'd like to test the impedance of a mixer oscillator input using diodes with 300-500mV fwd voltage. I'm guessing the Nano-VNA with 600mV should do the trick? 73 and thanks for everything you do
For a typical diode ring double balanced mixer, the LO port will typically NOT have a nice, well controlled impedance. The function of this port is to quickly and fully commutate the diodes. When this happens, then RF/IF ports should reflect the impedance at the other end, since the diode is making the connection from the IF to RF (or vice versa). Testing the IF or RF ports without a) terminating the undriven port, and b) properly driving the LO port, will lead to incorrect results.
Very nicely presented and clear. (a) Any caveats when measuring the output impedance looking backwards into the amplifier output? (b) Which firmware are you using in this video for the H4? Mine does not seem to have the "401 points" on the bottom..
As long as the output is AC coupled, then simply terminate the input and measure S11 on the output. I am currently running 1.0.39 from Dislord (see video #320: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-NcXzITPPTyA.html) on my H4.
