Measuring the Input Impedance of a Product, Circuit or System (070a)

There are two kinds of situations where you might want to measure the input impedance of a product, system or circuit:
1. At a Low frequency like an audio amplifier where there is no real concern about the complex aspects of impedance.
2. At Higher frequencies like a receiver pre-amp where we are very much interested in the complex impedance.
In this video I will be addressing the first of the two.
I will be measuring the input impedance of a small, inexpensive audio amplifier. I will discover how this input impedance is affected by:
The position of the volume control
The level of the input signal
The frequency of the stimulus
=======================================
Time Markers for Your Convenience
----------------------------
00:05 Introductory Comments
00:59 Why not use a DVM to measure Input Z?
01:53 Planning our Measurements
02:52 What is "Line Level?"
03:22 What is the difference between dBv and dBu?
04:20 Planning our Test Voltage
06:12 Three Important Questions ...
06:15 1. How does the Volume Control affect the Input Impedance?
06:43 2. How does the Signal Level affect the Input Impedance?
06:59 3. How does Frequency affect the Input Impedance?
07:36 Setting Up for the Test
07:39 Test Equipment Settings
09:00 Test Equipment Connections
10:00 First Test: How does the volume control affect the Input Impedance?
12:40 Second Test: How does the signal level affect the Input Impedance?
14:20 Third Test: How does Frequency affect the Input Impedance?
15:21 Bringing it all together
15:56 Parting comments and Toodle-Oots

[@15743_Hertz]

This was a refresher of my old job as a Tech Controller in the Air Force back in the eighties. Thanks for the reminder!

[@eie_for_you]

@@15743_Hertz You are very welcome! 😄

[@ernestb.2377]

Very helpful Ralph! I know 99.9% of this but still enjoy listening to you, and use it as a refresher 😊

[@eie_for_you]

I'm glad you found it helpful ... and refreshing ;-)

[@rubhern8187]

read many technical books always found it not very well explained. this was an excellent explanation and demo. 🎉🎉🎉🎉 this will be my reference point many thanks. liked and subs.

[@eie_for_you]

I'm so glad that you found this video helpful. 🙂Thank you for the encouragement! 🙂

[@octapc]

I haven't touched RF gear in over 30 years from when I was in the RAAF. Impedance matching theory has fallen out of my head and will probably never be needed. However, this video is the first of yours that I will watch solely for the purpose of reminiscing 😂

[@eie_for_you]

Thank you very much for your service to your country! I was in the U.S. Air Force. Enjoy! 🙂

[@W1RMD]

I always learn so much on your channel. Thanks and take care!

[@eie_for_you]

You are welcome! 😁😁😁😁😁😁

[@peterayearst23]

Nice explanation. I'm really looking forward to your experiments with impedance measurements at the higher frequencies.

[@eie_for_you]

Thank you! Yeah, the higher frequency stuff is a LOT trickier! As an adjunct to this, there is my video on measuring non-50 Ohm stuff with a 50 Ohm VNA. 🙂 kzbin.info/www/bejne/mpinhGeon8dgfbc

[@AuraARTStudio]

Спасибо. Очень подробно и понятно объясняете. 👍

[@eie_for_you]

Thank you! 🙂

[@DucatiMTS1200]

Super video Ralph. Great to get my brain refreshed by a clear and concise explanation. I suffer from TMB i.e. too many birthdays hi hi! 73 de GI8WFA.👍

[@eie_for_you]

LOL!🤣 TMB!! I gotta use this! Thanks! 🙂

[@stereophotog]

Great video and thank you for the efforts and time spent doing this for us.

[@eie_for_you]

Thank you and you are very welcome! 🙂

[@derdoktor7123]

Thanks for a very nice explanation, Ralph! If you have no idea what‘s coming next I would wish the process of matching a 50 ohms line to a crystal filter including the design process maybe 10 MHz 😊

[@eie_for_you]

You are welcome! Next up is the input impedance of an R.F. amplifier intended for use in a 50 Ohm environment. Yeah ... impedance transformation/matching. On my bucket list! 🙂

[@mikesradiorepair]

Don't have enough fingers and toes to count how many times I have tried to explain measuring a component or circuit's performance and the importance of matching input and output impedance. In my case it's usually a RF related circuit or component like a crystal or ceramic filter, band pass filter or mismatched amplifier stages with hi/low input output stages. You can't just hook up a filter or filter network, etc... to the input / output of a spectrum analyzer or vector network analyzer with a 50 Ohm impedance to a circuit or component with possibly thousands of Ohms of impedance and expect the result to be correct. In most cases people get it, a few are like talking to a brick wall. Mike KC3OSD

[@eie_for_you]

If you take in my video on measuring non-50 Ohm things using a 50 Ohm VNA (kzbin.info/www/bejne/mpinhGeon8dgfbc) you will see that I am in agreement. If we absolutely do not know what the input impedance is, where do we begin and how do we accommodate this? An initial measurement without impedance matching gives us a ballpark. From this ballpark, how precise do we need to be? Many times this ballpark is sufficient, but not always. Then we have to turn to impedance matching business. In my next video I do NOT do impedance matching because the amplifier I will be testing the input impedance of is SUPPOSED to be in a 50 Ohm system, though it turns out ... not so much. 🙂

[@TR3A]

Fantastic content! Thank-you!

[@eie_for_you]

Thank you so much! ... and you are very welcome! 🙂

[@ornithopterindia]

👍Thank you sir.

[@eie_for_you]

You are welcome! 🙂

[@carlosanvito]

Great video. Changes in input Z are likely due to having negative feedback directly in the first stage. Too bad the designer didn't use a high Z buffer with a fixed input resistance before the NFB stage(s). By the way, love the display monitor in the upper-left corner.

[@eie_for_you]

Thanks, man! I had to think about this a little... yeah, negative feedback with a capacitor in parallel with it to limit high end ... that would certainly do it. I agree with you on the fixed high input Z first stage; put the volume control between the first stage and the next. That is how I did it in my mixer project (Design End-to-End series). He is the reason I am here, my friend. That is my wallpaper on my desktop. 🙂

[@carlosanvito]

@@eie_for_you I'm completely with you, my friend - in my 'retirement' I'm an adjunct college prof, teaching engineering technology. While I'm not allowed to openly discuss my faith, I subtly include Christian values in the lectures and talk about the importance of integrity in the industries where they will likely work. Young people need to hear this from an old guy like me, as the rest of what they are constantly bombarded with is just the opposite.

[@jim9930]

@@carlosanvito May 21, 2011 was the beginning of judgement day on the world And it shall come to pass in that day, a great tumult from the Lord shall be among them; and they shall lay hold every one on the hand of his neighbor, and his hand shall rise up against the hand of his neighbor. Zechariah 14;13 "Tumult" = confusion & disbelief ...on purpose, by design for the punishment of mankind! Division is the result. Babylon is fallen(world systems). Division is worldwide in religions, politics, economics, news, industry, sciences, sports, medicine, entertainment, and even between the sexes. ...have you followed world events lately? The Lord is known by the judgement which he executeth, the wicked is snared in the work of his own hands. Higgaion. Selah. Psalm 9;16 Whoso keepeth the commandment shall feel no evil thing, and a wise man's heart discerneth both time and judgement. Ecclesiastes 8;5 And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign to ever and ever. And out of his mouth goeth a sharp sword, that with it he should smite the nations: and he shall rule them with a rod of iron: and he treadeth the winepress of the fierceness and wrath of Almighty God. Revelation 11;15 19;15 spoiler alert: The biblical calendar has been known, published(1974), and carefully studied for half a century now... There are forty+ time-paths from the bible pointing to 2033 as the last day. Go look up: Ebible2 timelines T - minus 9 years and counting... That is what young people need to know, Carlos FEED MY SHEEP

[@jim9930]

@@carlosanvito Because I have called, and ye refused; I have stretched out my hand, and no man regarded; But ye have set at nought all my counsel, and would none of my reproof: I also will laugh at your calamity, I will mock when your fear cometh. Proverbs 1; 24-26 GOD is our refuge and strength, a very present help in trouble He looketh on the earth, and it trembleth: He toucheth the hills, and they smoke. Psalm 103;32 Bow thy heavens, O lord, and come down: touch the mountains, and they shall smoke. Psalm 144;5 READ THE BOOK ! And an highway shall be there, and a way, and it shall be called the way of holiness; the unclean shall not pass over it; but is shall be for those, the wayfaring souls, though fools, shall not err therein. Isaiah 35;8

[@eie_for_you]

@@carlosanvito So very true! They've heard the message of "me first at all costs" ... "it is all about me and what I need" so much that selflessness is a rarity. 😞 Lying is OK if it is "for a good cause" (usually their own). Yes, integrity is so important. Keep up the good work! 🙂

[@Denis_F1GWO]

For @eie: Hello, My name is Denis, I am a French radio amateur (callsign F1GWO) and I want to tell you what I found on another KZbin channel (in French) the demonstration of a very nice and useful little LCR meter which is "completely relevant to the topic of this video". Among other things, it measures resistances (they call that "resistance" but in fact it is not, it is really impedance !) but not using DC, but AC, (You even cannot measure in DC, it is designed like this, even if you want, you cannot !) It works using its internal AC generator. Yes, really, and you can choose between 2 values for the voltage: 0.3V and 0.6V (RMS I think) and 3 values for the frequency: 100Hz, 1000Hz, and 10KHz. You can even use it as a small but effective audio generator with 2 output levels and 3 output frequencies (that’s not specified in the user manual but I tried, and it works nice !) The output is a very nice sine wave. So, it would be very interesting that you buy such a multimeter and make the measure with it. That would be a very nice experience. It would be very very easy to do it: just plug it at the input of the audio amplifier, and play with the potentiometer, that’s all ! And you can do that on the 3 frequencies. It is a very small but nice LCR meter. To find it just take a look at AliExpress and type: ZOYI ZT-MD1 And it is very cheap … About 20 bucks or less (I don’t know the price of Aliexpress in $, sorry) I hope this information will be very interesting for you. Give me your comment please (within your channel, I don’t have one) and if this does not interest you, I am sorry and I apologize having disturbed you. 73 and have a nice day. Denis

[@eie_for_you]

Very interesting. Amazon.com has it. Normally $40 but it is currently "on sale" for $34. This is within the $ limits of family buying Christmas presents, so I put it on my wish list to check it out. I will compare it to my VNA measuring components to see how it measures up (pun intended). 🤣🙂

[@ProAudioIQ]

👏👏👏👏👏

[@eie_for_you]

🙂

[@minazulkhan8287]

Hi Ralph, today I was trying to measure impedance of PCB track. With the help of ufl connectors I connected one end of track to port1 and other end to port2. I wanted to measure impedance at different frequence (836.5mhz , 897.5mhz, 1950 mhz etc..). As per supplier of PCB its a 50 ohm track . In VNA I used smith chart to get the readings. Again the VNA has smith chart for S11 and S21. Which smith chart should I consider ( though FYI i was getting readings closer to 50 ohm for S11 smith chart)?.... Thanks

[@eie_for_you]

I had to think on this for a moment or two. This is no different than measuring the characteristic impedance of a piece of coax or other transmission line. If you are looking for simple impedance, then this video would help: kzbin.info/www/bejne/iYTElYCrgrmfptE But it seems you are looking for complex impedance. First, measuring impedance is always S11. Second, you calibrated the VNA at the end of the port 1 cable (I am assuming) so you need to terminate the other end of the trace with your 50 Ohm termination, not port 2. The effects of the cable and so on looking into port 2 will truly mess up the readings. If you calibrated the VNA at the connector on the VNA you will have to do a "port extension" to move the reference plane for the measurement to the end of the cable right where you are connected to your trace. Note, however, *any* divergence from a 50 Ohm system when doing the port extension will render the port extension useless. See this video where I show how to do a port extension: kzbin.info/www/bejne/hqXMaXiorMR-ock I hope this helps. 🙂

[@minazulkhan8287]

@eie_for_you you r a great teacher ... Thanks

[@minazulkhan8287]

@eie_for_you Thank you... your a great teacher .... Also why cant I just measure complex impedance ( using s11 of smith chart) at required frequency and just find its magnitude ( root of sum of squares of real and imaginary).

[@eie_for_you]

@@minazulkhan8287 You are welcome! ... and thank you, too! What you propose is a perfectly legitimate way to do it. 🙂

[@fullwaverecked]

Here is something interesting, but completely off topic. I'm playing thumping music out of a '50s Alnico Zenith Satalite 12" with the tweeter and mid disconnected. No caps on the 12". My leg is next to the speaker and my leg hair is very active. IE static electricity. Why is that?

[@eie_for_you]

No clue 😕

[@fullwaverecked]

@@eie_for_you Silly question. But I think it has something to do with simple air movement, like how the shop vac hose gets static electricity built up. And the old grill cloth is rather conductive. Must be where the secret sauce tone comes from. Ha!

[@eie_for_you]

@@fullwaverecked Your guess is as good as mine ... maybe better. 🙂

[@geirha75]

can we somehow measure pure Z impedance of an efhw antenna up in the air. no unun connected. local conditions.

[@eie_for_you]

Check out this video: kzbin.info/www/bejne/mIHQpXyMotF9pc0 🙂

[@ssergeant9682]

could you help me troubleshoot my speakers? bought a set of JBL speakers to replace some old ones on my older yamaha receiver. the olders ones worked fine, but when I put a banana clip on speaker wire and plugged them in - no sound. I ran the test using my receiver and it said - no speaker found. I was not familiar with the speaker hookup on the new (er) speakers. wondering if using the banana plug was the issue? the knob on the speakers unscrews revealing a little hole that I read is one way to connect speaker wire. never done that before. I'm out of my element. could you help? (71 year old female and not afraid to experiment). but I get frustrated more easily that I used to when building my old PC's. thanks! wish I had a way to send photos to better explain what I'm dealing with.

[@eie_for_you]

Banana plug or using the hole ... both are equally valid ways to connect your speaker assuming the banana plug is installed properly. If these speakers are used, you *MAY* have bought a pair of blown speakers (someone used too much power or applied a large D.C. voltage across them - often the result of a failure of their amplifier). If I were you, the first thing I would do is to measure the resistance across the speaker terminals. From a D.C. perspective, this should be almost a short if they haven't been blown. Wish I could do more. By the way, age-wise, I'm not far behind you. 🙂

[@egonotto4172]

8:15 "The signal generator output impedance is set to 50 Ohm". I don't think so. All signal generators I know have a fixed output impedance of 50 Ohm. Only the voltage display is halved so that the voltage is displayed at a load of 50 Ohm.

[@eie_for_you]

Yes, you are right. This signal generator has a fixed 50 Ohm output impedance. However, there is a setting for the channel which we can set to the impedance that it will see on outs output which corrects the voltage as indicated on the display to accommodate the load impedance that actually exists on its output. It doesn't change the actual output voltage. It changes the voltage as *indicated* on the display. With that said, I DO personally own a signal generator whose output impedance is NOT 50 Ohms. While it is a good bet that most mainstream signal generators output impedance is 50 Ohms, it certainly in not a given. There is a whole class of them, for instance, which are 600 Ohms and 75 Ohms. So, let's be careful about our assumptions. 🙂

[@egonotto4172]

@@eie_for_you That's right, that's why I wrote “All signal generators I know”. But that wasn't right either, because I have an old Wien-Robinson oscillator that I hadn't thought of, whose output impedance I don't know. But I doubt that it has 50 ohms.

[@eie_for_you]

@@egonotto4172 True that! It is a good assumption, in general, that all signal generators of modern design will have an actual output impedance of 50 Ohms unless they have some specialized purpose. The cable TV industry is all about 75 Ohm as is the CCTV because 75 Ohm coax has less loss than their 50 Ohm counterpart. Parts of the audio industry was all about 600 Ohms; that is where the whole dBu steps in (1 mW into 600 Ohms is 0 dBu). 🙂

[@simon-d-m]

In the audio world, really low Z oscillators existed and are (were!) quite important. The BBC, for example, used to send "station tone" round its studio complexes, which would appear as a hole on jackfields. The tone source had to be low Z, so that several studios could use it simultaneously. Separately, I have an ex-BBC Comms. Dept. lines test oscillator/precision meter+attenuator set that has a 600 ohm output impedance switch (otherwise low-Z). I think the reason for using 600 Ohms as the measurement/matching impedance had to do originally with ease of design when transformer matching the component units in a signal chain: mic-to-line amp, mixing stage (constant impedance faders!), line send amp's input, and so on. Back in the day, these were often individual items in an equipment rack ("bay" in BBC speak - the reverse of US usage!). IIRC, the twisted pair outside broadcast circuits supplied to us by the Post Office were expected to be 75 Ohms, and our lines equalizers worked on that assumption. That said, the Post Office was notorious for putting in 'lump loading' - passive equalizers - at various points on long lines (i.e. in the telephone exchanges they passed through), so they would nominally retain the expected impedance at 1kHz. We worked on having around 70dB basic loss(!), but lines could have complex impedances caused by a string of inductors, etc. The usual approach was to call the telephone exchange and say, "take that lot out!". Our STC cans were 2k in each earpiece, so they could be used directly across a signal in a jackfield, but without double terminating it (or worse!). The other interesting operational impedance issue was mic matching. The usual assumption, still reasonably valid today, is that mics have a source Z of 300 Ohms, and a mixer mic input would be around 1.2k. When I started, there were a number of semi-consumer mics, mostly ribbon designs, still available with an output Z of 30 Ohms. This is way too low for the 1.2k mic amps. Studio wallboxes were thus equipped with impedance-changing 10x transformers (30-300 Ohms), and the mic connections at the wallbox were on 8-way Cannon EP-series connectors, rather than XLRs. Normal, 300 Ohm mics used pins that went straight through, whereas 30 Ohm mics made "automatic" use of the transformer on (presumably) six other pins. Thus operationally, as long as you used the correct cable for the mic, the impedance difference was rarely an issue. Why 30 Ohms? I think it's this: Ribbon mics are naturally an extraordinarily low impedance, and have to have significant impedance conversion. If you want a _small_ ribbon mic the old-fashioned way, the transformer's size becomes a significant nuisance to get the output Z up to 300 Ohms (and still have a decent signal from it). We used to use Reslo ribbons in TV studio mic booms, where both size and mass work against you. Although they made 300 Ohm versions, ours were 30 Ohms. You usually see the 30-ohm variants on secondhand sites and the 'youth' of today often wonder why they're so quiet! Sorry if my memory is playing tricks above - I was an operational engineer 40 years ago (I mixed programmes), not a comms man. The above goes back to my basic training in the 1970s (which was _extremely_ thorough), and conversations with colleagues in comms who I worked alongside.

[@eie_for_you]

@@simon-d-m Interesting! 🙂

[@y_x2]

Good info. You forgot to say that the equipment must be turn on!

[@eie_for_you]

I'm glad you liked the video! You made me think ... I thought for sure I said to turn on the little amplifier ... yup, at 10:29. 🙂