I'm currently studying Electrical Engineering and have had this issue in labs. The TA that was instructing just chocked it up to the signal generator just not being very accurate. I'm glad you set the record straight.
Oh, wow. I had no idea that's what that was -- I've just been told to put the signal generator into high-z mode and not known why! Thanks for the information, it's incredibly helpful.
Thank you, Alan. I really appreciate your taking the time to send me an answer. I need to go back to your earliest videos and do some catching up! Best wishes...Dan
Well thank you so much again, was just wondering about this because was trying to measure impedance using sig gen and oscilloscope, so look on your channel and here you are, a lifesaver
Very good! Thanks for explanation! I made it im my AFG1022, where the signals was displayed with twice voltage value on the osciloscope. Now, whith this setup, the measurement is ok!
I have to thank you, because i do have the same osciloscope,function generator and the same problem on my school lab. So very thank you for your video :)
For a split second, I thought I was watching a diff't channel. You sound diff't - it's allergy season here in NJ! Hope you feel better. Another good video.
Hi Alan, I have seen people tripped up by this before. A lot of old signal generators were marked "EMF" or "PD" to differentiate between the levels they would generate into Hi-Z or 50 ohms. It can be frustrating with modern test equipment where the termination options are hidden in menus somewhere and not permanently displayed on screen.
Recently I went from an audio function generator to a real one (Agilent 33220) and was wondering why it showed double the value until I realized: "sure it has 50 Ohm in series and shows for 50 Ohms termination". So I just added a 50 Ohm terminator. Searched for a menu item on my generator to set it up properly, didn't find a menu item. You video made me look into the manual, and what do you know: Its not in the Output menu, it's in the Utility menu. That's Agilent for you ;) (Where files are not deleted but purged :))
Well dang. I've wondered why there was a miss match. I didn't think about either of the scenarios that you gave, and I think my problem is a combination of both. Great video!!! (and I'm just getting over a cold...nasty stuff this year)
This is a good question! Something I'll have to do a video on. Basically, to determine output impedance, you would observe the amplitude change as you change the load, and then calculate the output z.
Thank you for nice tutorial. I can understand the difference between high and low impedance. I have one question. I wonder why can we ignore the impedance of the cable. In the video 2:15, simple circuit represents only function generator impedance and oscilloscope impedance. I can't understand why here cable impedance 50 ohm does not included.
The coax cable impedance really only begins to matter at high frequencies. Remember, the cable impedance is simply the impedance seen by the signal as it travels down the line (see my video on transmission lines and terminations). ua-cam.com/video/g_jxh0Qe_FY/v-deo.html
Beautiful generator AFG3252 costs "only" us$12,500 new and 6000 2nd hand. Tek has a new serie called AFG31000 @ uk.tek.com/signal-generator/afg31000-function-generator which is basicley the same the serie mentioned in the video.
The 50 ohm output impedance is used to provide a good source match to the line and load. Why 50 ohms, here's a good answer for that: www.microwaves101.com/encyclopedias/why-fifty-ohms
Thank you very much, this is the video i was looking for! But i was wondering, what if my load is 80 ohm or 200 ohm impedance, because my function generator doesn't have the "load" option like in yours (it can only switch between 50 ohm or Hi impedance) do i have to put a 80 or 200 ohm terminator? Sorry for the stupid question but i'm a beginner.
Hey - there's no such thing as a stupid question (only stupid answers, and I'll try to avoid giving them). Assuming that your switch simply adjusts the readout on your generator, you can set it to High Impedance, and then calculate the resulting amplitude by multiplying the reading by RL/(RL+50).
+w2aew "Assuming that your switch simply adjusts the readout on your generator" -> Can we assume that some generators are able to short their 50 ohm load, then?
+fabts4 I have seen some where you can change the output impedance from 50 ohms to 600 ohms. I don't think I've ever seen one where you can change it to (near) zero.
great video..im planning to get a sig gen, I wonder what are the best practices to not blow up the device..im concern if its ok for the sig gen output front end to see DC voltage from the device under test..
This is a bit after you recorded this video (and it is very helpful). Since I have rekindled my bench time, I have a related question. Often I see an omega symbol in the vertical indicator at the bottom of the screen. I use a Rigol MSO2024 with a BNC cable connected between my Siglen ARB and the scope. I saw the incorrect P-T-P voltage (hence your video was helpful), but I need help understanding the Omega 50mV, for example, displayed on the screen. I hope this makes sense to you. Can you give me a good explanation? Thanks, Dave
The omega symbol in the vertical indicator is typically use to indicate that this channel's 50 ohm termination is being used. I would imagine it is the same thing on the Rigol.
2 Questions if you don't mind: 1)When you terminated the coax with a 50 Ohm terminator at oscilloscope input, why does that help? The oscilloscope now sees a 50 Ohm resistor versus a 50 Ohm coax but they are both 50 Ohm either way! To me, by adding the 50 Ohm termination it is almost like you just added another inch of the same 50 Ohm extra coax at the end which I don't understand how that helps. Is it that terminator represented a parallel connection to scope while the coax represented a series 50 Ohms? 2)When you put the function generator to high-Z (output impedance) isn't that a mismatch with the 50 Ohm coax so the signal should get reflected before even before it leaves the generator?
To answer your questions: 1) The termination provides a "matched load" for the transmission line. To understand why, it will help to understand how transmission lines work (and why they have a characteristic impedance). This video should help with your understanding of that: ua-cam.com/video/g_jxh0Qe_FY/v-deo.html As you will learn from this video, a 50 ohm transmission line is NOT the same as a 50 ohm resistor. It only "looks like" a 50 ohm resistor to signals that are *changing* - so, crudely you could think of it as a AC-coupled 50 ohms. Again, the video should help clear this up... 2) When you change the function generator to high-Z, this does *NOT* change the output impedance of the generator. Let me say that again... it does *NOT* change the output impedance of the generator. This is a VERY common mis-understanding. The output impedance of the generator is *ALWAYS* 50 ohms. When you change the Output *load* setting on the generator, all this does is causes the generator user interface to change the output amplitude appropriately. Since the output impedance is 50 ohms, there is a 50 ohm resistor in series with the output at all times. So, the voltage at the load will depend on the load resistance because this load resistance forms a voltage divider with the generator's internal 50 ohm resistor. The *load* setting on the generator tells it how to calculate the voltage appearing at the load by properly calculating the effect of the voltage divider (50 || load_resistance).
It's not really clear to me why the resistor you connect to the scope is in parallel? (I calculated the resistance of a 50 and 1M ohm in parallel and got a value close to 50 ohm. That's clear to me) . But based on your drawing and my general understanding of the wirering then I persieve it as a series circuit and not a parallel. Can you help me understand? Thanks for your patience and videos 😊
The 50 ohm resistor that I connect to the scope is *in parallel* with the scope's 1Mohm input, and since it is so much lower than 1Mohm, the result is very nearly equal to 50 ohms. This 50 ohm combination then appears *in series* with the 50 ohm resistor that is inside the generator (in series with the output amplifier). Then end result is that the function generator's 50 ohm output resistance is in series with the 50 ohm termination at the scope, thus making a 2:1 voltage divider.
@@w2aew this is what I was able to figure out myself. My problem is that the schematic doesn't look like a parallel. Could you explain how it is a parallel connection with the scope? It looks like a serial connection to me.
@@Infinitesap The 1Mohm input impedance of the scope is not drawn explicitly. The 50 ohm thru terminator shown at about 3:10 in the video has a 50 ohm resistor from the center of the coax to the shield (ground). Thus, when connected to the scope input, this resistor is in parallel with the scope's 1M input impedance (since the scope will now be measuring across this 50 ohm resistor.)
Thanks this really made me understand. Could you make a video on how to use the sync and trigger in- and outputs on the fg/scope. Would also be awesome if you could show application with them on breadboard setups.
ok so the real question now is, why do i see 2x voltage on oscilloscope on a car charger rectifier on 30V-32V when i'm supposed to see 15V-15.6V or so... I wish i had RMS multimeter with correct AC measurements, since the waveform that is shown is kind of like a mix of full bridge rectifier with the pulse modulation by putting some weird cap-discharge in the middle (there is a spike in the rising part of half-wave that diminishes or rises above the visible part, that contributes to output RMS thus one can control RMS from about 10V up to 15.8V)
The function generator voltage would appear to be doubled because of the high impedance on the oscilloscope, so you would have to set it to 50-ohm mode or use 50-ohm feed-thru termination.
This got me bugged. My function generator is the cheap JDS6600 and it doesn't have a setting for output impedance. According to the manual, the output is 50 Ohm Impedance. I'm connecting it to the oscilloscope directly with a coax cable. And I see the correct Peek to Peek reading. Why would that be? The oscilloscope inputs are 1 MOhm...
can you make a video about a somewhat common issue which i cant find a way to fix, which is that my function generator and scope are both earth grounded and somehow ive read that causes issues..for example ive tried building a bridge rectifier but i cannot read the rectified wave, as soon as i try to do a common ground or anything my wave readings are totally off and awkward. It is now sunday, got my scope on friday , spent the entire weekend trying to figure this out... and i cant....scope noob :(
If you draw a schematic of your setup, including a "model" for the generator output and scope input, it'll be clear to see the problem with the common ground. A bridge rectifier expects an AC input at the input, where the input signal on each side swings above and below ground, while the output of the bridge typically has one side tied to ground. But, when you drive the input of the bridge with the function generator, that effectively ties one side of the input of the bridge to ground - so that node will no longer swing below ground (because it IS ground). A simply audio transformer can be used between the function generator and bridge rectifier input to solve this issue for your measurement purposes.
Hello Alan! I know you couldn't possibly know all the different oscilloscopes and signal generators. But, I will say that I have a Marconi Instruments 2022D Signal Generator, and my oscilloscope is a Fluke 196D Scopemeter (combination digital scope and multimeter). The manual for the SG says it has an RF output of between -127 to +13 dBm.(0.2uV to 2 V EMF). However when I connect the output of the SG to the probe on my scope, I do not get the readings to concur. Right now I have the SG set to 1000mV, but the scope is reading 4.1 mV+/- RMS. The scope shows 1:1 probe. What in the heck am I doing wrong? I want to connect the SG to the SO239 on my VHF radios and do some signal tracing. Thanks for any help...Dan
+Beretta96Dan The SG has to be loaded with a 50 ohm load in order to meet its specification. A 1X probe used with any scope will only give you less than 10MHz of bandwidth. Another problem you have is that the 196 only has 100MHz of bandwidth, which means that 100MHz signals will read 3dB down, and only if you use a 50 ohm thru termination and no probe.
Ok so the output on the wfg is setting up a voltage divider, that makes sense. So say we send a signal out of the wfg to a fairly busy circuit on a breadboard, since we have no way of knowing what the forward/reflected voltage will be on the overall circuit including the 75 ohm transmission line Ive sent my signal to, we should use the high Z output load if it has that ability to? Sending a signal straight to a scope seems simple, 1 meg probe = high impedance. Its thevinans theroem
Yes, a voltage divider, precisely. The generator uses the output-Z setting to properly compute the division so it can accurately show you the amplitude. It's always a good idea to measure/verify the output with a scope, etc.
Hi Alan Something else to be mindful of. The new generation of RF Function generators have much higher frequency capabilities. An excessive length of coax can turn into a Quarter Wave Transformer. At 150 Mhz that equates to a half meter multiplied by the velocity factor or 18 inches * .6 to .8. The equates to 10.75 inches to 14.4 inches...Something to think about.
Yes, excellent point. Anytime you get above a few MHz you have to start thing about transmission line effects, terminations, etc. I did a video on quarter wave lines as part of the video on tracking generators.
If the generator is set to Hi-Z, then it is expecting to see a high impedance load. Thus, the amplitude displayed on the generator would reflect that. But, since the load is actually 50 ohms, the actual voltage displayed on the oscilloscope would be 1/2 of the value displayed on the generator.
Bought a 50 ohm bnc feed through terminator which appears to have the 50 ohm in series instead of parallel to the output. What is the use of such a 'terminator'? Attenuation?
If I measured the signal i/p on a circuit under test, and this measurement is the same that is being displayed on the generator, I could be confident there is no conflict of impedances? Correct? Thanks.
Yes. It's always a good idea to measure the signals at the device under test, just as it's a good idea to measure/check power supply voltages at the circuit and don't always trust the power supply display.
Hi, long post so bear with me..thank you for all your content, I can binge watch your videos all day as you explain things in detail in a way it's very easy to understand. I know this is an older video, but have a request and forgive me if this has been covered. In regards to the 50 ohm termination from the function generator, can you explain the fundamentals of why a 50 ohm adapter is required (if so) at the scope from a function generator? What's confusing for me is if the FG has a 50 ohm output impedance, and this is before the output terminal, and the scope has a high impedance, why the 50 ohm at the scope input? My assumptions is the FG needs to see the 50 ohm load at the end of the cable to provide an accurate signal to the scope along the BNC cable. This likely comes down to basic electronics regarding load and current and I want to speculate I'm about 80-90% educated on this..lol. I have built a workstation for the kids as their HS has an engineering dept as well as a robotics dept, I'm moving my hobbyist electronics gear to this workstation so we can all get involved, and want to make sure I have a complete understanding of the setup procedures for the equipment. The kids and I love to get inside and see how and why things work, and hopefully build some projects with the arduino kits as well as trial and error on a breadboard. Hopefully I explained my request/lack of understanding correctly, thanks again and I plan to catch up on your videos.
The short answer to "why a 50 ohm termination is needed at the end of the coax" is to prevent signal reflections that can distort the signal. This is explained more fully in this video: ua-cam.com/video/g_jxh0Qe_FY/v-deo.html For low frequencies - below a few MHz, it really isn't that important since the length of the line is much much shorter than the signal's wavelength, and reflections don't pose any problem. For for higher frequencies, or for low frequency signals with high frequency content (such as fast rise/fall times), termination may be important. The function generator has a 50 ohm resistor in series with it's output to terminate and reflected signals that are coming back from an unterminated end. This ensures that these reflected signals to re-reflect back towards the load again. Since the generator has a 50 ohm series resistance, and the expectation is that the far end load will be 50 ohms, this sets up a basic 2:1 voltage divider between the output amplifier of the generator and the far end load. Again, for low frequency work, you can omit the 50 ohm far end termination. Just keep this in mind when setting the desired output amplitude. If the generator doesn't have a way for you to tell it that what the far end load impedance is, then you'll just have to remember that the output will be twice the set value if the far end load impedance is high.
@@w2aew Thank you, I did watch the linked video and your explanation helps alot. I forgot about the in-depth content of that video. Sidenote, I actually frequent your videos on a different account which has more kid friendly subscriptions so the sidebar videos stay clean for when my kids and I view them. Take care.
Thanks for this great video, it helps me to solve most of my questions. But one quesetion is left, when I have a load with an unknown impedance between 30 and 80 Ohm, should I set the impedance value of the funtion generator to 50 Ohm or to high z? I would be very happy about your help.
50 would be closest - but remember, all this is doing is adjusting the calculation that the generator does for the output amplitude to account for the voltage division of the 50 ohm output impedance and the load. In either case, if you want a specific amplitude applied to your load, MEASURE it AT the load and adjust the generator accordingly (regardless of what the generator is telling you the amplitude is).
"Load Impedance" just changes the amplitude voltage?! I don't have a Hi-Z option on my generator, I can use a 50 ohm attenuator or just set up half of the amplitude?
Yes, 'load impedance' changes the way the *displayed* amplitude is calculated. If your generator is expecting a 50 ohm load, you can use a 50ohm thru terminator, or simply account for the 2x voltage difference.
I am confused ... is peak-to-peak voltage on a square wave equal to true RMS, or is the RMS exactly half of peak-to-peak of a square wave? I have seen/read both. When hooking my Fluke 175 to the test output of my Tek 465 (300mv square wave), the Fluke measures approx half the peak-to-peak value displayed.
You also have to take into account any DC offset. But, in general, the RMS value of a 50% duty cycle square wave is equal to 1/2 the peak to peak value. This video might help: ua-cam.com/video/ue0wtlrmCJE/v-deo.html
Thanks Allan! Just discovered your amazing channel!! Am plowing through them at a good clip. Playing some multiple times. Playing and pausing others, and then running over to my scope. This is an amazing resource. Thanks so much!!!!
You may know that I maintain a complete index of all my videos, arranged by number and by topic. The link can be found on my channel's homepage, in the lower right corner of the banner graphic.
my signal/ function generator provide constant output voltage for all frequency range when analyzed through oscilloscope, its fine, but when i connect a load across signal generator its output voltage drop significantly why?
It will all depend on the impedance presented by the load (as well as your measurement setup when connected to the load). Be especially careful of coaxial connections, proper terminations and coupling techniques between the generator, the load, and the measuring device (scope, etc.).
who do i know what is the internal resistance of function generator if i don't have user manual , further for dynamic load how do i match my load resistance with internal resistance of function generator.
Hi Alan. I was wondering is there noticable difference in signal shape or amplitude depending on where you terminate your coax? I mean you can connect your through termination at the oscilloscope or at the signal generator will we notice a difference? Is it important to terminate your signal at the end of coax or it doesn't matter?
If you were to set the RF generator to +10dbm.. Does this really mean that the generator will provide that power of +10dbm at a 50 ohm LOAD... But in order for the generator to accomplish this it actually has to generate +16 dbm aka 40mW of Raw Power aka 2.00 Volts Peak to provide +10dbm aka 1.000 Volt peak to a 50 ohm load... Let me know if this correct...
You're dB calculations are off, but you have the idea correct. The generator has to produce twice the raw voltage (into 100 ohms) in order to achieve the desired voltage into 50 ohms. Remember that power is calculated from RMS voltage, not peak. Considering sinewaves only: 2Vpeak = 1.414Vrms, 20mW (+13dBm) across 100ohms, and 1Vpeak = 0.707Vrms, 10mW (+10dBm) across 50ohms.
I have a function generator that supposed to have a 50 output however, I tried a 50 Ohm terminator as suggested at the input of my scope and voltage was still wrong. I then built a variable 10 to 100 ohms terminator and I found that adjusting it to about 37 ohms worked just fine. Is it possible that the output of my signal generator is not exactly 50 ohms and using a variable terminator is the right thing to use? Please reply because I need to know what you think... Thank you
Does the generator have a setting where you can input the load impedance? If not, then the best practice would be to use a load impedance that matches the coax impedance (usually 50 ohms), and then always measure/verify the output amplitude with your scope.
What I am trying to accomplish with the variable terminator is adjusting it so that the signal generator Peak-Peak voltage match the scope Peak -Peak readings. If I use a 50 ohm terminator on the scope, the reading are not the same however, if I use my variable terminator set to about 37 Ohms, the readings match each other. I tried this on both ends of the 50 ohm coax and the readings match the same. I'm thinking maybe the signal generator voltage display reading is off. I realize that matching the terminator to the coax is the best thing to do. I guess I should dis-regard the voltage reading from the signal generator and make adjustment according to the scope is my thinking while using a proper 50 ohm terminator at the scope input. Oh! buy the way, the signal generator is a GW Instek SFG-1013 which of course is a entry level signal generator and not a lab grade device. It's frequency and voltage output is rock solid and good enough for what I need. Thank you for any reply.........
If it appears that the amplitude is "off" by the same factor at all frequencies, etc., maybe you can make a modification (insert a series resistor) inside of the generator to make the voltage division more accurate.
Fantastic as always! Please do a video on measuring impedance. When can I use a 50 ohm terminator attached to a tee instead of a 50 ohm through terminator? Thank you, oscillator!
You can generally use a 50 ohm terminator on a tee instead of a through terminator for HF frequencies and below, because the stub length will be insignificant. However, don't forget that even low frequency square waves and digital signals will have frequency components in the very high frequency range due to the fast rising and falling edges. If your signal's rise/fall times approach 1ns or less, then you'll start to see distortions due to using the tee instead of a through termination.
Yes, but that is the "surge" impedance. In other words, it appears like 50 ohms to rapidly changing signals, due to the distributed capacitance and inductance. At low frequencies, or where the line length is very short with respect to the wavelength of the signal, the coax simply looks like a pair of conductors.
attach 8 ohm 50 w resistor...to sig gen...which outputs 1 v rms... or higher.... seems that lots of so called sig gens are LIMITED at output current..... they play nice with voltages,,,,but ....they dont like to output even 100 mA current,...
Signal generators almost always have a 50ohm output impedance - THIS is what limits the current. If it is designed to deliver 1Vrms to a 50 ohm load, that means that is generating 2Vrms at the gen side of the internal 50ohm resistor in series with the output. Thus, the voltage delivered to the 8 ohm load would be 2V *(8/58) = 276mVrms.
@@kaybhee6 If you need to drive such low impedance loads, like loudspeakers, etc., then you'll likely want to stick and audio power amplifier between your siggen output and the load. There really isn't anything you can do with the siggen since the output impedance is generally fixed.
Ran into this problem many times. Lots of head scratching trying to figure what's wrong. You had the answer. Thanks OM.
I'm currently studying Electrical Engineering and have had this issue in labs. The TA that was instructing just chocked it up to the signal generator just not being very accurate. I'm glad you set the record straight.
Maybe you should share this video with your TA so that they'll know the right answer too!
Just ordered my first signal generator.... and I think you just saved me from my first confusion! Thx
Oscillator LOL. Hope the Cold clears up soon
Abasinia
Oh, wow. I had no idea that's what that was -- I've just been told to put the signal generator into high-z mode and not known why! Thanks for the information, it's incredibly helpful.
Alan -- you make everything so clear. Thank you for this and all your other videos.
Thank you, Alan. I really appreciate your taking the time to send me an answer. I need to go back to your earliest videos and do some catching up! Best wishes...Dan
Great vid. Sounds like a very sore throat, so extra thanks for battling through. Get well soon! 73's, EA5IGC
Thank you for explaining the purpose of the High Z option. You made that simple.
Well thank you so much again, was just wondering about this because was trying to measure impedance using sig gen and oscilloscope, so look on your channel and here you are, a lifesaver
"Thanks again for watching and oscillator"
Thanks for being here, I need people like you.
73
Very good! Thanks for explanation! I made it im my AFG1022, where the signals was displayed with twice voltage value on the osciloscope. Now, whith this setup, the measurement is ok!
I have to thank you, because i do have the same osciloscope,function generator and the same problem on my school lab.
So very thank you for your video :)
A timely and useful reminder. Thanks once again Alan.
Dial-in your desired impedance 😲 Sweet! 👍😁 Excellent vid. Thank you!
For a split second, I thought I was watching a diff't channel. You sound diff't - it's allergy season here in NJ! Hope you feel better. Another good video.
Sir, as always you teach me something that i never knew, thank you,
Hi Alan, I have seen people tripped up by this before. A lot of old signal generators were marked "EMF" or "PD" to differentiate between the levels they would generate into Hi-Z or 50 ohms.
It can be frustrating with modern test equipment where the termination options are hidden in menus somewhere and not permanently displayed on screen.
Recently I went from an audio function generator to a real one (Agilent 33220) and was wondering why it showed double the value until I realized: "sure it has 50 Ohm in series and shows for 50 Ohms termination". So I just added a 50 Ohm terminator. Searched for a menu item on my generator to set it up properly, didn't find a menu item. You video made me look into the manual, and what do you know: Its not in the Output menu, it's in the Utility menu. That's Agilent for you ;) (Where files are not deleted but purged :))
On my Rigol, it's also Utility
Well dang. I've wondered why there was a miss match. I didn't think about either of the scenarios that you gave, and I think my problem is a combination of both. Great video!!! (and I'm just getting over a cold...nasty stuff this year)
Thanks for that lucid explanation.
Invaluable information as always! Thank you!
Next request: Lock the door into the lab, rest up, get well!
Great video Alan. This brings up the question: how does one go about determining the input or output impedance of a circuit?
This is a good question! Something I'll have to do a video on. Basically, to determine output impedance, you would observe the amplitude change as you change the load, and then calculate the output z.
Using the new cam by the looks of it?
Yes! I still have to work on the lighting, but for the first time out with the new cam, I think it came out ok.
***** The video is great, Alan!
Another great video! Sounds like you are nursing a head cold..Hope you get better fast!
73,
U saved my day, thank u so much. Really appreciated!!!
Thank you so much for this video!
Thank you for nice tutorial. I can understand the difference between high and low impedance.
I have one question.
I wonder why can we ignore the impedance of the cable.
In the video 2:15, simple circuit represents only function generator impedance and oscilloscope impedance.
I can't understand why here cable impedance 50 ohm does not included.
The coax cable impedance really only begins to matter at high frequencies. Remember, the cable impedance is simply the impedance seen by the signal as it travels down the line (see my video on transmission lines and terminations).
ua-cam.com/video/g_jxh0Qe_FY/v-deo.html
Dear w2aew,
Thank you for your reply.
I'm watching the link.
Thank you for your nice tutorial again!
I want to generate a "noisy dc" signal. Say a 5Hz 0v-5v primary and a 0.5v p2p 100Hz ripple on that 5Hz signal. How do I do that?
Program a noise signal of 0.5v p/p on your generator with an offset of 0-5V. That’s it!
dc volt ref sources are plenty... any suggestions for accurate to 1 mv sine or square ac volt ref.. please... now working on errors,, etc
Oh i get it now. I love your channel. Thanks for this.
Beautiful generator AFG3252 costs "only" us$12,500 new and 6000 2nd hand. Tek has a new serie called AFG31000 @ uk.tek.com/signal-generator/afg31000-function-generator which is basicley the same the serie mentioned in the video.
Very informative as always. Thanks for sharing.
Hello Alan,
Why is 50 Ohms resistor value widely used? And what is the use of having 50 Ohms internal resistance in the function generator?
The 50 ohm output impedance is used to provide a good source match to the line and load. Why 50 ohms, here's a good answer for that: www.microwaves101.com/encyclopedias/why-fifty-ohms
Gosh this was explained well. Cheers m80!
Thank you very much, this is the video i was looking for!
But i was wondering, what if my load is 80 ohm or 200 ohm impedance, because my function generator doesn't have the "load" option like in yours (it can only switch between 50 ohm or Hi impedance) do i have to put a 80 or 200 ohm terminator?
Sorry for the stupid question but i'm a beginner.
Hey - there's no such thing as a stupid question (only stupid answers, and I'll try to avoid giving them). Assuming that your switch simply adjusts the readout on your generator, you can set it to High Impedance, and then calculate the resulting amplitude by multiplying the reading by RL/(RL+50).
+w2aew "Assuming that your switch simply adjusts the readout on your generator" -> Can we assume that some generators are able to short their 50 ohm load, then?
+fabts4 I have seen some where you can change the output impedance from 50 ohms to 600 ohms. I don't think I've ever seen one where you can change it to (near) zero.
excellent explanation
great video..im planning to get a sig gen, I wonder what are the best practices to not blow up the device..im concern if its ok for the sig gen output front end to see DC voltage from the device under test..
I guess it would really depend on the particular signal generator (and how much voltage you're talking about). There is no universal yes/no answer.
This is a bit after you recorded this video (and it is very helpful). Since I have rekindled my bench time, I have a related question. Often I see an omega symbol in the vertical indicator at the bottom of the screen. I use a Rigol MSO2024 with a BNC cable connected between my Siglen ARB and the scope. I saw the incorrect P-T-P voltage (hence your video was helpful), but I need help understanding the Omega 50mV, for example, displayed on the screen. I hope this makes sense to you. Can you give me a good explanation? Thanks, Dave
The omega symbol in the vertical indicator is typically use to indicate that this channel's 50 ohm termination is being used. I would imagine it is the same thing on the Rigol.
2 Questions if you don't mind:
1)When you terminated the coax with a 50 Ohm terminator at oscilloscope input, why does that help? The oscilloscope now sees a 50 Ohm resistor versus a 50 Ohm coax but they are both 50 Ohm either way! To me, by adding the 50 Ohm termination it is almost like you just added another inch of the same 50 Ohm extra coax at the end which I don't understand how that helps. Is it that terminator represented a parallel connection to scope while the coax represented a series 50 Ohms?
2)When you put the function generator to high-Z (output impedance) isn't that a mismatch with the 50 Ohm coax so the signal should get reflected before even before it leaves the generator?
To answer your questions:
1) The termination provides a "matched load" for the transmission line. To understand why, it will help to understand how transmission lines work (and why they have a characteristic impedance). This video should help with your understanding of that: ua-cam.com/video/g_jxh0Qe_FY/v-deo.html
As you will learn from this video, a 50 ohm transmission line is NOT the same as a 50 ohm resistor. It only "looks like" a 50 ohm resistor to signals that are *changing* - so, crudely you could think of it as a AC-coupled 50 ohms. Again, the video should help clear this up...
2) When you change the function generator to high-Z, this does *NOT* change the output impedance of the generator. Let me say that again... it does *NOT* change the output impedance of the generator. This is a VERY common mis-understanding. The output impedance of the generator is *ALWAYS* 50 ohms. When you change the Output *load* setting on the generator, all this does is causes the generator user interface to change the output amplitude appropriately. Since the output impedance is 50 ohms, there is a 50 ohm resistor in series with the output at all times. So, the voltage at the load will depend on the load resistance because this load resistance forms a voltage divider with the generator's internal 50 ohm resistor. The *load* setting on the generator tells it how to calculate the voltage appearing at the load by properly calculating the effect of the voltage divider (50 || load_resistance).
@@w2aew I see now. Really liked your transmission line video!
It's not really clear to me why the resistor you connect to the scope is in parallel? (I calculated the resistance of a 50 and 1M ohm in parallel and got a value close to 50 ohm. That's clear to me) . But based on your drawing and my general understanding of the wirering then I persieve it as a series circuit and not a parallel.
Can you help me understand? Thanks for your patience and videos 😊
The 50 ohm resistor that I connect to the scope is *in parallel* with the scope's 1Mohm input, and since it is so much lower than 1Mohm, the result is very nearly equal to 50 ohms. This 50 ohm combination then appears *in series* with the 50 ohm resistor that is inside the generator (in series with the output amplifier). Then end result is that the function generator's 50 ohm output resistance is in series with the 50 ohm termination at the scope, thus making a 2:1 voltage divider.
@@w2aew this is what I was able to figure out myself. My problem is that the schematic doesn't look like a parallel. Could you explain how it is a parallel connection with the scope? It looks like a serial connection to me.
@@Infinitesap The 1Mohm input impedance of the scope is not drawn explicitly. The 50 ohm thru terminator shown at about 3:10 in the video has a 50 ohm resistor from the center of the coax to the shield (ground). Thus, when connected to the scope input, this resistor is in parallel with the scope's 1M input impedance (since the scope will now be measuring across this 50 ohm resistor.)
@@w2aew thank that made it clear for me. Please make more videos. I'm really learning a lot from you 🙂.
Thanks this really made me understand. Could you make a video on how to use the sync and trigger in- and outputs on the fg/scope. Would also be awesome if you could show application with them on breadboard setups.
ok so the real question now is, why do i see 2x voltage on oscilloscope on a car charger rectifier on 30V-32V when i'm supposed to see 15V-15.6V or so... I wish i had RMS multimeter with correct AC measurements, since the waveform that is shown is kind of like a mix of full bridge rectifier with the pulse modulation by putting some weird cap-discharge in the middle (there is a spike in the rising part of half-wave that diminishes or rises above the visible part, that contributes to output RMS thus one can control RMS from about 10V up to 15.8V)
You need something that can show/calculate the true-RMS value - because if the waveform is not sinusoidal, then the 0.707x factor is meaningless.
The function generator voltage would appear to be doubled because of the high impedance on the oscilloscope, so you would have to set it to 50-ohm mode or use 50-ohm feed-thru termination.
@Ralph Reilly I'm not sure why do you think I need to know, but it is one year old post.
This got me bugged. My function generator is the cheap JDS6600 and it doesn't have a setting for output impedance. According to the manual, the output is 50 Ohm Impedance. I'm connecting it to the oscilloscope directly with a coax cable. And I see the correct Peek to Peek reading. Why would that be? The oscilloscope inputs are 1 MOhm...
This simply means that the reading assumes a high load impedance. I bet if you connect a 50 ohm load, the the actual output would be 1/2 the setting.
can you make a video about a somewhat common issue which i cant find a way to fix, which is that my function generator and scope are both earth grounded and somehow ive read that causes issues..for example ive tried building a bridge rectifier but i cannot read the rectified wave, as soon as i try to do a common ground or anything my wave readings are totally off and awkward. It is now sunday, got my scope on friday , spent the entire weekend trying to figure this out... and i cant....scope noob :(
If you draw a schematic of your setup, including a "model" for the generator output and scope input, it'll be clear to see the problem with the common ground. A bridge rectifier expects an AC input at the input, where the input signal on each side swings above and below ground, while the output of the bridge typically has one side tied to ground. But, when you drive the input of the bridge with the function generator, that effectively ties one side of the input of the bridge to ground - so that node will no longer swing below ground (because it IS ground). A simply audio transformer can be used between the function generator and bridge rectifier input to solve this issue for your measurement purposes.
Great video. Love all of them. Sounds like you have a cold on this one though.
Hello Alan! I know you couldn't possibly know all the different oscilloscopes and signal generators. But, I will say that I have a Marconi Instruments 2022D Signal Generator, and my oscilloscope is a Fluke 196D Scopemeter (combination digital scope and multimeter). The manual for the SG says it has an RF output of between -127 to +13 dBm.(0.2uV to 2 V EMF). However when I connect the output of the SG to the probe on my scope, I do not get the readings to concur. Right now I have the SG set to 1000mV, but the scope is reading 4.1 mV+/- RMS. The scope shows 1:1 probe. What in the heck am I doing wrong? I want to connect the SG to the SO239 on my VHF radios and do some signal tracing. Thanks for any help...Dan
+Beretta96Dan The SG has to be loaded with a 50 ohm load in order to meet its specification. A 1X probe used with any scope will only give you less than 10MHz of bandwidth. Another problem you have is that the 196 only has 100MHz of bandwidth, which means that 100MHz signals will read 3dB down, and only if you use a 50 ohm thru termination and no probe.
How is the iinput impedance of oscilloscope connected to its terminals? In series or in parallel?
In parallel with the input.
Ok so the output on the wfg is setting up a voltage divider, that makes sense. So say we send a signal out of the wfg to a fairly busy circuit on a breadboard, since we have no way of knowing what the forward/reflected voltage will be on the overall circuit including the 75 ohm transmission line Ive sent my signal to, we should use the high Z output load if it has that ability to?
Sending a signal straight to a scope seems simple, 1 meg probe = high impedance.
Its thevinans theroem
Yes, a voltage divider, precisely. The generator uses the output-Z setting to properly compute the division so it can accurately show you the amplitude. It's always a good idea to measure/verify the output with a scope, etc.
@@w2aew thanks as always Alan. Happy new year. Enjoy some good propogation
Hi Alan Something else to be mindful of. The new generation of RF Function generators have much higher frequency capabilities. An excessive length of coax can turn into a Quarter Wave Transformer. At 150 Mhz that equates to a half meter multiplied by the velocity factor or 18 inches * .6 to .8. The equates to 10.75 inches to 14.4 inches...Something to think about.
Yes, excellent point. Anytime you get above a few MHz you have to start thing about transmission line effects, terminations, etc. I did a video on quarter wave lines as part of the video on tracking generators.
What voltage will be displayed on the oscilloscope if we set the function generator to high Z while having a 50ohm through terminator.
If the generator is set to Hi-Z, then it is expecting to see a high impedance load. Thus, the amplitude displayed on the generator would reflect that. But, since the load is actually 50 ohms, the actual voltage displayed on the oscilloscope would be 1/2 of the value displayed on the generator.
Thank you!
Thank you for letting us know this useful info, Any idea on how to set high Z for 33120A AWG, keysight ?
page 40 of this manual: www.keysight.com/upload/cmc_upload/All/6C0633120A_USERSGUIDE_ENGLISH.pdf
When using a function generator for cb/ham radio allignment would you need a generator with a dc offset??
Usually not - in most cases, you'd want to AC-couple a test signal that you're injecting to a circuit.
Thank you sir
Great explanation. Thanks a lot!
What Arbitrary Waveform - Function Generator would you recommend to an amateur radio hobbyist for $350 or less?
The SIGLENT SDG1032X is probably a good buy.
Many thanks. I was stumped until I watched this video and discovered I had a bad 50 Ohm Terminator. Doh... Not it all lines up.
Bought a 50 ohm bnc feed through terminator which appears to have the 50 ohm in series instead of parallel to the output. What is the use of such a 'terminator'? Attenuation?
It's most commonly used as a "source terminator" when the source has a low output impedance.
When the source impedance is lower than say 50 ohm. So it acts like the 50 ohm resistance in your schematic here.
If I measured the signal i/p on a circuit under test, and this measurement is the same that is being displayed on the generator, I could be confident there is no conflict of impedances? Correct? Thanks.
Yes. It's always a good idea to measure the signals at the device under test, just as it's a good idea to measure/check power supply voltages at the circuit and don't always trust the power supply display.
Hi, long post so bear with me..thank you for all your content, I can binge watch your videos all day as you explain things in detail in a way it's very easy to understand.
I know this is an older video, but have a request and forgive me if this has been covered. In regards to the 50 ohm termination from the function generator, can you explain the fundamentals of why a 50 ohm adapter is required (if so) at the scope from a function generator? What's confusing for me is if the FG has a 50 ohm output impedance, and this is before the output terminal, and the scope has a high impedance, why the 50 ohm at the scope input? My assumptions is the FG needs to see the 50 ohm load at the end of the cable to provide an accurate signal to the scope along the BNC cable. This likely comes down to basic electronics regarding load and current and I want to speculate I'm about 80-90% educated on this..lol.
I have built a workstation for the kids as their HS has an engineering dept as well as a robotics dept, I'm moving my hobbyist electronics gear to this workstation so we can all get involved, and want to make sure I have a complete understanding of the setup procedures for the equipment. The kids and I love to get inside and see how and why things work, and hopefully build some projects with the arduino kits as well as trial and error on a breadboard. Hopefully I explained my request/lack of understanding correctly, thanks again and I plan to catch up on your videos.
The short answer to "why a 50 ohm termination is needed at the end of the coax" is to prevent signal reflections that can distort the signal. This is explained more fully in this video: ua-cam.com/video/g_jxh0Qe_FY/v-deo.html
For low frequencies - below a few MHz, it really isn't that important since the length of the line is much much shorter than the signal's wavelength, and reflections don't pose any problem. For for higher frequencies, or for low frequency signals with high frequency content (such as fast rise/fall times), termination may be important.
The function generator has a 50 ohm resistor in series with it's output to terminate and reflected signals that are coming back from an unterminated end. This ensures that these reflected signals to re-reflect back towards the load again. Since the generator has a 50 ohm series resistance, and the expectation is that the far end load will be 50 ohms, this sets up a basic 2:1 voltage divider between the output amplifier of the generator and the far end load.
Again, for low frequency work, you can omit the 50 ohm far end termination. Just keep this in mind when setting the desired output amplitude. If the generator doesn't have a way for you to tell it that what the far end load impedance is, then you'll just have to remember that the output will be twice the set value if the far end load impedance is high.
@@w2aew Thank you, I did watch the linked video and your explanation helps alot. I forgot about the in-depth content of that video.
Sidenote, I actually frequent your videos on a different account which has more kid friendly subscriptions so the sidebar videos stay clean for when my kids and I view them.
Take care.
Thanks for this great video, it helps me to solve most of my questions. But one quesetion is left, when I have a load with an unknown impedance between 30 and 80 Ohm, should I set the impedance value of the funtion generator to 50 Ohm or to high z? I would be very happy about your help.
50 would be closest - but remember, all this is doing is adjusting the calculation that the generator does for the output amplitude to account for the voltage division of the 50 ohm output impedance and the load. In either case, if you want a specific amplitude applied to your load, MEASURE it AT the load and adjust the generator accordingly (regardless of what the generator is telling you the amplitude is).
@@w2aew thank you for responding!
"Load Impedance" just changes the amplitude voltage?! I don't have a Hi-Z option on my generator, I can use a 50 ohm attenuator or just set up half of the amplitude?
Yes, 'load impedance' changes the way the *displayed* amplitude is calculated. If your generator is expecting a 50 ohm load, you can use a 50ohm thru terminator, or simply account for the 2x voltage difference.
@@w2aew Thank you so much.
I am confused ... is peak-to-peak voltage on a square wave equal to true RMS, or is the RMS exactly half of peak-to-peak of a square wave? I have seen/read both. When hooking my Fluke 175 to the test output of my Tek 465 (300mv square wave), the Fluke measures approx half the peak-to-peak value displayed.
You also have to take into account any DC offset. But, in general, the RMS value of a 50% duty cycle square wave is equal to 1/2 the peak to peak value. This video might help:
ua-cam.com/video/ue0wtlrmCJE/v-deo.html
Thanks Allan! Just discovered your amazing channel!! Am plowing through them at a good clip. Playing some multiple times. Playing and pausing others, and then running over to my scope. This is an amazing resource. Thanks so much!!!!
You may know that I maintain a complete index of all my videos, arranged by number and by topic. The link can be found on my channel's homepage, in the lower right corner of the banner graphic.
my signal/ function generator provide constant output voltage for all frequency range when analyzed through oscilloscope, its fine, but when i connect a load across signal generator its output voltage drop significantly why?
It will all depend on the impedance presented by the load (as well as your measurement setup when connected to the load). Be especially careful of coaxial connections, proper terminations and coupling techniques between the generator, the load, and the measuring device (scope, etc.).
who do i know what is the internal resistance of function generator if i don't have user manual , further for dynamic load how do i match my load resistance with internal resistance of function generator.
In the vast majority of cases, the internal resistance is 50 ohms.
Well explained. Thank you
no way.
I thought there was something wrong with my tektronics awg2005 generator cause my other one siglent didn't have that. wow. thank you
Hi Alan. I was wondering is there noticable difference in signal shape or amplitude depending on where you terminate your coax? I mean you can connect your through termination at the oscilloscope or at the signal generator will we notice a difference? Is it important to terminate your signal at the end of coax or it doesn't matter?
It is almost always best to terminate at the load.
Very helpful. Thank you.
If you were to set the RF generator to +10dbm.. Does this really mean that the generator will provide that
power of +10dbm at a 50 ohm LOAD... But in order for the generator to accomplish this
it actually has to generate +16 dbm aka 40mW of Raw Power aka 2.00 Volts Peak
to provide +10dbm aka 1.000 Volt peak to a 50 ohm load... Let me know if this correct...
Thanks for such informative videos... Sounds like you have a bad cold.... Hope ya feel better...
You're dB calculations are off, but you have the idea correct. The generator has to produce twice the raw voltage (into 100 ohms) in order to achieve the desired voltage into 50 ohms. Remember that power is calculated from RMS voltage, not peak. Considering sinewaves only: 2Vpeak = 1.414Vrms, 20mW (+13dBm) across 100ohms, and 1Vpeak = 0.707Vrms, 10mW (+10dBm) across 50ohms.
I have a function generator that supposed to have a 50 output however, I tried a 50 Ohm terminator as suggested at the input of my scope and voltage was still wrong. I then built a variable 10 to 100 ohms terminator and I found that adjusting it to about 37 ohms worked just fine. Is it possible that the output of my signal generator is not exactly 50 ohms and using a variable terminator is the right thing to use?
Please reply because I need to know what you think...
Thank you
Does the generator have a setting where you can input the load impedance? If not, then the best practice would be to use a load impedance that matches the coax impedance (usually 50 ohms), and then always measure/verify the output amplitude with your scope.
Would I be doing the right thing using a variable terminator?
Thank you
It would be best to have the termination match the coax impedance (i.e. 50 ohms) to prevent any reflections.
What I am trying to accomplish with the variable terminator is adjusting it so that the signal generator Peak-Peak voltage match the scope Peak -Peak readings. If I use a 50 ohm terminator on the scope, the reading are not the same however, if I use my variable terminator set to about 37 Ohms, the readings match each other. I tried this on both ends of the 50 ohm coax and the readings match the same. I'm thinking maybe the signal generator voltage display reading is off. I realize that matching the terminator to the coax is the best thing to do. I guess I should dis-regard the voltage reading from the signal generator and make adjustment according to the scope is my thinking while using a proper 50 ohm terminator at the scope input.
Oh! buy the way, the signal generator is a GW Instek SFG-1013 which of course is a entry level signal generator and not a lab grade device. It's frequency and voltage output is rock solid and good enough for what I need.
Thank you for any reply.........
If it appears that the amplitude is "off" by the same factor at all frequencies, etc., maybe you can make a modification (insert a series resistor) inside of the generator to make the voltage division more accurate.
My antique generator is 600 ohms as was my uncle's at his old Tv shop. Wish I still had his old heathkit. ☹️
Lol, oscillator! Nice one and good video.
i mean, it is so basic. If some one does this mistake maybe he/she shouldn't use AWG in first place
Hi Do I terminate at the generator or at the scope?
For low frequencies (below a few MHz), it can be either place. Above a few MHz, it's best to terminate at the end of the transmission line.
Great tip! Thanks!
Great video! thanks!
Thank you, very impressive for me
What's the ttl output stand for ?
The TTL output designation on the signal generator means that this output is designed to be compatible with TTL logic signal levels.
Fantastic as always! Please do a video on measuring impedance. When can I use a 50 ohm terminator attached to a tee instead of a 50 ohm through terminator? Thank you, oscillator!
You can generally use a 50 ohm terminator on a tee instead of a through terminator for HF frequencies and below, because the stub length will be insignificant. However, don't forget that even low frequency square waves and digital signals will have frequency components in the very high frequency range due to the fast rising and falling edges. If your signal's rise/fall times approach 1ns or less, then you'll start to see distortions due to using the tee instead of a through termination.
I see what you did there at the end of the video
Is the output AC or DC ? ..or both ?
Both - it is an AC signal with a user adjustable DC offset
Thank you sir!
Thank you.
Thanks a lot
jakie to proste, dziękuję
Oscillator! That's a new one on me..... Moscow!
Aother reason the voltage is wrong is when a signal is applied to inductive or capacitive loads.
- But isn't the coax also 50 ohms?
Yes, but that is the "surge" impedance. In other words, it appears like 50 ohms to rapidly changing signals, due to the distributed capacitance and inductance. At low frequencies, or where the line length is very short with respect to the wavelength of the signal, the coax simply looks like a pair of conductors.
@@w2aew Thanks this solved a mystery for me :-)
attach 8 ohm 50 w resistor...to sig gen...which outputs 1 v rms... or higher.... seems that lots of so called sig gens are LIMITED at output current..... they play nice with voltages,,,,but ....they dont like to output even 100 mA current,...
Signal generators almost always have a 50ohm output impedance - THIS is what limits the current. If it is designed to deliver 1Vrms to a 50 ohm load, that means that is generating 2Vrms at the gen side of the internal 50ohm resistor in series with the output. Thus, the voltage delivered to the 8 ohm load would be 2V *(8/58) = 276mVrms.
@@w2aew many kind thanks.. now i know my stuff works... what do u suggest,, to increase current output..
@@kaybhee6 If you need to drive such low impedance loads, like loudspeakers, etc., then you'll likely want to stick and audio power amplifier between your siggen output and the load. There really isn't anything you can do with the siggen since the output impedance is generally fixed.
awesome!!!!!!
👍🌷👍
Oscillator! LOL!