How to connect function generator to scope? - Page 1
Forum section: electronics / beginners

author: jdb90
author_rank: Newbie
timestamp: February 05, 2025, 12:26:48 am
content: Hi,Beginner here.I've bought a function generator (TTI TG550):https://resources.aimtti.com/manuals/TG550_Instruction_Manual-Iss8.pdfI want to see it's output on my oscilloscope (OWON SDS1022):https://files.owon.com.cn/probook/SDS1000_series_Oscilloscope_USER_MANUAL_.pdfThe function generator has a 600 ohm and a 50 Ohm BNC outputs.I have some 50 Ohm BNC cables (as my other hobby is amateur radio)The scope input is I believe 1M Ohm so I assume I needs some sort attenuator/impedence matcher? Rather than just hooking it up directly with the aforementioned BNC cable? (Or breaking it out and attaching the probe hook/ground clip to the centre conductor/shield)?Will this do the trick?:https://www.picotech.com/accessories/oscilloscope-accessories/50r-terminator-bnc?

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2025, 12:35:53 am
content: You don't need to match impedances in this case and it will probably make things worse if you try. Just connect the 50R BNC output of your sig gen directly to the BNC input of your scope. If you do use a termination adapter (the one you listed would be OK) the best case is you will simply reduce your amplitude by 1/2.

author: TimFox
author_rank: Super Contributor
timestamp: February 05, 2025, 12:41:06 am
content: In general, it is a good idea to connect a low-output-impedance voltage source to a high load impedance.(For current sources, it is a good idea to connect a high-impedance current source to a low load impedance.)For the voltage source, you get maximum loadvoltagefor a high load impedance, but maximum loadpowerwhen you match the source impedance (e.g., 50\$\Omega\$).

author: jdb90
author_rank: Newbie
timestamp: February 05, 2025, 12:43:32 am
content: Thanks!Will I get an accurate reading of amplitude with the mismatch though?Also, since the max output of the generator is as high as 20v according to the manual (and I assume that's more than my scope can handle?) what level should I dial it down to? No more than a couple of volts?I don't want to blow anything up

author: ledtester
author_rank: Super Contributor
timestamp: February 05, 2025, 12:54:44 am
content: Quote from: jdb90 on February 05, 2025, 12:43:32 amYour scope can handle 20V.In any case you should set your probes to 10x mode. That's the recommended seeing for general use and it will also reduce the voltage seen by your scope by a factor of 10.

author: jdb90
author_rank: Newbie
timestamp: February 05, 2025, 12:55:21 am
content: They are already on 10x (so that's good)But I was meaning when connecting with BNC

author: ledtester
author_rank: Super Contributor
timestamp: February 05, 2025, 01:15:27 am
content: Look at it what it says around the BNC inputs...Peak voltage is actually 1.4 times RMS voltage.There are other caveats when measuring high voltage but low voltage is no problem even on 1x.

author: jdb90
author_rank: Newbie
timestamp: February 05, 2025, 01:20:08 am
content: Sweet, thanks everyone.I'll give it a go tomorrow then!

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2025, 01:27:12 am
content: Quote from: jdb90 on February 05, 2025, 12:43:32 amYour scope can handle 20V. That 20V would be for a high-impedance load like your scope. If you added 50R termination, you'd get exactly half of that. In either case, the voltage your scope indicates will be correct as that will be the actual voltage at its inputs.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 05, 2025, 02:23:22 am
content: Quote from: jdb90 on February 05, 2025, 12:43:32 amYou only need to be concerned about mismatch if the length of the cable is longer than 0.2 wavelengths.If the function generator has 600and 50the output amplitude will be accurate when driving these loads.The oscilloscope has a 1Minput, so this can ignored.To measure the 50 Ohm output from the function generator, connect the generator output to two 100 Ohm resistors connected in parallel. These will make a 50 Ohm load.The power rating of the resistors can be calculated:Power = V2/ R = V2/ 50Alternatively, if you connect the function generator to the oscilloscope without a termination the displayed signal amplitude on the oscilloscope will be double the output set on the function generator.Regards,Jay_Diddy_B

author: radiolistener
author_rank: Super Contributor
timestamp: February 05, 2025, 06:15:19 am
content: Quote from: jdb90 on February 05, 2025, 12:43:32 amThere are at least two issues with impedance matching:1) Amplitude depends on load impedance, so with impedance mismatch you will get different amplitude. In your case low impedance generator loaded with high impedance oscilloscope input leads to about 2x times higher amplitude on your oscilloscope. So, you can simply divide it by 2 and get actual amplitude for proper load.2) Impedance mismatch leads to reflections and ringing in the cable, it leads to waveform distortion. But for low frequencies, when wavelength is more than 100-200 times larger than cable length, you can just ignore it, because reflections effect will be very low.So, for low frequencies, for example up to 20-100 kHz you can use it with no impedance matching.But for more reliable results, you can put tee-connector on oscilloscope input and put generator cable on its one side and 50 Ω load terminator on the other side. I said more reliable, because when you measure output of your circuit with incorrect impedance it may works incorrectly. Another possible issue is that your circuit may works ok with high impedance load but fails with low impedance load and you will not see this issue without proper termination. But lab signal generators usually can support pretty wide load impedance from about 30-40 Ω to infinity with no issues.Also note, that 300V max rating limit marked on oscilloscope input is specified for low frequency or DC signals, usually it is valid up to 1-100 kHz but at higher frequency max rating limit is dramatically derated and usually is about 40V max for 1 MΩ input at very high frequency. So be careful to not burn your oscilloscope input. But lab signal generators which have much less max output amplitude is safe to use it with no care.

author: jdb90
author_rank: Newbie
timestamp: February 05, 2025, 06:27:01 am
content: Yeah, it would be more than a few 10s of kHz for sure - I'm wanting to sweep some filters/traps/etc for the 80m amateur band (so 3-4MHz ish)I'll dial the amplitude down to less than 5v on the generator, and order that passthrough adaptor to be safe, then.

author: radiolistener
author_rank: Super Contributor
timestamp: February 05, 2025, 06:36:19 am
content: At 3-4 MHz you can notice some reflection effect in the cable with no proper termination. It can be noticed as some amplitude varying depends on frequency/cable length.

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2025, 06:50:44 am
content: Quote from: radiolistener on February 05, 2025, 06:15:19 amFor a simple, straight connection of the FG to the scope reflections won't matter even at higher frequences because they'll just be reflected once and absorbed back at the FG. This is "back termination". You can go much, much higher than 100kHz with this arrangement.Here's a sig-gen connected to a scope with a 6-foot BNC coax cable. The sig-gen has a 50R output and is set for 2.5Vp-p(open circuit) and the signals are at 20MHz and 100MHz. There's no distortion and no attenuation at 20MHz. The 100MHz signal is attenuated some 1dB because the response of the sig-gen starts to roll off there and is -3dB @ 120MHz. Reflections are back terminated and don't affect the signal.

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2025, 06:55:26 am
content: Quote from: jdb90 on February 05, 2025, 06:27:01 amFor direct connections just to test things out and practice using the tools you can just use a BNC coax cable with no adapters and no worries. If you are introducing something else in the circuit that you are testing, then we'd have to know the details of that to advise you better as impedance matching will probably become relevant.