50 ohm termination, is there rule of thumb for maximum voltage? - Page 1
Forum section: electronics / beginners

author: AlessandroAU
author_rank: Regular Contributor
timestamp: February 04, 2016, 06:32:58 am
content: Hi all,This is just a general question, I have a DG1022u signal generator and I was wondering, is there a maximum safe voltage you can 50 ohm terminate into? I can't find any details in the manual.Are signal sources designed to handle enough current to terminate into 50ohm across their entire output range? I have seen oscilloscopes that specify 5v max into 50ohm, does this also apply equally to signal sources?Thanks!

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 04, 2016, 07:02:49 am
content: the rated 5v for a dso with built in 50ohm is due to wattage limit. You pass more than 5v you'll burn the small resistor inside.or mostly often it will heat up and goes out of spec... the trick is in power rating and resistive drift. If you can provide those 2 in spec, you can feed it any voltage you like...

author: KM4FER
author_rank: Regular Contributor
timestamp: February 04, 2016, 05:54:37 pm
content: Allan, W2AEW, made a video which may help answer your question.#137; Why your Function Generator's output voltage reading can be wrong.https://m.youtube.com/?#/watch?v=tClE8s6RZdgEarl

author: uncle_bob
author_rank: Unknown
timestamp: February 04, 2016, 06:03:11 pm
content: Quote from: AlessandroAU on February 04, 2016, 06:32:58 amHiAs in you want to back feed a signal into the generator?For a variety of reasons, that can be a bad idea. One is the simple one of powering the generator off with a back fed signal applied. Some generators disconnect the output with a relay when powered down. About 99.9% of them do not do this.If for some reason (there are good reasons) you need to do this, put an attenuator between the generator and whatever you are driving. If it's a 10db pad, and you are not going crazy (2.7KV back feed ...) the generator should be protected no matter what goes on.Bob

author: AlessandroAU
author_rank: Regular Contributor
timestamp: February 04, 2016, 10:17:56 pm
content: Hi all,Sorry, I might have been unclear! I understand 50ohm termination, let me rehash.I want to know if it considered o.k to put a 50ohm resistor at the end of the coax coming from the signal gen at its max output voltage (20v pk-pk). (just 50ohm terminate the signal).The reason I ask is because often I've seen oscilloscopes that have a 50ohm ohm input specify a max voltage of 5v into 50ohms. I want to know if a similar limit applies to a signal generator, in this case a rigol DG1022u, I feel like it might but is preheaps undocumented as I've found no mention in the manual.Would it be reasonable to expect a signal generator to handle 50ohm termination on its output up to its maximum output voltage?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: February 04, 2016, 10:28:09 pm
content: Quote from: AlessandroAU on February 04, 2016, 10:17:56 pmOf course, but the maximum output voltage is only 10 Vpp for channel 1, and 3 Vpp for channel 2 when driving a 50 ohm load:http://www.rigolna.com/products/waveform-generators/dg1000/dg1022/The rated voltage is half, because half of the voltage is dropped across the internal 50 ohm output resistor. The specs clearly say it can do 10 Vpp at 50 ohm, it would be a terrible design if that output resistor was not sized correctly to handle the power dissipation under those conditions (about 250 mW).edit: fixed a math error

author: AlessandroAU
author_rank: Regular Contributor
timestamp: February 04, 2016, 10:52:49 pm
content: doh.I did not see that table in the user guide. Thanks very much. That is exactly what I wanted to know.I agree it would be a terrible design if it couldn't handle 50ohm termination at max output, but I also could easily see some sleazy manufacturer skimp out and put a 1/4 watt resistor in there instead.

author: TimFox
author_rank: Super Contributor
timestamp: February 05, 2016, 04:37:22 pm
content: Both attenuators and terminations are available at different power ratings. 5 V into 50 ohms dissipates 1/2 W, which is a typical low-power resistor rating.For example, Weinschel makes high-power coaxial units up to 1000 W. Seehttp://weinschel-catalog.apitech.com/category/s-terminations-loads-high-power-terminations-loads?pcat=weinschel.Similar attenuators are more useful, since one can either terminate the output with a normal terminator or run it into a CRO or voltmeter (with appropriate terminator) to measure the input voltage.

author: uncle_bob
author_rank: Unknown
timestamp: February 05, 2016, 06:02:37 pm
content: HiBe careful generalizing this. There is a LOT of RF test gear out there that will blow out with a +/- 5V signal on the input. I have a lot of examples sitting in piles here and there. Some of the damage is repairable, some of it is not. Spectrum analyzers, frequency counters, even signal generators, all can fall into this category. It depends very much on the specific piece of gear and how you have the knobs and dials set.Bob

author: TimFox
author_rank: Super Contributor
timestamp: February 05, 2016, 06:17:36 pm
content: It is a good idea to use a fixed coaxial attenuator to protect sensitive inputs on high-frequency gear.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: February 05, 2016, 06:37:39 pm
content: I think your manual/device-labels are missing the information that they are rated for 5VRMSmaximum.I think it is perfectly fine to dump very high voltages into a 50Ohm terminator-resistor (up to the 300V Peak-Voltage-Rating), if you don't exceed 5V RMS.Btw. my PM3320A has circuitry inside that detects if the RMS-Voltage exceeds safe parameters and switches off the 50Ohms-Termination automatically to protect itself.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 06, 2016, 05:36:52 am
content: Quote from: blueskull on February 05, 2016, 01:15:19 amNote that the first attenuator takes the brunt of most of the power.I know,it is obvious,but I got caught one time,wondering why one attenuator was getting warmer than the other.