Oscilloscope noise floor thread - Page 1
Forum section: electronics / testgear

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 08:50:47 am
content: Hey guys,I'm just curious about what you guys are getting for noise floors out of your scopes. Please share some screenshots. I just bought an entry level R&S RTS1002 for $975USD. Here's what I'm getting set at a 50Mhz bandwidth, and 1Mohm input:

author: borjam
author_rank: Unknown
timestamp: September 20, 2019, 10:48:55 am
content: Siglent SDS1202X-E50 ohm terminator connected through BNC-SMA adapter

author: seronday
author_rank: Regular Contributor
timestamp: September 20, 2019, 11:09:41 am
content: Mivera,The RTC1002 screen shot has HI Resolution mode enabled.Can you show another screen shot with Hi Resolution mode turned Off.

author: Performa01
author_rank: Super Contributor
timestamp: September 20, 2019, 11:10:05 am
content: Quote from: borjam on September 20, 2019, 10:48:55 amWhat kind of comparison is this going to be?96.9kSa (what's this? Sample rate or record length?) vs. 3.5Mpts @ 50MSa/s?RMS measurement (that includes the DC offset)?Here's one rare instance where a serious attempt has been made to do it properly:https://www.eevblog.com/forum/testgear/oscilloscope-input-noise-comparison/

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 12:09:42 pm
content: The horizontal range makes a huge difference and needs to be specified as well.My DSO9104A is an 8 bit scope, but when I set the horizontal range to 5 ps, the average noise with 50 MHz bandwidth limit, 50 Ohm input (terminated) is 16 uVppBut when I set the horizontal range to 100 ns, noise goes up to 173 uVppAt 500 ns, it's 210 uVpp

author: nctnico
author_rank: Super Contributor
timestamp: September 20, 2019, 12:14:09 pm
content: Quote from: Performa01 on September 20, 2019, 11:10:05 amAgreed. Just showing screendumps is going nowhere.

author: borjam
author_rank: Unknown
timestamp: September 20, 2019, 12:42:14 pm
content: Quote from: Performa01 on September 20, 2019, 11:10:05 amYes, you are right. I hope we will be awarded an Ig Nobel in MetrologyThat said, just trying to match the settings of the first sceenshot without giving it much thought.

author: Fungus
author_rank: Super Contributor
timestamp: September 20, 2019, 12:59:05 pm
content: Quote from: Gyro on September 20, 2019, 11:52:38 amIf we all turn on our 20MHz bandwidth limiters we'll get something similar, yes.(that's what it's for!)

author: srce
author_rank: Regular Contributor
timestamp: September 20, 2019, 01:52:36 pm
content: Have a look at this thread:https://www.eevblog.com/forum/testgear/oscilloscope-input-noise-comparison/

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 02:26:44 pm
content: I'm just learning how to use this scope still. If the goal is to get the lowest noise out of the machine, wouldn't it be best to put in a mode that achieves the lowest noise?

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 02:46:33 pm
content: I'll play with it more today. I just bought it yesterday and have no idea what I'm doing. I also don't have any 50 ohm BNC adapters to put on the inputs. But just ordered these:https://www.digikey.com/product-detail/en/CT2944C-50/CT2944C-50-ND/9826334?WT.z_cid=sp_22_buynowShould I be able to get the noise even lower when those connected?

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 03:39:53 pm
content: Quote from: Mivera on September 20, 2019, 02:26:44 pmYou must put the scope in a mode that allows the intended measurement to be made. That mode might not be the lowest noise mode. For example, if that is a 100 MHz scope and you need to measure an 80 MHz sine wave, the scope will be in a high noise mode because of all the available bandwidth. The higher the bandwidth the more noise.But in order to measure and compare oscilloscope noise, the measurement has to be done in a carefully prepared, deterministic test setup.

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 03:43:14 pm
content: I don't need 80mhz bandwidth for what I'm measuring. My intention of this thread is to see the lowest noise possible you can achieve from your machine.Here's another shot of the R&S1002 with 20mhz bandwidth, and 1GSa refresh. HR mode off. No 50ohm BNC adapter:

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 03:43:23 pm
content: Quote from: Mivera on September 20, 2019, 02:46:33 pmIdeally, you would want to use just a terminator. What you bought is a thru terminator with a BNC connector, so it isn't much different from the oscilloscope's own input in 50 Ohm mode. By terminating the input with a normal terminator, you almost completely eliminate noise injection into the scope's input through the BNC's center pin (not that it will be much, but still, it is a source of non-determinism).

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 03:45:16 pm
content: My scope doesn't have a 50ohm mode. So did I buy the right adapter to add a 50ohm mode?

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 03:50:02 pm
content: Quote from: Mivera on September 20, 2019, 03:45:16 pmYes you can use that adapter to measure the output of a 50 Ohm source.

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 03:51:13 pm
content: So using this adapter will achieve the same thing as if my machine had a 50ohm mode and I enabled it?

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 04:04:48 pm
content: Quote from: Mivera on September 20, 2019, 03:51:13 pmIt will not be 100% the same thing. If the scope had a 50 Ohm mode, then you would be terminating the signal source with a near perfect 50 Ohm load. Here, you terminate it with a 50 Ohm load, but on top of that, the oscilloscope's input will modify that near perfect match somewhat (consider internal capacitance of scope, the 1 MOhm impedance and other factors). But for low frequencies it will be good enough.

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 04:08:15 pm
content: Ok should be good enough for me. The max bandwidth I actually need for my purposes is 1mhz.Can you send me a link to the BNC termination everyone is using to lower noise? So would I put one of those together with the 50ohm adapter for best results?

author: taydin
author_rank: Frequent Contributor
timestamp: September 20, 2019, 04:15:31 pm
content: Here is a link from pasternack:https://www.pasternack.com/2-watts-bnc-male-rf-load-up-to-4-ghz-pe6156-p.aspxBut you gotta understand that my purpose of using a terminator in the noise measurement was just to short the BNC live to ground in a nice way, so that it doesn't pick up noise from the air. This eliminates measurement variations when the location has high RF noise or low RF noise.

author: nctnico
author_rank: Super Contributor
timestamp: September 20, 2019, 04:27:39 pm
content: But if you measure a power supply the signal source you are measuring has such a low impedance that 50 Ohm termination doesn't matter. 50 Ohm termination is only necessary when the connection to your DUT is a transmission line (rule of thumb: when it is longer than 1/10 of the shortest wavelength of the signal). Again: when measuring low lovel signals noise in the ground connection will dominate the measurement.

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 04:30:28 pm
content: Ok thanks. My scope has a button to enable ground. But when I enable it the run mode stops.

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 04:32:31 pm
content: Quote from: nctnico on September 20, 2019, 04:27:39 pmI plan to follow the recommended methods in this video:And Dave recommended the 50ohm mode. This is why I bought these adapters. So you don't think they will help?

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 05:00:17 pm
content: Actually that's not where I read that. It was other places such as in this application brief:http://literature.cdn.keysight.com/litweb/pdf/5992-0946EN.pdf?id=2653138But now on 2nd read not sure if the external adapters will change anything.

author: egonotto
author_rank: Super Contributor
timestamp: September 20, 2019, 05:42:58 pm
content: Quote from: Mivera on September 20, 2019, 03:43:14 pmHello,20 Mhz bandwidth and 2ns/div makes no sence.Use better 1ms/div.Please 1 mHz = 0.001 Hz, 1 MHz = 1000000 Hz.Best regardsegonotto

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 06:01:11 pm
content: Ok here's with those settings and nothing connected to my BNC's:

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 06:21:08 pm
content: in this review of the Siglent SDS1104X-E, he had the time scale set at 50ns with 20mhz bandwidth. And terminated the BNC. His result was 28uV RMS and 150PP:https://www.eevblog.com/forum/testgear/siglent-sds1104x-e-in-depth-review/?action=dlattach;attach=397075Here's what I'm getting from the R&S RTC1002 with nothing terminated on the BNC and with the same settings:

author: egonotto
author_rank: Super Contributor
timestamp: September 20, 2019, 06:42:20 pm
content: Hello,that look very good.But please don't use mHz if you mean MHz.Best regardsegonotto

author: nctnico
author_rank: Super Contributor
timestamp: September 20, 2019, 06:48:49 pm
content: Quote from: egonotto on September 20, 2019, 06:42:20 pmNote mean (average) is a useless number. At least RMS gives some information about the actual signal level but this will also include DC offset. If you want to get a ball-park number then use peak-peak measurement and use statistics to average that over a couple of hundred sweeps.

author: Mivera
author_rank: Regular Contributor
timestamp: September 20, 2019, 07:30:56 pm
content: Well not bad for a rebranded piece of junk that's only worth $400 at the most anyways.