Is my scope too noisy? - Page 1
Forum section: electronics / testgear

author: billfernandez
author_rank: Regular Contributor
timestamp: November 08, 2015, 05:08:26 am
content: I'm finding that both channels on my Fluke ScopeMeter 199C, on the 2mV/div vertical scale, with 50Ohm BNC terminator caps on the inputs, have about 1mVpp of relatively random noise over the short term, and almost 2mVpp of noise over a period of a few minutes.I've attached some screen shots of this, and of the spectrum of the noise.does anyone know if this much noise in normal for this scope, or for this kind of scope?

author: mafio
author_rank: Contributor
timestamp: November 08, 2015, 05:15:22 am
content: Try testing filtering circuits. RC. If you see a difference then you maybe able to rule out your test bed.

author: Fungus
author_rank: Super Contributor
timestamp: November 08, 2015, 06:40:22 am
content: See this thread:https://www.eevblog.com/forum/testgear/ds1000z-periodic-'noise'/

author: billfernandez
author_rank: Regular Contributor
timestamp: November 08, 2015, 08:26:50 am
content: Quote from: mafio on November 08, 2015, 05:15:22 amThat would be a good idea if I was hooked up to a test bed, but in fact I only have 50 Ohm shunts connected directly to the scope inputs.

author: billfernandez
author_rank: Regular Contributor
timestamp: November 08, 2015, 08:27:29 am
content: Quote from: Fungus on November 08, 2015, 06:40:22 amThank you. That's an amazingly timely and relevant thread.

author: EEVblog
author_rank: Unknown
timestamp: November 08, 2015, 08:27:58 am
content: Also

author: vk6zgo
author_rank: Super Contributor
timestamp: November 08, 2015, 10:02:32 am
content: As Dave points out,there is a lot of noise around,which the DSO sees,& displays,without the averaging of an analog 'scope display.It does happen with analog Oscilloscopes,but not as obviously,tending to look like a thickening of the trace.Years back,we were disappointed by the "thick trace" on our new 7000 series CRO compared to a 545B.The problem?--------a bloody great pair of TV Transmitters on CH2---right in the middle of the 'scope's passband.(No problem for the 545B which rolled off at 35MHz)The 7000 series only had two plugins fitted,with a big hole between them---when we fitted a cover,the "thick trace" went away.I would have thought that the Scopemeter would have a narrower analog passband than the average Bench DSO,hence would belesslikely to appear "noisy".

author: billfernandez
author_rank: Regular Contributor
timestamp: November 08, 2015, 10:31:15 am
content: vk6zgo: Reminds me of the story of the radio telescope installation that finally tracked down some intermittent glitches to when folks in the break room would open the microwave oven before the cycle had completed.Dave: Thanks for your very informative video. Nicely done. Reminded me that I have bandwidth limiting and averaging on my scope.So with the inputs essentially shorted, and on the 2mV range, and the scope set to accumulate an envelope over a five minute period:o At the full 200MHz bandwidth the height of the envelope is 2mV.o Bandwidth limited to 20MHz, the height of the envelope is 1mV.o Bandwidth limited to 10kHz, the height of the envelope is 0.5mV.(these are my only choices)That tells me something about the character of the noise inherent in my scope. I would have expected it to be about an order of magnitude better, but I guess it's not.And for the 5 to 10Vpp signals that I'm preparing to measure, turning on averaging gives me very clean, sharp waveforms, while turning it off and adding envelope accumulation tells me something about the noise riding on the incoming signal. In either case the 2mVpp inherent noise of the scope should not affect my results. That seems like the best I'll be able to do with this scope.Thanks everyone.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2015, 10:48:49 am
content: It claims 200MHz bandwidth, so expect a similar noise level.Both instruments have excess noise. For reference, my TDS460 has something like 0.8mV RMS (350MHz BW, 50 ohms).What's ideal?Nothing is perfectly clean, there is thermal noise everywhere. Electrons undergo random walks, leading to a noise voltage and current (the ratio being the system impedance; in this case, 50 ohms) proportional to sqrt(BW). This is 0.919 nV/rtHz, or for 350MHz BW, 17uV RMS.The average op-amp is in the 10nV/rtHz range, give or take. (By the way, noise also goes as sqrt(R), so this would be typical noise for a 5kohm resistor.) So you can see, it's fairly tricky to measure true thermal noise.Likely, quantization noise dominates in most instruments. An 8 bit ADC is typical, with a few LSBs of internal noise. ADCs typically take a 1Vpk signal, so 1 LSB = 3.9 mV.But scopes also have preamplifiers, since 2mV/div (~20mV FS) is 50 times too small to read properly in the ADC. Even with the ADC being so noisy, most of the excess noise will be due to the amplifier. (If the input stage is a modest 5nV/rtHz, rolling off with a 200MHz passband, you expect 70uV input referred noise, or 3.5mV at the ADC, comparable to 1 LSB.)Understandably, any teeny fraction of supply ripple or noise on the front end (uV worth!) will contribute large errors on these small scales, so you want to make sure those circuits are well designed -- they dictate the true performance of the system!By the way, you don't need to sample at the full bandwidth to measure the noise level of the system. If you have an old school DSO, you can measure RMS with input GND'd, and see that the noise level is essentially constant on all ranges, except for the shortest time/divs (where samples are being interpolated, and you're seeing the roll-off of the system bandwidth). This is because the entire noise bandwidth is aliased into the sample band. Regardless of sample rate (given that it's low enough), the expected value of a given sample is V = <v_n>, and you're just doing statistics on V. (If the sample rate is comparable to BW, V is no longer uncorrelated, but changes less between nearby samples, because of bandwidth. This is good, because we need Fourier and statistics domains to agree!)On a modern DSO (with large memory and DPO effects and stuff), the sample rate and bandwidth may be quite different from the old school minimums, but the same measurement can still be made.If "High Res" mode is used, the sample rate is pushed slightly higher than what's visible, and a DSP filter is applied. If the sample rate happens to extend above the bandwidth, then no aliasing occurs, and noise is particularly low (having the best tradeoff against bandwidth). Otherwise, the noise level (in terms of nV/rtHz) will not be the noise floor of the system, but BW/Fs times greater, because all the noise above Fs gets folded into the pass band (at, say, Fs/8 or whatever the High Res filter is doing). Noise still goes down, because bandwidth goes down. But this is certainly something a modern DSO can have an advantage on.Speaking of bandwidth, pay close attention to the bandwidth setting as you zoom in -- many instruments did not offer full bandwidth on their most sensitive scales! I think a number of current Tektronix scopes still do this. I forget if the DS1054Z does.(Even back in the Tek 475 days -- mine gets extra-noisy on the 2mV/div range, because that range enables an extra 2.5x preamp in the signal chain. The offset also shifts, for the same reason. Though the bandwidth remains, so that's nice.)Tim

author: G0HZU
author_rank: Super Contributor
timestamp: November 08, 2015, 11:36:22 am
content: The noise floor is going to vary from model to model of scope.My TDS2012 is fairly noisy on the 5mV/div setting (full 100MHz BW) and it shows about 3mV pkpk noise at a casual glance.My old Tek 465 has quite low noise on 5mV/div. It appears to have a much lower noise front end when used on sensitive settings like 5mV/div when compared to the TDS2012. Small signal performance is one area where it outperforms the TDS2012 with a useful margin.

author: Rupunzell
author_rank: Frequent Contributor
timestamp: November 10, 2015, 08:44:49 am
content: Tek 7104. 7A26, 200Mhz with 1 Meg ohm & 22pf input impedance has about 0.8mV of noise displayed (DC to 200 Mhz). 7A29 with 50 ohm input impedance has about 2mV noise displayed (DC to 1Ghz) which is quite excellent given the board bandwidth.Noise performance of these "high impedance" inputs are limited by JFET device physics. They offer similar noise performance over a given bandwidth regardless if they are used in a analog scope or DSO. There are inherent noise adders due to signal conversion into digits (previous post already discussed) There are numerous tricks that can be played in DSO post processing that can aid in apparent noise performance, BUT be aware of how and what the post processing might do to signal fidelity on the display. Actual noise performance does vary a LOT between instrument brands, models and base technologies used in the specific time domain instrument.In real world T&M conditions, there are many things that can and will affect noise on the display including RF pickup from the environment, power line noise and more with nothing connected to the inputs.Once a cable or DUT has been applied to the input connector, this adds to the instrument performance degradation to varying degrees due to cable reflections, cable noise, EM field pick up and more. This is why anything HF and required high signal integrity ends up with the DUT at the input connector.Bernice

author: billfernandez
author_rank: Regular Contributor
timestamp: November 11, 2015, 03:09:11 am
content: Quote from: Rupunzell on November 10, 2015, 08:44:49 amThanks Bernice. This is why I capped the inputs of my scope with shielded, BNC terminators for my evaluation: So that all I would see would be noise and artifacts completely internal to the scope. I figured that 50 Ohm terminators (which I had on hand) were close enough to dead shorts.