Oscilloscope sensitivity question - Page 1
Forum section: electronics / general

author: pfm
author_rank: Regular Contributor
timestamp: March 18, 2012, 06:40:36 pm
content: Need some oscilloscope sensitivity 101 -Typically sensitivity spec is listed as "2mv/div to 10v/div". What does this mean in terms of input voltages less than 2mv ? What happens if the scope is fed a 1mv or 0.1mv signal ? Is it virtually blind to it ?

author: robrenz
author_rank: Super Contributor
timestamp: March 18, 2012, 06:49:04 pm
content: At 2mV/ div and1x probea 1mVpp sine wave will be half a division high. It is just a measure of the maximum amplification occuring in the scope, not a cutoff where it does not work below that level.

author: Unknown
author_rank: Unknown
timestamp: March 18, 2012, 07:04:39 pm
content: Note that a real scope will have some non-zero noise floor, so that 1 mV signal, and certainly a 0.1 mV signal, may disappear in the noise. The 0.1 mV signal may only be a few pixels high on a QVGA display, and would be very close to the theoretical resolution of an 8-bit ADC of 0.063 mV.In addition, people often use a 10x attenuating probe for frequencies beyond the audio range, this will reduce the sensitivity by a factor of ten.

author: tekfan
author_rank: Frequent Contributor
timestamp: March 18, 2012, 07:07:07 pm
content: Don't forget that at the highest sensitivity settings an oscilloscope will rarely have full specified bandwidth.

author: pfm
author_rank: Regular Contributor
timestamp: March 18, 2012, 10:23:35 pm
content: Quote from: alm on March 18, 2012, 07:04:39 pmYes visually it would be impractical to see it but how well would it show up on the fft ? The table on page 5 of this Rigol 'How to Guide for FFTs' indicates that on adbscale it would go down to microvolts. Perhaps its just anexampleof the scale assuming the adc has that much resolution. Its a bit misleading in that case.http://na11.salesforce.com/servlet/servlet.FileDownloadPkb?file=00PA0000001Ax4c&orgId=00DA0000000IWhw&pPid=501A0000000DTWIbtw does anyone know what is the minimum real world limit of amplitude(voltage) measured for fft(not waveform) viewing by a Rigol DS1052 or Owon SDS6062 for a signal of say 1Mhz or 10Mhz with a 1x probe ?

author: Unknown
author_rank: Unknown
timestamp: March 18, 2012, 11:37:38 pm
content: Quote from: pfm on March 18, 2012, 10:23:35 pmSomeone just messed up the units. It should clearly read Vrms instead of mVrms, -60 dBVrms would correspond to 1 mVrms, or 0.001 Vrms.Quote from: pfm on March 18, 2012, 10:23:35 pmForum user rf-loopmeasured them for the Owon SDS7102. FFT would have exactly the same issues since it's based on the same data and noise tends to be quite broadband. Averaging might help if the noise is completely uncorrelated with the signal, but will not improve performance beyond the resolution limit unless the scope has a high-resolution mode (I don't believe any of the cheap Asian scopes support this) and only works if your signal is perfectly continuous (it will also smooth out any noise in the signal).This post contains an FFT of a Rigol DS1052E at 2 mV/div, although it may not be perfectly representative since it was specifically taken to show excessive noise. Noise floor appears to be about -50 dBVrms across the spectrum, ignoring the peaks.Don't expect your average source to drive 10 MHz in an 1x probe without attenuation, the capacitive loading at 10 MHz will be ~160 ohms, hence my statement that you probably shouldn't expect great fidelity beyond audio frequencies (100 kHz may be OK, 1 MHz only if you have a low-impedance source).

author: chscholz
author_rank: Regular Contributor
timestamp: March 19, 2012, 04:03:32 am
content: Good point, plus the highest sensitivity is often just a zoom on a lower sensitivity.Quote from: tekfan on March 18, 2012, 07:07:07 pm

author: rf-loop
author_rank: Super Contributor
timestamp: March 19, 2012, 06:24:05 am
content: Quote from: alm on March 18, 2012, 11:37:38 pmLater today, I try send some simply test result with FFT.It seems that noise floor is around -60 dB below full level.If I adjust oscilloscope so that 100MHz signal level is p-p around just full screen (around -10dBm signal @ 50ohm) and then look it with FFT. Not any averaging. Then drop signal level so that it is still clearly visible over noise floor. Signal -60dB below full level (-70dBm) is well visible. around 3-6dB over noise level (1). (Using infinite or 5 second peristence it is very good to find where is real noise level.) I will test later with some other settings and after have time I take some pictures.(1) it is littlebit difficult to say exactly what is signal peak level becouse near noise floor there is same amount of noise also "over" signal itself. But with persistence on, it can also visualize somehow. In this individual test it looks like noise floor (noise peak level floor) is under -60dB .. -65dB compared to maximum level.)This is also checked with signal itself for proof that it is real. (dropping signal down near to bnoise floor and looking point where signal just disappear under noise.)Always it need also remember that tests are made with one individual oscilloscope and it can not tell that every unit are same.(also SDS7102 input capacitance is very low (in new improved HW with also Fujitsu RF relays)).

author: pfm
author_rank: Regular Contributor
timestamp: March 19, 2012, 02:10:18 pm
content: rf-loop, it would be great to see the results. Please do post here.Maybe you could also try a frequency that’s in in the middle of the scale rather than at the edge of the scope capability ( I am assuming 100mhz scope) ? Say less than 50Mhz ?You could try different windows too and pick one that gives the best magnitude resolution. From the limited options I guess Blackman might be best.See this is where I’d prefer to just export the data out to an external fft analyser program that throws open options for several other Windows(Blackman-Harris, or B-H 7 term or Kaiser..) and configurations (zero pad, overlap,…). At that point the hardware has done its part the best it could, then its all limited by the software.

author: rf-loop
author_rank: Super Contributor
timestamp: March 19, 2012, 09:00:22 pm
content: Here:Signal source HP8644BFrequency around 100MHzLevel. For oscilloscope full display but not anything over for avoid clipping distortion.Signal with coaxial to scope and ended with Tektronix 50ohm.Scope setting: Input CH1 20mV/div AC.Capture normal, no average.Persistence infinity.FFT, zoomed 5x so that one div is 5MHz, center 100MHz and scale 10dB/div, Hanning.1. signal itself ( around 72mVrms)2. FFT same signal3. FFT same settings but signal level dropped 60dB and now around 72uVrmspersistence used for better show noise peak floor.Result: not bad for cheap oscilloscope.

author: pfm
author_rank: Regular Contributor
timestamp: March 20, 2012, 03:16:51 am
content: Very cool. Thanks for doing this.Would be great to see an apples to apples comparison done same way for other comparable scopes from Rigol, Atten, as well as mainstream like Tektronix, BKP.

author: Unknown
author_rank: Unknown
timestamp: March 20, 2012, 04:06:03 am
content: Quote from: rf-loop on March 19, 2012, 09:00:22 pmThat's quite impressive, so noise is less than -60dB, or less than 0.25 bits at 8-bits vertical resolution. Does the Owon apply some sort of averaging? Is performance from a high-impedance source (eg. 10x probe with probe tip to BNC adapter) at highest sensitivity similar?

author: rf-loop
author_rank: Super Contributor
timestamp: March 21, 2012, 12:45:09 pm
content: Quote from: pfm on March 20, 2012, 03:16:51 amMaybe this need wait forever... specially if want that mesurements meets even some acceptable good lab practice and is repeatable in some other lab. But yes I hope...

author: rf-loop
author_rank: Super Contributor
timestamp: May 17, 2012, 07:51:52 am
content: Pop up.It is nice if get result from some other entry-level oscilloscope so that measurement is comparable. Aplles to apples.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 17, 2012, 10:30:10 am
content: Quote from: pfm on March 18, 2012, 10:23:35 pmThe average noise spectral density from a digitizer is 1 LSB / (sqrt(12 * f_nyquist)). If you have sharp spectral features you easily can see them on an FFT even if they are well below 1 LSB. The only caveat is that you need sufficient dither/noise to spread the signal over several digitizer values.So in principle, an oscilloscope input with an 8 bit digitizer and 20 mV full scale and a 1 GS/s sample rate could have a noise floor of around 1 nanovolt / sqrt(Hz). You won't reach that in practice of course due to other noise sources, and it can be difficult to sufficiently dither an 8 bit ADC without clipping which will create horrible spectral artifacts. Also, most oscilloscope FFT implementations suck terribly, and don't do the front end justice.

author: rf-loop
author_rank: Super Contributor
timestamp: May 17, 2012, 01:59:37 pm
content: Quote from: alm on March 20, 2012, 04:06:03 amI made tiny test.Owon SDS7102 connected with original probe (T5100) to HP5644B output.Output is not high impedance of course. HP 50ohm output N/ N-BNC / Tektronix 50ohm feed thru/ BNC - Probe adapter 28P230-1/ Owon probe (this make direct GND connection to probe tip)Pictures:-----------------------------bProbe set 10:1Scope set: probe 10x, AC, 100mV/divFFT 0-500MHz, 10dB, vertical adjusted so that signal area is ok in displayCursors adjusted so that other is top level and other is -2.8dB from top.Generator set for level 3.3dBm (for level control there is also this 50ohm termination)Sweep 1 - 500MHz.After sweep stopped to 60MHz so that there are some harmonics on the display, just for fun.Near 430MHz there can see small drop out in frequency response. I have no idea what it is.(some kind of match - resonate or what ever... with normal direct 50 ohm coaxial cable there is not this. But... this is "100MHz" cheap oscilloscope. )----------------dConnection as before.Generator 10dBmScope 200mV/div 10x, (probe 10x) (same as 20mV/div if 1x)Sinewave p-p level nearly full 10div.-----------------ePersistence now infinite so that all noise peaks can see.FFT same signal. Same input settings, 10dB and zoom 5xCursors around signal peak and noise peak level.-----------------iFFT same, signal level dropped exactly 60dB (-50dBm)(of course signal top looks like more now, due to noise and of course some inaccuracies with these low level. (signal is now 708uV)Signal peak can easy detect from noise.------------------oNow oscilloscope input change to 50mV/div (10x) generator and all same.level adjusted so that nearly full 10div (gen out around -2dBm)--------------------pFFT vertical adjusted and agen scale 10dB.Signal level is now -2dBm.-------------------rSame but signal level dropped 50dB (to -52dBm)Now agen signal can detect easy from noise.Maximum value to noise peak level is now around 10dB less... (more moise)-------------------gFFT as picture e but now average 16.-------------------With own "cheap" probe. Not bad result at all imho..

author: w2aew
author_rank: Super Contributor
timestamp: May 17, 2012, 02:02:26 pm
content: Keep in mind that some scopes (especially some older analog scopes) have a buffered copy of the signal from one of the input channels available at the rear panel. At the low v/div settings, this output may actually provide some gain. Thus, connecting this back to a second input channel, you can effectively increase the sensitivity even more. Here's an example of that in this video (this example is shown starting about 4 minutes in...)