| Products > Test Equipment |
| How much noise floor and other things matter in oscilloscope usability |
| << < (9/63) > >> |
| David Hess:
--- Quote from: Fiorenzo on December 24, 2021, 09:38:12 am ---What typology of circuits/signals need measurements with a 1x probe? --- End quote --- 20 MHz AC coupled power supply noise and ripple measurements work well with a 1x probe. This is actually specified in the ATX power supply standard. A 50 ohm cable can be used in place of a 1x probe but it will deliver worse performance. Audio measurements are another area where 1x probes are useful. --- Quote ---Does a higher sample rate give a better visual resolution of square wave signals at high frequency or its benefit is reduced by the amount of noise of the front end? --- End quote --- It does give better fidelity but how it affects noise depends on the implementation. High end instruments now use an ADC which can trade sample rate with noise and resolution, but for most, sample rate has no effect. The reason for this is that if the ADC is not doing noise shaping, then it operates at a constant (maximum) sample rate and different sample rates are produced by discarding samples during decimation, which has no effect on noise within the input bandwidth of the ADC. --- Quote from: nctnico on December 23, 2021, 01:28:17 pm ---Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty. --- End quote --- Most DSOs these days only have a single input divider. Older DSOs have at least two which allows the input buffer to operate over 1/10th of input range so the input full power bandwidth does not limit performance. Separately there is a low impedance output divider, usually now in the form of a PGA (programmable gain amplifier), with its own noise characteristics. At high sensitivity noise is dominated by the input buffer, and at low sensitivity noise is dominated by the preamplifier and ADC. --- Quote from: Kleinstein on December 24, 2021, 10:15:06 am ---For the noise relative to the input the amplifier a 10:1 probe can be a major noise source. The 1 M resistor in the divider has a natural noise of some 130 nV/sqrt(Hz), which is higher than a reasonable JFET based input stage (more like 10 nV/sqrt(hz) range). A low noise for the input is nice, but not relevant when using a 10:1 probe. --- End quote --- For a typical tip capacitance of a 10x probe, the 1 megohm shunt resistance is in parallel with about 100 picofarads of compensation capacitance producing a noise bandwidth of only 2.5 kHz, so its noise contribution is only about 6.5 microvolts RMS over a wide bandwidth which is close to insignificant. For the same reason, the noise contribution from the roughly 500 kilohm resistance in series with the gate of the input transistor for protection adds basically no noise. It is bypassed with about 1000 picofarads reducing its noise bandwidth to an insignificant level. --- Quote ---The lower gain settings usually use an internal input divider and if not designed good this may add some noise to 1 or 2 of the higher ranges, which can be a bit annoying as it is avoidable. So ideally a full noise testing would test all ranges, at least with 1 BW setting. --- End quote --- The internal high impedance input dividers are also capacitively compensated limiting their high frequency noise. The output dividers are low impedance so require no compensation, but have low noise anyway. In a modern DSO, these are part of the PGA. --- Quote ---For the ADC noise the sampling rate can make a difference. So the same DSO may look noisy at the higherst sampling rate but looks much better at a lower sampling rate when more samples are averaged. The noise relevant bandwidth is different from the 3 dB bandwith and can be quite a bit higher if the sampling rate is high, or closer to the -3dB BW when the sampling rate is barely sufficient. So it needs some case for the comparison to get comparable condictions (e.g. same sampling rate, relatively close to the maximum, like some 1 Gs/s for the scopes in question). With the high speed scopes the ADC noise can be a factor. --- End quote --- I have only seen high end DSOs take advantage of noise shaping in the ADC. There is probably some effect on noise for DSOs which use an interleaved ADC for multiple channels. Some old DSOs with relatively low real time sample rates, like 100s of MSamples/second, have less ADC noise (and preamplifier noise) than the quantization noise of their ADC. When I first saw this on my 2232, I thought something was broken or misconfigured. This might actually be considered a disadvantage when averaging where added noise would produce a better result and I have actually seen this happen with the averaged signal producing a stair-step from the ADC's quantization noise. |
| tautech:
Here's a simple example where the noise increases at the higher input sensitivities required to properly scale the higher attenuation probes. Done some years back and grabbed from an old post. SDS1104X-E with 1x, 10x, 100x and 1000x probes all connected to the probe Cal output. |
| Fiorenzo:
--- Quote from: tautech on December 24, 2021, 01:42:30 pm --- --- Quote from: Fiorenzo on December 24, 2021, 12:29:55 pm --- --- Quote from: bdunham7 on December 23, 2021, 04:11:46 pm --- --- Quote from: Fiorenzo on December 23, 2021, 03:36:01 pm ---What circuits works with such a low signal? This is my question from the beginning. I understand that noise sucks but it is the practical application of a low noise oscilloscope that i cannot figure out. For example, if I work with on an old valve radio am I going to encounter such kind of low signal? It Is only an example.... --- End quote --- On an old valve radio you might want to use a 100X probe, in which case a 'small signal' that would merit using the lowest range of the scope might be hundreds of millivolts. With a 10X probe, which is what you will almost always use, you might start caring about front-end noise at 50mV. Low front end noise gives you better FFT performance as well. There are all sorts of cases where noise is an issue and if you are just starting out, I can't predict which ones you will run into. --- End quote --- Thank you for all your explanations. Very usefull. I am trying to understand: what kind of electronics work with such kind of low signals? --- End quote --- Few but if ever needing to measure low values indeed noise can get in the way. When probing high impedance circuits and connection can effect the circuit operation and as 10x probes are most commonly used a 1mV/div scope setting need be used for a 10mV signal however it will only be displayed ~1div high. To take this to extremes a popular tablet DSO has just 50mv/div max sensitivity which when coupled with a 10x probe permits only 500mV/div sensitivity which is useless for anything but the simplest of tasks. When we need higher sensitivity 1x is used but at the expense of higher probe capacitance loading on the circuit so such use is often restricted to low impedance measurements like the ripple on a DC rail or that of a PSU. When most move to a DSO forgetting to set the input attenuation to match the probe is a common newbie error instead of letting the scope display the correct measured value. This is where probe autosense like in the other DSO you are looking at is of substantial value. --- End quote --- Thank you tautech and everyone again for the time spent replying. I find everything you wrote very usefull and interesting |
| Fiorenzo:
--- Quote from: David Hess on December 24, 2021, 02:05:21 pm --- --- Quote from: Fiorenzo on December 24, 2021, 09:38:12 am ---What typology of circuits/signals need measurements with a 1x probe? --- End quote --- 20 MHz AC coupled power supply noise and ripple measurements work well with a 1x probe. This is actually specified in the ATX power supply standard. A 50 ohm cable can be used in place of a 1x probe but it will deliver worse performance. Audio measurements are another area where 1x probes are useful. --- Quote ---Does a higher sample rate give a better visual resolution of square wave signals at high frequency or its benefit is reduced by the amount of noise of the front end? --- End quote --- It does give better fidelity but how it affects noise depends on the implementation. High end instruments now use an ADC which can trade sample rate with noise and resolution, but for most, sample rate has no effect. The reason for this is that if the ADC is not doing noise shaping, then it operates at a constant (maximum) sample rate and different sample rates are produced by discarding samples during decimation, which has no effect on noise within the input bandwidth of the ADC. --- Quote from: nctnico on December 23, 2021, 01:28:17 pm ---Also note that the noise doesn't only apply to the most sensitive V/div setting, it applies similar to all V/div settings. The noise level is usually specified in Volts using the most sensitive V/div setting but it would be more accurate to specify it as a percentage of a division or full range. In the end the V/div setting adjusts an input divider but the actual noise level (what goes into the ADC and what gets added by the ADC) stays the same; it is just scaled differenty. --- End quote --- Most DSOs these days only have a single input divider. Older DSOs have at least two which allows the input buffer to operate over 1/10th of input range so the input full power bandwidth does not limit performance. Separately there is a low impedance output divider, usually now in the form of a PGA (programmable gain amplifier), with its own noise characteristics. At high sensitivity noise is dominated by the input buffer, and at low sensitivity noise is dominated by the preamplifier and ADC. --- Quote from: Kleinstein on December 24, 2021, 10:15:06 am ---For the noise relative to the input the amplifier a 10:1 probe can be a major noise source. The 1 M resistor in the divider has a natural noise of some 130 nV/sqrt(Hz), which is higher than a reasonable JFET based input stage (more like 10 nV/sqrt(hz) range). A low noise for the input is nice, but not relevant when using a 10:1 probe. --- End quote --- For a typical tip capacitance of a 10x probe, the 1 megohm shunt resistance is in parallel with about 100 picofarads of compensation capacitance producing a noise bandwidth of only 2.5 kHz, so its noise contribution is only about 6.5 microvolts RMS over a wide bandwidth which is close to insignificant. For the same reason, the noise contribution from the roughly 500 kilohm resistance in series with the gate of the input transistor for protection adds basically no noise. It is bypassed with about 1000 picofarads reducing its noise bandwidth to an insignificant level. --- Quote ---The lower gain settings usually use an internal input divider and if not designed good this may add some noise to 1 or 2 of the higher ranges, which can be a bit annoying as it is avoidable. So ideally a full noise testing would test all ranges, at least with 1 BW setting. --- End quote --- The internal high impedance input dividers are also capacitively compensated limiting their high frequency noise. The output dividers are low impedance so require no compensation, but have low noise anyway. In a modern DSO, these are part of the PGA. --- Quote ---For the ADC noise the sampling rate can make a difference. So the same DSO may look noisy at the higherst sampling rate but looks much better at a lower sampling rate when more samples are averaged. The noise relevant bandwidth is different from the 3 dB bandwith and can be quite a bit higher if the sampling rate is high, or closer to the -3dB BW when the sampling rate is barely sufficient. So it needs some case for the comparison to get comparable condictions (e.g. same sampling rate, relatively close to the maximum, like some 1 Gs/s for the scopes in question). With the high speed scopes the ADC noise can be a factor. --- End quote --- I have only seen high end DSOs take advantage of noise shaping in the ADC. There is probably some effect on noise for DSOs which use an interleaved ADC for multiple channels. Some old DSOs with relatively low real time sample rates, like 100s of MSamples/second, have less ADC noise (and preamplifier noise) than the quantization noise of their ADC. When I first saw this on my 2232, I thought something was broken or misconfigured. This might actually be considered a disadvantage when averaging where added noise would produce a better result and I have actually seen this happen with the averaged signal producing a stair-step from the ADC's quantization noise. --- End quote --- Mr. David you gave a lot of explanations, this is very kind and usefull. Trying to do a recap: at this point It seem to me that a "low noise" oscilloscope is important when working with FFT analysis, power supply ripple, audio signal, and high impedance circuits? In regard of the mso5000 with its high sample rate of 8GSa/s I am not sure if in the balance It is an advantage due to its apparently noisy front end. As an ignorant, at the beginning I thought: the Rigol is better because It has better specs, so I bought it. Could you suggest me a different model if you think It could be better? |
| Fungus:
--- Quote from: Fiorenzo on December 24, 2021, 07:30:31 pm ---In regard of the mso5000 with its high sample rate of 8GSa/s I am not sure if in the balance It is an advantage due to its apparently noisy front end. --- End quote --- It definitely is. It will help reduce the Gibbs Phenomenon on your digital circuits. (Ever wonder how "ringing" can occur before a signal starts to rise? Undersampling combined with sin(x)/x reconstruction...) |
| Navigation |
| Message Index |
| Next page |
| Previous page |