Products > Test Equipment
How much noise floor and other things matter in oscilloscope usability
<< < (21/63) > >>
rf-loop:

--- Quote from: bdunham7 on December 25, 2021, 10:44:12 pm ---

I believe that many very-high-BW scopes now have more than a first-order roll off and thus a flatter frequency response but poorer step response. I

--- End quote ---

As is also of course in not-very-high-BW  Siglent SDS6k.
"poorer" step response...  what can also turn to "less aliasing" step response.
jonpaul:
Bonjour just seen this long thread now.

Suggest the OP check the many fine books and papers on noise, measurement, reduction, definition.

The noise "floor" is a function of the resistance, bandwidth, temperature.
Averaging is possible only on repetitive signals.
Noise may be irrelevant in some digital systems, but or primary importance in fine instrumentation, audio, etc.
Think of microphone preamps, seismic pickups, photomultipliers, etc.

A  digital scope and analog are "different animals" and have various benefits and downsides.

Many options for preamps, diff amps, etc. The best we have seen are the TEK 7000  plugin 7A22.

Finally the Chine scopes  may have misleading specs and hidden faults,   the cheapest implementation.

We have used the classic Tektronix scopes since 1967.

Just my reflections!

Bon Chance,

Jon




gf:

--- Quote from: Performa01 on December 25, 2021, 10:10:33 pm ---The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.

--- End quote ---

The spectrum reminds me on the typical frequency response of a 8-tap moving average filter (possibly in addition to other filters).
Is this the well-known HiRes mode, or yet a different mode?
David Hess:

--- Quote from: Performa01 on December 26, 2021, 07:12:27 am ---
--- Quote from: bdunham7 on December 25, 2021, 10:44:12 pm ---
--- Quote from: Performa01 on December 25, 2021, 10:10:33 pm ---Myth #2: “Frontends with higher bandwidths are always noisy, even when bandwidth limited.”
--- End quote ---

I think the statement, at least the one I'm thinking of, was that the noise density was higher for higher-BW capable amplifiers.  The noise will still be a function of the noise density and the actual bandwidth, so limiting BW will  still reduce noise as expected.
--- End quote ---

Well, the FFT plots in my screenshots show nothing but the noise density. Of course, the total noise is actually higher for the 2 GHz instrument at full bandwidth than what it could ever be on the SDS2kX Plus.

I strongly suggest that, other than in the seventies of the last century, for modern semiconductors the noise density remains fairly constant over frequency. In my screenshots it can be seen that it gets rather lower at higher frequencies and four times the system bandwidth doesn’t mean higher noise density at all.
--- End quote ---

For a given transistor technology and construction, there is a tradeoff between bandwidth and noise density, so for instanced a 2N3822 JFET supporting a bandwidth up to 115 MHz (1) has a noise density of about 3.5 nV/Sqrt(Hz) while a 2N4416 JFET supporting a bandwidth up to 250 MHz has a noise density of about 6 nV/Sqrt(Hz).  If the later is used in a 100 MHz amplifier, it results in higher noise than the lower performance part even with the same bandwidth.

Further, in general MOSFETs are noisier than JFETs which are noisier than bipolar transistors, which may be an issue with modern instruments which are more likely to rely on RF MOSFETs instead of RF JFETs for their input buffer.  It is difficult to make an analytical comparison here even if we know what part is being used because the RF MOSFETs are not as well characterized for noise.  This also means that the noise from a bipolar stage following the high impedance input buffer should be of no significance.

The above does not apply to higher bandwidth instruments that use exotic and effective unavailable to us technologies.  Specialized transistors on exotic processes will have a completely different figure of merit for bandwidth and noise compared to silicon MOSFETs, JFETs, and bipolar transistors, but the general rule about the tradeoff between them still applies.  But these inexpensive DSOs up to 350 or maybe even 500 MHz are not using anything like that.

And of course none of the above says anything about poor design.  Noise could be in excess of the predicted front end noise for lots of different reasons.  Empirical measurement is king here and easy to do in this case if the oscilloscope can report peak-to-peak or AC RMS (standard deviation) measurements.  (2)

(1) As a source follower where Ft = Gm / (2 Pi C); the transistor used for the high impedance buffer needs high transconductance and low capacitance.  High transconductance reduces noise, up to a point where other internal noise sources dominate, but the construction for low capacitance increases it.

(2) Which reminds me to suggest being a little cagey about Rigol's RMS and standard deviation measurements on a noise waveform, or any instrument which makes measurements on the display record.  I have seen evidence in that past that the processing to produce the display record corrupts these measurements when applied to noise.
David Hess:

--- Quote from: G0HZU on December 25, 2021, 09:42:58 pm ---
--- Quote ---The picture show 10 Ms/s. So there is anditional BW limit (~ 5 MHz) there. To do a fair comparison one would have the switch the faster scope also a slower hirizontal rate to get the lower sampling rate.
--- End quote ---

I'm not sure there will be a 5MHz bandwidth limit. With the 30MHz limiter enabled I think the bandwidth limit for signals is still 30MHz on this scope even at low sample rates. One would have to be wary of aliasing but the signal being viewed here is noise.
--- End quote ---

At low sample rates the noise within the ADC input bandwidth simply gets aliased to lower frequencies.  The total noise remains the same.


--- Quote from: Performa01 on December 25, 2021, 10:10:33 pm ---For low frequencies, things are a lot more complex than just a FET buffer, because of the split path design of all contemporary wideband frontend designs. The practical consequence is, that general purpose (wideband) oscilloscopes generally aren’t well suited for low frequency tasks below about 10 kHz regardless of the probes used. There are specialized instruments for this.
--- End quote ---

Split path high impedance buffers started showing up not long after integrated low input bias current operational amplifiers in the 1970s.  The split path actually reduces low frequency noise because even a noisy operational amplifier has lower flicker noise than the RF FET used for the high impedance buffer.  Sometimes it is a lot lower.

The disadvantage of the split path design is that without careful consideration, overload recovery can be horrible.


--- Quote from: gf on December 26, 2021, 11:09:48 am ---
--- Quote from: Performa01 on December 25, 2021, 10:10:33 pm ---The SDS2000 series has an excellent software enhanced 10 bit mode, which limits the bandwidth to 100 MHz and lowers the noise floor even more. See the next screenshot.
--- End quote ---

The spectrum reminds me on the typical frequency response of a 8-tap moving average filter (possibly in addition to other filters).
Is this the well-known HiRes mode, or yet a different mode?
--- End quote ---

High resolution mode is usually or always implemented as a boxcar averaging filter for simplicity since it must operate at the maximum sample rate during decimation, so it should produce something like a sinc response which is what is shown.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod