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| Why do oscilloscopes have bandwidth limits? |
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| alm:
Read the Tektronix Circuit Concepts book on vertical amplifiers that Jon referenced, at least chapter one. Obviously no one makes vertical amplifiers with discrete transistors or tubes anymore, but the description about the relationship between bandwidth and transient response is still relevant. The bandwidth is limited by the design of the amplifier. In the case of an op-amp, it would be related to the gain-bandwidth product and the gain it is configured for in the circuit (very simplified). There might be additional limitations, like the configurable bandwidth limit (e.g. 20 MHz), or bandwidth limits added so they can offer lower bandwidth models without much additional engineering. That's why some scopes can be 'hacked' to a higher bandwidth, but this is generally limited to the highest bandwidth model in the same series, like 50 MHz Rigol DS1052E to 100 MHz (DS1102E equivalent), but not higher. One important distinction is if you want to look at the bus to decode what is being sent, or if you are verifying the signal integrity of the bus where you want to look at details like meeting setup-and-hold timing requirements. There's a rule-of-thumb chart posted here with the necessary bandwidth for decoding and checking signal integrity for common buses. |
| jonpaul:
à moment please analog and digital scopes are completely different in the archetecure design and components All blocks in the signals path affect the overall observed bandwidth For example an analog scope CRT with a simple deflection plate has perhaps 10..50 MHz BW At 100..300 MHz a dome mesh PDA For 400 MHz to 1 ghz distributed deflection with controlled delay and Zo Tektronix 2467/B and 7104 use that and microchannel intensifier plate see the Tektronix CRT concepts book Bon courage Jon |
| robert.rozee:
i am quite certain that i will regret asking this, but... if you're just looking at the rise-time of digital (0-5v) signals, why do you need an amplifier in front of the ADC? most oscilloscopes are designed to handle input signal levels ranging from 10's of mV up to 10's of volts, but in the specific case outlined in this thread it seems that this sort of flexibility is not required. might you just need perhaps an impedance buffer, and possibly a differential driver (in the case of the ADC having a differential input). and i'm wondering if you even need more than ONE channel? yes, this might end up being a very specialized instrument, one that can barely be called an oscilloscope. cheers, rob :-) |
| TheUnnamedNewbie:
I'll risk going over board here, but as my dayjob is working against these limits, I'll give a brief insight in what is going on. For scopes, I think there are a multitude of factors: Cost, interconnections, and technology. Cost is simple: faster needs more design expertise, fancier technologies, more hardware, more storage, and at the end of the day things are more expensive. Interconnections: You can't just send 10 GHz signals over cheap connectors. The connector starts to have a big impact, as do the cables. Everything starts becoming part of the circuit and that requires more finicky interconnections. I wouldn't want to use our 110 GHz UXR scope to measure an arduino output, because it would load it with 50 ohm which the arduino probably wouldn't be very happy with. High speed scopes start using connectors like 3.5 mm, 2.4 mm, etc, which are very fragile, require care in connecting-disconnecting, and are very expensive - a 2.4 mm to 3.5 mm metrology-grade adapter might cost several hundred euros a piece. I don't want that on a scope I want to use to measure some SPI busses with, so there is little point making those scopes that fast, in a right-tool-for-the-job kind of way. At the upper limit is technology. There are two metrics with regards to the 'speed' of an active device, the 'transit frequency', or ft, and the 'maximum power gain frequency', or fmax. This is the point where, due to the fundamental physical limits (things like the mobility of carriers in semiconductors) make it so a device no longer produces current gain (for ft) or power gain (for fmax). At this frequency, you need more current into a device than you get out, so you no longer have the ability to make signals larger - let alone do more complex processing of this signal. This puts a first-order boundary on a scope. (actually above-fmax circuits are a thing but lets not open that can of worms) For discrete active electronics this is going to be in the few GHz (usuall) at best. It goes higher when you use integrated circuits - small CMOS goes to about 300 GHz (depends on the technology, but most people agree that it peaks at around 40nm CMOS nodes). SOI can go further, to 400 GHz, SiGe can do 500 GHz, InP has technologies that go up to 1 THz (but that technology is trash for anything but a very simple amplifier). On top of that, a significant additional factor is that designing these high speed 100 GHz circuits is by no means trivial. It is by many considered a black art, and us millimeter-wave and RF designers are sometimes treated as voodoo magicians, wrestling waves out of devices. Sparameters, matching, smith charts... An on top of that, to get to these points, you need to start canceling out poles with zeros, or as an RF designer would see it, tune out capacitance with inductance. This introduces a bandwidth limit - you only tune out the capacitance at a certain frequency. Make it more wideband? That requires more complex interconnections of inductors and capacitors, which in turn leads to lower gain, and at some point you lose all the benefits you were trying to get. Even though you can design a 230 GHz to 250 GHz amplifier in 45 nm CMOS, you cannot design a DC-200 GHz amplifier in the same technology |
| Brumby:
--- Quote from: TheUnnamedNewbie on November 02, 2022, 12:20:24 pm ---I wouldn't want to use our 110 GHz UXR scope to measure an arduino output... --- End quote --- Thank you for that. That was a brilliant mental picture! :-DD |
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