Electronics > Beginners
Oscilloscope bandwidth - is jump from 100MHz to 200 MHz significant?
EEVblog:
--- Quote from: bmdaly on March 02, 2018, 11:28:22 am ---Before finalizing the decision, I wonder if there are any use cases where the extra bandwidth would really make a significant difference to understanding what's going on in a circuit?
--- End quote ---
Not hugely, no.
Signal integrity on a 10MHz square wave for example you get some more harmonics adding to the detail, but nothing to write home about. Probing is vastly more important at this point.
Maybe if you were actually measuring amplitudes of >100MHz signals it helps a lot of course, but few people actually have that requirement.
Most people simply won't notice any real usable difference between between 100MHz and 200MHz bandwidth.
rhb:
It might be worth pointing out that in the 60's the 50 MHz Tek 547 was the main work horse for the Apollo program. So you can do a lot with a 50 MHz scope. And the 100 MHz Tek 465 was the standard for a long time after that. When I worked for Amoco in the early 80's there was typically a row of 2-3 465s by the window where I took my punch card decks.
tggzzz:
--- Quote from: EEVblog on March 05, 2018, 12:56:18 am ---
--- Quote from: bmdaly on March 02, 2018, 11:28:22 am ---Before finalizing the decision, I wonder if there are any use cases where the extra bandwidth would really make a significant difference to understanding what's going on in a circuit?
--- End quote ---
Not hugely, no.
Signal integrity on a 10MHz square wave for example you get some more harmonics adding to the detail, but nothing to write home about. Probing is vastly more important at this point.
Maybe if you were actually measuring amplitudes of >100MHz signals it helps a lot of course, but few people actually have that requirement.
Most people simply won't notice any real usable difference between between 100MHz and 200MHz bandwidth.
--- End quote ---
The frequency (or more accurately baud rate) of digital signals is completely unimportant w.r.t. signal integrity. I've even seen a problem on line with <<1Hz "frequency".
How so? The line signalled an infrequent error condition. When an error was signalled the risetime was sufficiently fast to cause overshoot which caused internal diodes to conduct - and circuit malfunction.
If an anthropomorphic explanation helps... When a circuit "receives" a transition it responds to that transition. Since it doesn't "know" when the next transition might be arriving, it doesn't "know" the frequency and hence doesn't "care" about frequency.
Here's a quick, dirty and somewhat unrealistic example designed to emphasise the problem. It is modern jellybean logic (3* 74lvc1g gates in parallel) closely decoupled, 5V supply, driving a <1inch unterminated line. The probe is a 1.5GHz low impedance Z0 probe (0.7pF) and a 350MHz scope. The scope is set to 10ns/div and 1V/div.
You can see that there is a 1.6V overshoot, which is more than enough to turn on diodes in a receiver (causing malfunction and/or long-term overstress) The overshoot occurs on a transition, and the frequency is so low that it is not visible in that trace; in fact it ~100kHz.
Zero999:
--- Quote from: bmdaly on March 04, 2018, 05:45:34 pm ---
--- Quote from: Hero999 on March 04, 2018, 10:47:41 am ---
--- Quote from: capt bullshot on March 02, 2018, 12:03:17 pm ---For your applications, you'll be fine with a 100MHz scope. My most used scope is an old and crusty 100MHz TDS220, next stop is the good old analog 7603.
4 channels is pretty useful, most of the time you'll find yourself using one or two channels.
Having more than 100MHz BW also requires more advanced probing techniques, and often 50 Ohm terminated inputs. Your vanilla 10x scope probe won't make any use of more than 100MHz BW - you'll just see more ringing. So 100MHz, or even as low as 50MHz is enough for the most tasks in debugging analog and uC circuitry.
--- End quote ---
Yes, the ordinary x10 probe won't work well at those frequencies. Fortunately it's easy to make your own low-Z probe. I made mine from 75R co-axial cable and some resistors, giving a total input impedance of 750R. 50R cable will work too, but the total impedance will be 500R.
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May I ask, why did you choose 75Ohm over 50Ohm? Do you have a 75Ohm input impedance on your scope?
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My oscilloscope has an input impedance of 1M, in parallel with 13pF.
I chose 75Ohm, because it would result in less loading, on the device under test, than 50R. If your 'scope has a 50R input, then use the built-in resistor and 450R on the end of the cable.
One thing to note is that the input impedance of the 'scope becomes a factor, at high frequencies. In my case 675R in parallel with 75R is 67.5R, forms a low pass filter with the 13pF capacitance on my oscilloscope's input FC = 1/(2pi*13*10-12*67.5) = 181*106 = 181MHz, which is a non-issue for a 100MHz 'scope.
Harb:
From my experience, few people need more than 100Mhz.........
Usually the extra money is forked out because the bigger the bandwidth number the "Cooler" it looks to have one.......same goes with most test equipment actually......
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