Electronics > Beginners
Oscilloscope bandwidth - is jump from 100MHz to 200 MHz significant?
bmdaly:
--- 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.
--- End quote ---
May I ask, why did you choose 75Ohm over 50Ohm? Do you have a 75Ohm input impedance on your scope?
bmdaly:
Thanks everyone for your comments.
--- Quote from: tggzzz on March 02, 2018, 01:48:27 pm ---It depends on what frequency analogue signals you will be using, what you will be looking for, and what logic family you will be using.
Sometimes analogue circuits can oscillate at a much higher frequency than you are expecting ("amplifiers oscillate, oscillators won't").
In some analogue applications, harmonics of the fundamental frequencies are important; factor that into your assessment.
A 100MHz scope will (just about) allow you to see a 3.5ns risetime; a 200MHz scope 1.8ns. Logic families from the early 80s (LSTTL, STTL) have that kind of risetime. Some modern jellybean logic families have sub-nanosecond risetimes (e.g. 74lvc). If using similar families, a principal use of a scope is to check "signal integrity", which needs all the bandwidth you can get.
Most "high" impedance *10 scope probes with a 6"/15cm ground lead self-oscillate at ~100MHz. A 200MHz scope will show you that, but a 100MHz scope will disguise it :) At such frequencies construction techniques and probing techniques become important.
--- End quote ---
Thanks for raising these limitations/pitfalls - some links that address some of them, that might be of interest to others:
* some interesting info on probing here - https://youtu.be/zodpCuxwn_o - from You Tuber Alan Wolke (w2aew)
* ... and here, tggzzz's own blog - https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ (Surprisingly, Tektronix's http://w140.com/tek_ABCs_of_Probes_1990.pdf incorrectly uses S (Siemens) instead of s (seconds) at various points throughout the article!)
* some useful articles on unexpected/unwanted oscillations and how to deal with them in this series of articles - https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/05/23/why-op-amps-oscillate-an-intuitive-look-at-two-frequent-causes
In the absence of any way of seeing high frequency oscillations (frequency beyond limits of scope/probes), are there any standard tell tale signs to look out for? Is it possible the circuit would still function, but waste power at unobserved high frequencies, or are the signs more obvious?
tggzzz:
--- Quote from: bmdaly on March 04, 2018, 11:18:41 pm ---Thanks everyone for your comments.
--- Quote from: tggzzz on March 02, 2018, 01:48:27 pm ---It depends on what frequency analogue signals you will be using, what you will be looking for, and what logic family you will be using.
Sometimes analogue circuits can oscillate at a much higher frequency than you are expecting ("amplifiers oscillate, oscillators won't").
In some analogue applications, harmonics of the fundamental frequencies are important; factor that into your assessment.
A 100MHz scope will (just about) allow you to see a 3.5ns risetime; a 200MHz scope 1.8ns. Logic families from the early 80s (LSTTL, STTL) have that kind of risetime. Some modern jellybean logic families have sub-nanosecond risetimes (e.g. 74lvc). If using similar families, a principal use of a scope is to check "signal integrity", which needs all the bandwidth you can get.
Most "high" impedance *10 scope probes with a 6"/15cm ground lead self-oscillate at ~100MHz. A 200MHz scope will show you that, but a 100MHz scope will disguise it :) At such frequencies construction techniques and probing techniques become important.
--- End quote ---
Thanks for raising these limitations/pitfalls - some links that address some of them, that might be of interest to others:
* some interesting info on probing here - https://youtu.be/zodpCuxwn_o - from You Tuber Alan Wolke (w2aew)
* ... and here, tggzzz's own blog - https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ (Surprisingly, Tektronix's http://w140.com/tek_ABCs_of_Probes_1990.pdf incorrectly uses S (Siemens) instead of s (seconds) at various points throughout the article!)
* some useful articles on unexpected/unwanted oscillations and how to deal with them in this series of articles - https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/05/23/why-op-amps-oscillate-an-intuitive-look-at-two-frequent-causes
In the absence of any way of seeing high frequency oscillations (frequency beyond limits of scope/probes), are there any standard tell tale signs to look out for? Is it possible the circuit would still function, but waste power at unobserved high frequencies, or are the signs more obvious?
--- End quote ---
Thanks for the video URL; it is a useful practical demo of the effect of the ground connections. (I also like his taste in using a Tek 485 as the demo vehicle :) ) If you are interested in the theory behind that, plus practical examples, have a look at:
https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/
and the later higher bandwidth variant:
https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/
I'm no longer surprised at Tek's historic misuse of SI units; I was appalled about them 40 years ago :(
In extreme cases even a probe's tip capacitance can cause an amplifier to oscillate, but in such cases that is probably a benefit since it highlights something that is too close to the edge of the envelope.
Don't forget that a 15pF capacitor (think probe tip) has an impedance of ~100 \$\Omega\$ at 100MHz (50 \$\Omega\$ at 200MHz). The 0.7pF "low" impedance Z0 resistive divider probe is still ~450 \$\Omega\$ at 500MHz)
rhb:
The "digital black magic" books by Graham and Johnson go into considerable detail about the probing problem. Their recommendation is to design in test points that can be enabled with 0 ohm resistors. They discuss using pins, but U.FLs seem a better choice to me. With an ARM clocked at 200 MHz, the harmonics extend to well over 1 GHz.
tggzzz:
Ignore the frequency since it is only indirectly relevant. Concentrate on the rise/fall time since that is directly relevant.
Why? Because if you have an Xns risetime, the bad effects are the same with a 1Hz or 100MHz frequency.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version