creen,
If you ever had to do complicated scope data capture with no end to the num ber of conditions upon which the capture should be conditioned, you would a ppreciate a modern scope more, and maybe realize it's still not enough. The same goes for modern logic analyzers, never enough counters.
perspective,
Don't buy a scope unless it has a DEFAULT SETUP button that's always in plain view.
But if you are really anxious to minimise reflections, "double termination" at both ends can be worth the extrra effort, and the halving of the signal amplitude involved. Balanced pairs of signal can give you back the lost amplitude.
Except that no termination is ever perfect. Figure on a +/-2% tolerance on the cable impedance. I was once offered cables that were custom made to a more tightly specified tolerance, but I've never seen anything like that from a distributor.
Analog TV Distribution Amps typically had 150, 225 or 300 ohm source resistors, because the output was 2, 3 or 4 volts at the source, and a
75 ohm resistor at the other end. SDI (Digital) video is flat from DC to 270 MHz, and the better quality DAs measure the roll off at the input & equalize the response so the output is flat. Grass Valley is one popular brand for TV stations
tolerance too. Just pretend you don't see the 10mV overshoot, it's just monitor front end parasitics ....
1% terminators were common in the '70s. .1% are common now.
perspective,
(letters-on-screen/trace-on-screen,
That's why decent TV shop grade scopes had switch selectable H & V settings, and TV stations used video waveform monitors.
1% termination resistors sort of make sense. 0.1% don't unless you've got peculiarly precisely constructed coaxial cable/transmission line. The tolerance on the characteristic impedance of standard coaxial cable is about +/-2%.
Terminating something like that with a O.1% tolerance terminating resistor would be a complete waste of money.
both ends can be worth the extrra effort, and the halving of the signal amplitude involved. Balanced pairs of signal can give you back the lost amplitude.
cable impedance. I was once offered cables that were custom made to a more tightly specified tolerance, but I've never seen anything like that from a distributor.
I designed a source-terminated pulse generator. The only way we could guarantee the customers wouldn't return it for "transient response out of spec" was to ship a tight tolerance cable with it.
meters times megahertz equals 300 so 1m 1/4 wavelength is 75 MHz
Propagation speed in coax is slower than free space.
But if you source terminate, all that the cable adds is time delay. At the end, it looks like 50 ohms resistive at any frequency. Given a 50 ohm signal generator, extending its output with a 50 ohm (lossless) coaxial cable is transparent.
In vacuum. Dave forgot (or doesn't know about) the propagation constant for the coax. Phil didn't >:-} ...Jim Thompson
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Cool, Thanks. I'll have to think a bit before I understand how that works... no don't tell me! I'll ask if I get stuck.
George H.
f
to
n l s dNice, Thanks. But that's going to take even more thought than the circuit Jeroen posted. (I can get lost going through those transformers :^)
George H.
Grin, as the saving goes, "you can lead a horse to water but you can't make him read."
George H.
y
Sure... but you can only teach one (maybe two) thing(s) at a time. In this case there's lots of other stuff going on so that just giving them an 'easy' monitor pulse is not that bad.
In retrospect I should have done as Joerg (and maybe others) suggested and added more poop. A real line driver would not have been much more.
George H.
esign perspective,
-screen,
Well don't be too hard on the latest generation of DSO's. They are much better than those earlier generations that were all buttons and maybe one knob. Those were impossible to use. I really like having colored traces that match the color on the gain and offset knobs... that is nice.
I guess the one button I'd like to remove is the autoset. (I'd rather have them think!)
George H.
tolerance too. Just pretend you don't see the 10mV overshoot, it's just monitor front end parasitics ....
Here's a cheap 51 ohm carbon film resistor:
and its TDR step response
The main error seems to be from the length of the resistance element, which is over in about 200 picoseconds. Then it rings at 9.6 GHz for a bit.
Plenty good up to maybe 1 GHz. Carbon comps are probably better.
tolerance too. Just pretend you don't see the 10mV overshoot, it's just monitor front end parasitics .... On PCB layout, we used to use two 100 ohm 1/8W carbon comps flat against the ground plane spread 60 degrees from the RF connector. Carbon comps were typically 2 percent low with a one percent spread. 40 years ago.
a 5% tolerance too. Just pretend you don't see the 10mV overshoot, it's jus t monitor front end parasitics ....
.ed text -
Neat, and if you stick a metal film in there does it take longer? (I assume 50 ohm MF's are spiral cut.)
George H.
tolerance too. Just pretend you don't see the 10mV overshoot, it's just monitor front end parasitics ....
text -
Aren't the carbon films spiral cut?
More things to do to avoid real work!
Surface mounts, even the cheapies, are very good.
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