why the signal from a 50-ohm output impedance device should be checked by a scope channel with a 50-ohm input impedance?

I know the source signal (10 periods of 3MHz sinusoidal wave in a square window ). The signal was transmitted through a medium and received by a sensor. I used scope to check the receiving signal. Since the receiving pre-amplifier is a device with 50-ohm output impedance, I'm debating with myself why feeding it into a 50 ohm coupling channel is better than a 1meg ohm coupling channel.

Very easy to "phut" some circuit you are probing too.

........... Phil

I guess it also depends on whether your scope is forming the

You said that already. I guess the program is now repeating. ;-)

Hmm. I am wiggling, aren't I? Like a fish on a line.

Not familiar with that type of fish. Is it at all like cod?

Not entirely. If 50-Ohm switchable inputs are a feature found on most 'scopes, it is likely that it is not a "dangerous" option, otherwise customers would complain, and manufacturers would stop including this dangerous option.

I'm not sure, but I do feel a hook tugging at my mouth. I'll be more careful about taking the bait next time. ;-)

--Mac

It's hard to build coax with a 1 megohm characteristic impedance. Mechanically, the center conductor has to be tiny, and a tiny chunk of stray capicatance gives big reflections.

It's reasonably easy to build good coax at 50 ohms.

I remember melting the insulation on the wire going to a ground clip.

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Millions of them in use.

Connecting arbitrary internal circuit points to ground via 50 ohms is lethal.

What do you expect the 120/240 AC supply might do to a 50 ohm 1/2 watt load resistor ??

.......... Phil

Keep wiggling.

Keep wiggling - d*****ad.

The resistor will be a bulk material, RF type.

YOU *asked* Meindert for an explanation of the danger aspect.

Forget that - Mac ?????

The latter is more likely with someone so prickly & bloated.

......... Phil

In article , Mac wrote: [...]

The only scopes I've seen that lacked this feature either (a) were extreme low lost ones or (b) used banana plugs for the inputs.

My first scope didn't have a 50 Ohms option. It did however have an option to bypass the amplifiers and drive the plates from external circuits. This was handy since the 3dB bandwidth of the vert. and horz. circuits was about 30KHz.

In article , Tim Shoppa wrote: [...]

Or:

a) about 100 feet of ribbon cable running across the shop bursting into flames and filling the whole room with very smell smoke. (it seems that the equipment on the far end had a hot ground)

b) a deep thumping sound out of the power supply just after several chips were toast. Logic running from +(5+5) and +(5) didn't like suddenly running from +(5+5) and GND.

In article , Hal Murray wrote: [...]

I vote "hard" in the above as the understatement of the year.

You have to read the specs carefully. Sometimes they tell you that it wants a 50 Ohm load. Sometimes they tell you that its impedance is really

Putting a 50 ohm impedance at either end of a line will eat up the energy in the reflected waves and make the signal at the load end look good.

BTW: If you are sending squarewaves, you can get the same effect with clamping the load end to the voltage that a 50 Ohm load would have made it. The advantage is that you get to put the energy back into the power supply.

At the risk of adding some signal to the noise of this thread...

There exist scopes that attempt to limit the damage when you stuff too much signal into the 50 ohm input.

Here's a real live actual quote from the TEK TDS540 circuit description:

Begin Quote The 68020 interprets user commands and initiates changes to the settings. The Acquisition Processor mointors the input overload sense (OVLS1-OVLS4) and the probe data (PRDATA1-PRDATA4) communications lines. End Quote

The above leads me to the conclusion that some attempt is made to limit damage. No, it won't protect against a lightning strike, but should prevent serious damage to the resistor for moderate overloads.

I'd state that many other TEK scopes also protect their 50 Ohm inputs, but that would require me to look it up and confirm my statement...so I won't.

It's still NOT good to overload your input.

And it's nice to see that we haven't forgotten to start name calling when we run out of technical arguments and FACTS. Sigh!!

mike

"mike"

So what ?

There exists a bunch of others that do not.

You are using a logical fallacy.

Must be one of them rocket scientist types.....

............ Phil