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

[Mac]

"Mac"

** It would be very easy to forget the switch was set to the 50 ohm
position
and connect the input to a source of high voltage. Phut 50 ohm resistor.

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

[snip]

The scope input can be set to 50-Ohms. The PROBE input cannot.

** Probes that are 1:1 or use clips or plugs on a length of co-ax
certainly can.

Millions of them in use.

Yeah, but I don't hook generic probes up to my 1 GHz scope.

** Do you have any experience with electronics at all ???

Most logic gates can drive 50 Ohms without going phut, at least for a
while. Lots of op-amps can endure indefinite short-circuit to ground, so
50 Ohms won't phase them at all.

Do you have any experience with electronics? ;-)

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

Well, not literally, lethal, most of the time. Besides, all equipment
requires some skill on the part of the operator.

As for phutting the input termination, I don't know how much power it
takes to do that, but I'm sure it is possible.

[snip]

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

Phil, there is obviously a power limit. I never said there wasn't. But
there is no reason to assume that it is 0.5 Watts. It is also possible
that the 'scope employs some mechanism to protect against overloads.

[snip]

.......... Phil

Just about all modern high-bandwidth scopes have a 50-Ohm option. Do you
seriously not know that? Haven't you seen a nice modern scope with a
50-Ohm termination option?

If you don't like it don't buy one. But for what I do, it is very useful,
and I doubt I am the only one who sees it that way.

--Mac

 

[Phil Allison]

"Mac"
Phil Allison wrote:


** OK - we have ourselves a " live one " here folks !!!!

Yeah, but I don't hook generic probes up to my 1 GHz scope.

** Irrelevant to the general question.

Keep wiggling.

Most logic gates can drive 50 Ohms without going phut, at least for a
while. Lots of op-amps can endure indefinite short-circuit to ground, so
50 Ohms won't phase them at all.

** Electronics is just a tad more diverse than you imagine.

Keep wiggling - dickhead.

Well, not literally, lethal, most of the time.

** Lethal to the circuit - dickhead.

Besides, all equipment
requires some skill on the part of the operator.

** So do we have a live "red- herring " on the line ??

Phil, there is obviously a power limit. I never said there wasn't. But
there is no reason to assume that it is 0.5 Watts.

** Irrelevant if it 0.5, 1 or 2 watts.

The resistor will be a bulk material, RF type.

It is also possible
that the 'scope employs some mechanism to protect against overloads.

** You are clutching at straws now.

Just about all modern high-bandwidth scopes have a 50-Ohm option. Do you
seriously not know that? Haven't you seen a nice modern scope with a
50-Ohm termination option?

** Irrelevant to the issue of general use.

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

Forget that - Mac ?????

If you don't like it don't buy one.

** Are you are a red herring or a toad fish - Mac ??

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




.......... Phil

 

[Paul Burridge]

I guess it also depends on whether your scope is forming the
*termination* of such a stage or merely probing the signal path
between matched stages. In the former case, the scope needs to
present 50 ohms to the stage; in the latter, it will be in parallel
with 50 ohms already, so should present as high an impedance as
possible so one would use the 25pf || 1meg option or something of that
order so the nominal interstage impedance isn't unduly affected.

 

[Tim Shoppa]

I've been using scopes with built-in switchable 50-ohm terminations for
many years and never fried one.

Seems to me that a much more likely disaster mode is somebody hooking
the ground clip of a probe to a high-current source. At least when
this happens the sparks usually let you know what happened .

Just about all modern high-bandwidth scopes have a 50-Ohm option. Do you
seriously not know that? Haven't you seen a nice modern scope with a
50-Ohm termination option?

There are some modern "bench" scopes that are really just glorified
scopemeters. These seem to lack switchable 50-ohm terminations
(probably
the heritage of originally being scopemeter designs, with nearly
indestructible input circuitry being a design requirement). They
aren't
high-bandwidth really anyway.

Tim.

 

[for_idea]

Thanks, so that means a device with output impedance 50 ohm only means
this device would behave better with a 50 ohm load, itself may not
really have 50 ohm output impedance?

 

[for_idea]

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.

 

[Mac]

"Mac"
Phil Allison wrote:


** OK - we have ourselves a " live one " here folks !!!!

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

** Irrelevant to the general question.

Keep wiggling.

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

** Electronics is just a tad more diverse than you imagine.

Keep wiggling - dickhead.

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

** Lethal to the circuit - dickhead.




** So do we have a live "red- herring " on the line ??





** Irrelevant if it 0.5, 1 or 2 watts.

The resistor will be a bulk material, RF type.





** You are clutching at straws now.




** Irrelevant to the issue of general use.

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.

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

Forget that - Mac ?????





** Are you are a red herring or a toad fish - Mac ??

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. ;-)

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




......... Phil

--Mac

 

[Hal Murray]

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.

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.

 

[Hal Murray]

Seems to me that a much more likely disaster mode is somebody hooking

the ground clip of a probe to a high-current source. At least when
this happens the sparks usually let you know what happened .

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

 

[Ken Smith]

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.[/QUOTE]

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.

 

[Ken Smith]

Seems to me that a much more likely disaster mode is somebody hooking
the ground clip of a probe to a high-current source. At least when
this happens the sparks usually let you know what happened .

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.

 

[Ken Smith]

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.

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

 

[Ken Smith]

Thanks, so that means a device with output impedance 50 ohm only means
this device would behave better with a 50 ohm load, itself may not
really have 50 ohm output impedance?

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
50 Ohms and sometimes its both.

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.

 

[mike]

"Mac"
Phil Allison wrote:


** OK - we have ourselves a " live one " here folks !!!!






** Irrelevant to the general question.

Keep wiggling.







** Electronics is just a tad more diverse than you imagine.

Keep wiggling - dickhead.







** Lethal to the circuit - dickhead.






** So do we have a live "red- herring " on the line ??







** Irrelevant if it 0.5, 1 or 2 watts.

The resistor will be a bulk material, RF type.







** You are clutching at straws now.

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

** Irrelevant to the issue of general use.

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

Forget that - Mac ?????







** Are you are a red herring or a toad fish - Mac ??

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




......... Phil

--
Wanted, Serial cable for Dell Axim X5 PDA.
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/

 

[Phil Allison]

"mike"

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

** Very little risk of that from you.

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

** Yawn.

So what ?

There exists a bunch of others that do not.

You are using a logical fallacy.

It's still NOT good to overload your input.

** Yawn.....

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

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

** Technical stuff don't get you far when dealing with the incoirrighably
illogical.

Sigh!!

** Wanker.




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