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

If the source is designed to be operated into a 50-ohm load, that's the way it should be tested. Most scopes these days have a switchable

Nothing wrong with doing it with a scope with 1Mohm input impedance as long as you've got a 50-ohm terminator Teed off at the input too (that's the way it used to be done before scopes had built-in 50-ohm terminations.)

Tim.

Thanks. but what would be wrong if I don't use a 50 ohm terminator and connect the signal directly to a 1meg ohm channel? I guess I will get as much as twice of the signal observed by a 50 ohm channel since most of the voltage will be dropped on the 1meg ohm.

I have a Lecroy 9341 which has several coupling modes including 1 meg and 50 ohm.

For DC and low frequencies (

What happens will depend on the source. If your source has a 50 ohm source impedance then your prediction will be true. But if your source is something like an RF power amplifier then it will be to work _into_ a

So if the source wants to see 50 ohms -- give it what it wants.

"if the source wants to see 50 ohms -- give it what it wants"... sounds reasonable

as I understand, with 1MHz, the wavelength is 300meters, if the interconnecting length is short (such as 1 meter or less), the transmission line effect is insignificant and the reflection is not a major problem to the detecting signal. Even with the signal I'm using,

Thanks,

Zhi

If it's a 3MHz square wave with 10ns rise/fall times, then you really need to consider the edges at least as having 300MHz components.

Tim.

what if it is sinusoidal wave? thanks.

Sinusoids are no problem if the wavelength is a lot longer than your cable (use a velocity of 2/3 speed of light to calculate wavelength from frequency).

In grad school I was using rf signals where the wavelength was comparable to the cable length. With no 50 ohm terminator, the scope would show resonances as the frequency was tuned due to standing waves building up in the cable at the resonances. Even though it was a "constant amplitude" source, the amplitude varied wildly as the frequency was tuned.

Putting a 50 ohm at the scope put an end to that problem. As others have said, the 50 ohm prevents the signal from reflecting back and forth within the cable. This is important for short pulses, and square waves with short transitions between steps, and high frequency sine waves.

Mark

Dangerous how?

--Mac

If you know that already, what do you need the scope for?

Cheers,

Phil Hobbs

Well, if the source really has a 50-Ohm source impedance, then you will see twice the Voltage signal, yes.

But you may often find sources which are designed to drive 50-Ohms but which do not actually have a 50-Ohm source impedance. Such a source may become squirrelly if it sees a high-impedance load.

Even if the source doesn't mind driving a high-impedance load, probing it that way will not tell you what the signal looks like with a 50-Ohm load, obviously.

Anyway, why don't you ask the person who told you this to explain his/her reasoning?

--Mac

Oh? Wich ones? My Tek doesn't and it seems a dangerous feature to me...

Meindert

This has never happened to me, and in practice, I don't think it is as easy to do as you suggest.

The scope input can be set to 50-Ohms. The PROBE input cannot. Also, the probes usually have some extra pins on them that force the scope into the correct mode for that probe. If it is an active probe, then the input will be set to 50 Ohms, and if it is a passive, it will be set to 1M. If you connect your own coax, then you can set it to either one.

So to phut something, you would probably have to jury rig a probe onto some coax. Besides, most things don't phut on 50 Ohms.

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. Like I said, I've never done it.

--Mac

Nice one, Tim. That's by far the most outstandingly clear answer to this (entirely creditable) question yet!

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

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

Do you have any experience with electronics? ;-)

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

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.

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

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

If you're scoping a signal then it's somewhat unlikely that the scope is the intended load in actual use.

The signal should be terminated at its normal destination.

Graham

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 :-).

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.