My Siglent SDS1202X-E shows wrong Pk-Pk voltage

[Fungus]

Is there a rule of thumb?

No.

Termination is needed at all frequencies. It's more needed at high frequencies because you can create resonances and interference patterns inside the cable but you have to go a DC signal before you can say "termination wouldn't improve this".

You should always terminate when you're talking about test gear.

https://www.ti.com/lit/an/snla034b/snla034b.pdf

[pope]

The 50 ohm terminator isnt that only for higher signals in the MHz range?

Yes, but only for short cables.

Slightly longer answer: It's to do with the ratio of wavelength to cable length.

Can you elaborate a bit?

Is there a rule of thumb? For example, do we need a 50 Ohm terminator with 1m cable for frequencies up to 200kHz ?

Is the cable 50 ohm impedance? is the source 50 ohm impedance? The frequency is one of the considerations, but not the only one. Rules of thumb are best left for when you understand the underlying issues.

When talking about audio gear the answer to your questions is usually "no". So? Does this mean you can't measure audio gear?

[Fungus]

The frequency is one of the considerations, but not the only one. Rules of thumb are best left for when you understand the underlying issues.

When talking about audio gear the answer to your questions is usually "no". So? Does this mean you can't measure audio gear?

It means you have to understand the underlying issues.

[bdunham7]

You should always terminate when you're talking about test gear.

Without understanding the underlying issues? 

If the source impedance matches the cable characteristic impedance, you are probably good to go no matter what the sink impedance is, presuming the sink is a scope, DMM or similar measuring instrument.  The reflected signal will be damped by the termination at the source and the superimposition principle will keep our direct and reflected waves travelling smoothly even in a single lane of coax.   

And at 200kHz and a 1 meter cable, termination of such a system will halve your amplitude and that's pretty much it.  I have 'test gear' with input bandwidth specifications of 1 and 2 MHz that use binding posts for the inputs.  And scraggly looking ordinary wires of random and unequal lengths going from there to the circuit board.

Terminating a signal generator at the source and then connecting a probe does nothing except provide a 25R source impedance and perhaps give the correct indicated output amplitude if the source is expecting the 50R load.

[Fungus]

You should always terminate when you're talking about test gear.

Without understanding the underlying issues? 

Every rule has exceptions, obviously. 

[Someone]

And at 200kHz and a 1 meter cable, termination of such a system will halve your amplitude and that's pretty much it.  I have 'test gear' with input bandwidth specifications of 1 and 2 MHz that use binding posts for the inputs.  And scraggly looking ordinary wires of random and unequal lengths going from there to the circuit board.

Depends what the output impedance is, just because its audio frequency doesn't mean is so low in impedance you can ignore it, plenty of mundane things have a high enough source impedance that you do need to consider it when probing/measuring. Halving the amplitude because of impedance is as stupid a "rule of thumb" as frequency vs length.

[Someone]

For example, do we need a 50 Ohm terminator with 1m cable for frequencies up to 200kHz ?

Is the cable 50 ohm impedance? is the source 50 ohm impedance? The frequency is one of the considerations, but not the only one. Rules of thumb are best left for when you understand the underlying issues.

When talking about audio gear the answer to your questions is usually "no". So? Does this mean you can't measure audio gear?

To return the question back to you: if the audio system is not 50 ohm source impedance, why are you asking about a 50 ohm termination?

If one part of the system cant be changed, then you need to understand how the things you can change will affect it. No measurements are perfect and have zero effect on what you are observing (except trivial counting number of objects on a table).

[bdunham7]

Halving the amplitude because of impedance is as stupid a "rule of thumb" as frequency vs length.

No, it is a mathematically correct statement provided you have a system worthy of termination--as in the source and cable characteristic impedance are the same.  So 50R source, 50R cable, if you terminate that at the other end with 50R, your observed amplitude is half.  If the source and cable don't match or you terminate it with something else, then the discussion isn't really worth halving having. 

You can connect your meter/scope/analyzer to an audio system with 50R 1M coax, whether it is the very low impedance speaker outputs or something like a phono cartridge, and it will just act as a pair of wires.  Or you can just use a pair of wires.  Obviously you are correct that in this case adding a 50R terminator to the 50R coax would decrease the amplitude by some factor other than half, as well as being totally pointless.

[Someone]

And at 200kHz and a 1 meter cable, termination of such a system will halve your amplitude and that's pretty much it.  I have 'test gear' with input bandwidth specifications of 1 and 2 MHz that use binding posts for the inputs.  And scraggly looking ordinary wires of random and unequal lengths going from there to the circuit board.

Halving the amplitude because of impedance is as stupid a "rule of thumb" as frequency vs length.

No, it is a mathematically correct statement provided you have a system worthy of termination--as in the source and cable characteristic impedance are the same.  So 50R source, 50R cable, if you terminate that at the other end with 50R, your observed amplitude is half.  If the source and cable don't match or you terminate it with something else, then the discussion isn't really worth halving having. 

You can connect your meter/scope/analyzer to an audio system with 50R 1M coax, whether it is the very low impedance speaker outputs or something like a phono cartridge, and it will just act as a pair of wires.  Or you can just use a pair of wires.  Obviously you are correct that in this case adding a 50R terminator to the 50R coax would decrease the amplitude by some factor other than half, as well as being totally pointless.

Lets take what you said in full:

You should always terminate when you're talking about test gear.

Without understanding the underlying issues? 

If the source impedance matches the cable characteristic impedance, you are probably good to go no matter what the sink impedance is, presuming the sink is a scope, DMM or similar measuring instrument.  The reflected signal will be damped by the termination at the source and the superimposition principle will keep our direct and reflected waves travelling smoothly even in a single lane of coax.   

And at 200kHz and a 1 meter cable, termination of such a system will halve your amplitude and that's pretty much it.  I have 'test gear' with input bandwidth specifications of 1 and 2 MHz that use binding posts for the inputs.  And scraggly looking ordinary wires of random and unequal lengths going from there to the circuit board.

Terminating a signal generator at the source and then connecting a probe does nothing except provide a 25R source impedance and perhaps give the correct indicated output amplitude if the source is expecting the 50R load.

Its these sorts of half assed explanations that lead to the confusion these posters are having. You've had to come back and add a lot of extra detail to correct it, and its still not complete. There is no "rule of thumb" that is easily distilled down to a short statement like you made and I called it out. The actual explanation needs to be much longer, and fully understood, before the quick version makes sense. Leaving out the context is exactly why the posters are confused. And you're still only scratching the surface of their questions, impedance matching is beyond the idealised discrete models and leads into transmission line/waveguide effects.

A 50 ohm coax 1m long connected to a high impedance source might cause significant changes to the signal, even if it is low frequency (<<20kHz ) and the sink is 1M or 10M. The non-ideal effects are ignored, until they get in the way. Which is why people need to understand why they are ignored before they choose dismiss the effects as insignificant in that specific situation.

[mawyatt]

Things about impedance matching and termination impedance usually relates to maximum power transfer, not maximum voltage or current. A simple analysis shows that when the load impedance is conjugately matched to the source impedance this is the condition for maximum power transfer from the source to the load.

Unfortunately most folks don't bother to take this into account unless they are working in RF, MW or MMW where things are generally specified in power (dBm for example) rather than voltage or current.

Best,

[bdunham7]

Its these sorts of half assed explanations that lead to the confusion these posters are having. You've had to come back and add a lot of extra detail to correct it, and its still not complete. There is no "rule of thumb" that is easily distilled down to a short statement like you made and I called it out. The actual explanation needs to be much longer, and fully understood, before the quick version makes sense. Leaving out the context is exactly why the posters are confused. And you're still only scratching the surface of their questions, impedance matching is beyond the idealised discrete models and leads into transmission line/waveguide effects.

I've not really added detail, just restated the context of this discussion which was connecting an AWG with a 50R source impedance to an oscilloscope using a 50R coax.  Within that context, my three observations about 'halving' and so on are correct and readily proven so by simple experiment.  These aren't generalized 'rules of thumb', they are specific assertions that I'll defend empirically, but only precisely as stated.  I haven't attempted to produce a treatise on transmission lines and I'm mainly addressing comments which seem to indicate a desire to needlessly add unnecessary terminations that won't solve the OPs problem or any other.   

A 50 ohm coax 1m long connected to a high impedance source might cause significant changes to the signal, even if it is low frequency (<<20kHz ) and the sink is 1M or 10M. The non-ideal effects are ignored, until they get in the way. Which is why people need to understand why they are ignored before they choose dismiss the effects as insignificant in that specific situation.

I suppose it might--it could pick up noise, it's capacitance might matter enough and so on.  In fact the capacitance of a meter of RG-58 would probably be approaching the point of concern with something like a phono preamp input and it might even be a show-stopper in some really high impedance circuits.  But in that regard, it isn't any different than any other pair of wires--or a 1X oscilloscope probe--which might have similar parasitics.  Are you claiming that a 1M 50R coax connected to such a circuit would cause "significant changes to the signal" due to reflections or other transmission-line characteristics (other than parasitics like capacitance or leakage that can exist in any wires or components) and that termination would be appropriate to remedy that issue? 

[Fungus]

Some questions simply can't be answered by a soundbite in a forum.

[Someone]

Its these sorts of half assed explanations that lead to the confusion these posters are having. You've had to come back and add a lot of extra detail to correct it, and its still not complete. There is no "rule of thumb" that is easily distilled down to a short statement like you made and I called it out. The actual explanation needs to be much longer, and fully understood, before the quick version makes sense. Leaving out the context is exactly why the posters are confused. And you're still only scratching the surface of their questions, impedance matching is beyond the idealised discrete models and leads into transmission line/waveguide effects.

I've not really added detail, just restated the context of this discussion which was connecting an AWG with a 50R source impedance to an oscilloscope using a 50R coax.  Within that context, my three observations about 'halving' and so on are correct and readily proven so by simple experiment.  These aren't generalized 'rules of thumb', they are specific assertions that I'll defend empirically, but only precisely as stated.  I haven't attempted to produce a treatise on transmission lines and I'm mainly addressing comments which seem to indicate a desire to needlessly add unnecessary terminations that won't solve the OPs problem or any other.   

But thats it, there is no halving. The OP starts off by asking why the measurement doesn't come closer than several % of the exact number, a few posters reply with possible causes for that. The OP comes back and asked about termination (without ever mentioning the specific frequency), clearly not understanding when/why termination is important.

The thread stated with several % error, how accurate are 50 ohm sources/terminations? how accurate are 50 ohm cables? Tolerancing of those could easily explain a few % error from reflections, a cheap/poor BNC connector could be enough, even in your suggested open termination. Halving is about as relevant to engineering as Zero, only ever an approximation and vague reference. Thinking about voltages like assigning numbers to a variable in software gets people stuck like this all the time. Its an electronic circuit, resistances are inherent and non zero, currents are unavoidable and non zero, tolerances of the parts/equipment used are non-zero, there will be noise/interference that is non-zero. They should not be abstracted away with "rules of thumb" and ignored, when the original post is questioning a few %.