Scope Wars

[SilverSolder]

That pulse is 100 ps wide, so most of the energy is lost because the scope has only 350 MHz BW.  The time base is 1 ns/div which is as fast as the old girl can go.   However, in an ideal lossless case with a Dirac function which has unit area, the area of the  minimum phase BW limited impulse response is proportional to the area of the transfer function.

All the energy is being forced into sine waves which start at T0.

The requirement of  signal to be causal is that the real and imaginary parts in the frequency domain of a pure real function in the time domain be the Hilbert transform of each other.

I just found a fast one shot pulse generator I built 40 years ago out of a 7400 NAND gate.  The pulse width is governed by the propagation delay of the gates.  I couldn't measure it then, but I can now.

Have Fun!
Reg

Is the energy actually lost, though?  E.g. imagine an ideal filter made of just capacitors and inductors - the energy cannot be dissipated...  it kind of has to be present in the output, even if it doesn't look the same as it did going in...   ?

Edit:  If the filter is an RC filter, I can see that energy can be dissipated in the resistive part of the filter.  That dissipation must be frequency dependent (or the filter wouldn't work as a filter).  I am beginning to see how the shape of the response to a single pulse can tell you what the filter has to look like.   In the case of the ideal LC filter above, the response would presumably be a sine wave at the resonant frequency that goes on forever...  with an amplitude proportional to the energy in the original pulse.   Wish I was better at math, all this is probably obvious if you understand the underlying equations...

[Elasia]

That pulse is 100 ps wide, so most of the energy is lost because the scope has only 350 MHz BW.  The time base is 1 ns/div which is as fast as the old girl can go.   However, in an ideal lossless case with a Dirac function which has unit area, the area of the  minimum phase BW limited impulse response is proportional to the area of the transfer function.

All the energy is being forced into sine waves which start at T0.

The requirement of  signal to be causal is that the real and imaginary parts in the frequency domain of a pure real function in the time domain be the Hilbert transform of each other.

I just found a fast one shot pulse generator I built 40 years ago out of a 7400 NAND gate.  The pulse width is governed by the propagation delay of the gates.  I couldn't measure it then, but I can now.

Have Fun!
Reg

Is the energy actually lost, though?  E.g. imagine an ideal filter made of just capacitors and inductors - the energy cannot be dissipated...  it kind of has to be present in the output, even if it doesn't look the same as it did going in...   ?

Edit:  If the filter is an RC filter, I can see that energy can be dissipated in the resistive part of the filter.  That dissipation must be frequency dependent (or the filter wouldn't work as a filter).  I am beginning to see how the shape of the response to a single pulse can tell you what the filter has to look like.   In the case of the ideal LC filter above, the response would presumably be a sine wave at the resonant frequency that goes on forever...  with an amplitude proportional to the energy in the original pulse.   Wish I was better at math, all this is probably obvious if you understand the underlying equations...

Except every inductor and capacitor is actually a combination of all 3.. no such this as the ideal component and thus the energy dissipates eventually if nothing else.. as heat

[rsjsouza]

Six pages already? Let me bookmark this.

[rhb]

Much of the energy is reflected back into the source because of the impedance mismatch.

I've started work on a 40 m ultra portable QRP transceiver and an immediate concern that has arisen is the harmonics reflected back into the PA.  Having never designed a transceiver from scratch I don't know what matters and what doesn't.   I've got books on filter design on order, but they seem to be taking a long time to arrive.  It may be that a pair of diplexers dumping the unwanted spectrum into a 50 ohm load will perform better.

Have Fun!
Reg

[RoGeorge]

Is the energy actually lost, though?  E.g. imagine an ideal filter made of just capacitors and inductors - the energy cannot be dissipated...  it kind of has to be present in the output, even if it doesn't look the same as it did going in...   ?

Edit:  If the filter is an RC filter, I can see that energy can be dissipated in the resistive part of the filter.  That dissipation must be frequency dependent (or the filter wouldn't work as a filter).  I am beginning to see how the shape of the response to a single pulse can tell you what the filter has to look like.   In the case of the ideal LC filter above, the response would presumably be a sine wave at the resonant frequency that goes on forever...  with an amplitude proportional to the energy in the original pulse.   Wish I was better at math, all this is probably obvious if you understand the underlying equations...

That's correct, with the mention that voltage is not the same thing as energy.  Also, the area of a voltage in time (Volt*second) is not the same thing as energy (Joule).

But, yes, energy is never lost, can not be destroyed, nor created.  That is what the Energy Conservation Law says.  Energy can only be moved from one place to another, or transformed from one form to another.

Speaking about energy in an LC circuit, indeed, if you somehow manage to put a "blob" of energy into an ideal LC tank, that blob of energy will start to "slosh", back and forth, between the capacitor and the inductor, forever, as you said.

That sloshing forever comes with another mention.  Even if the coil and the capacitor and the connections between them have no resistance (are all ideal, so no rezistive losses), we will still have some losses because the circuit will radiate energy as radio waves.  So, to keep the energy in the LC tank forever, we will need to somehow isolate the LC tank from the surrounding space, in order to prevent energy losses in the form of radio waves photons.

Such an isolation will be hard to imagine for a wire as a coil and a pair of plates as a capacitor, let's say we will just use an ideal resonant cavity ended in perfect mirrors that will reflect back all the radio waves photons, so the oscillations will last forever.

But even so, with all those ideal parts and materials, our Universe continuously expands with time.  It is said that space itself expands, therefore the mirrors of our LC tank will keep going further and further apart.  The further away an object is, the more speed that object will gain because of space expansion, and at some point the speed of the distant object will become faster than light.

To our resonant cavity ended in mirrors, that will mean that at some moment the photons will bounce on one mirror, but in their way to the other mirror, they will never reach the other mirror, because the other mirror is speeding away faster than light.  So, our lump of energy will reflect for the last time, and that energy will never come back.  The oscillations will end.

Whatever we do, it seems like it won't oscillate forever and ever, not even in the ideal case of no energy dissipation.
Nothing last forever.   

[rhb]

Oooh!  That was a very fine explanation!

I'd been getting frustrated by all the BS noise, but that makes up for it.

Reg

[rhb]

So overshoot is actually useful. It shows you you are trying to look at the signal that is too fast for your scope, instead of happily hiding anything it doesn't like... You fight it by getting faster scope, not one that hides it better...

I had initially objected to this, but in fact having thought about it, it makes very good sense.  The overshoot tells you if you have significant signal above 50% of Nyquist.  If that's the case, you do need a faster scope to get an accurate waveform.

In analog scopes there was no real choice about the matter.  The BW had to extend far past the -3 dB point to be able to build it.  That's not the case with DSOs, so I'll have to amend my criteria.

A DSO can allow aliasing and then slap a boxcar filter on the signal in the FPGA which eliminates the aliased part of the spectrum.  Point taken.

Thanks,
Reg

[David Hess]

Not really. There is some ringing in the signal and in linear/dot mode you just get a large smeared area. Even with the extra ringing sin x/x interpolation gives the most accurate representation given the circumstances.

Some DSOs do not allow sin(x)/x reconstruction to be disabled and even if they do, sin(x)/x reconstruction is still present for the digital trigger and *that* is what smears the results; the trigger point is changing depending on the relationship between the sample trigger and input edge.  DSOs with analog triggers do not suffer from this problem and sampled points align correctly.  If they did not, then equivalent time sampling would not work.

[tomato]

Such an isolation will be hard to imagine for a wire as a coil and a pair of plates as a capacitor, let's say we will just use an ideal resonant cavity ended in perfect mirrors that will reflect back all the radio waves photons, so the oscillations will last forever.

But even so, with all those ideal parts and materials, our Universe continuously expands with time.  It is said that space itself expands, therefore the mirrors of our LC tank will keep going further and further apart.  The further away an object is, the more speed that object will gain because of space expansion, and at some point the speed of the distant object will become faster than light.

To our resonant cavity ended in mirrors, that will mean that at some moment the photons will bounce on one mirror, but in their way to the other mirror, they will never reach the other mirror, because the other mirror is speeding away faster than light.  So, our lump of energy will reflect for the last time, and that energy will never come back.  The oscillations will end.

Ooooh ... lots of problem with the physics in this analogy.  No good way to fix it, so best to just discard it. 

[0culus]

Such an isolation will be hard to imagine for a wire as a coil and a pair of plates as a capacitor, let's say we will just use an ideal resonant cavity ended in perfect mirrors that will reflect back all the radio waves photons, so the oscillations will last forever.

But even so, with all those ideal parts and materials, our Universe continuously expands with time.  It is said that space itself expands, therefore the mirrors of our LC tank will keep going further and further apart.  The further away an object is, the more speed that object will gain because of space expansion, and at some point the speed of the distant object will become faster than light.

To our resonant cavity ended in mirrors, that will mean that at some moment the photons will bounce on one mirror, but in their way to the other mirror, they will never reach the other mirror, because the other mirror is speeding away faster than light.  So, our lump of energy will reflect for the last time, and that energy will never come back.  The oscillations will end.

Ooooh ... lots of problem with the physics in this analogy.  No good way to fix it, so best to just discard it.

Then perhaps you'd be happy to provide a better analogy?

[SilverSolder]

Such an isolation will be hard to imagine for a wire as a coil and a pair of plates as a capacitor, let's say we will just use an ideal resonant cavity ended in perfect mirrors that will reflect back all the radio waves photons, so the oscillations will last forever.

But even so, with all those ideal parts and materials, our Universe continuously expands with time.  It is said that space itself expands, therefore the mirrors of our LC tank will keep going further and further apart.  The further away an object is, the more speed that object will gain because of space expansion, and at some point the speed of the distant object will become faster than light.

To our resonant cavity ended in mirrors, that will mean that at some moment the photons will bounce on one mirror, but in their way to the other mirror, they will never reach the other mirror, because the other mirror is speeding away faster than light.  So, our lump of energy will reflect for the last time, and that energy will never come back.  The oscillations will end.

Ooooh ... lots of problem with the physics in this analogy.  No good way to fix it, so best to just discard it.

Then perhaps you'd be happy to provide a better analogy?

Probably the non-zero resistance in any real LC circuit, and the losses due to radiation, will have caused the oscillations to die out long before the expansion of the universe gets to a point where it influences our scope tests! 

[gf]

Some DSOs do not allow sin(x)/x reconstruction to be disabled and even if they do, sin(x)/x reconstruction is still present for the digital trigger and *that* is what smears the results; the trigger point is changing depending on the relationship between the sample trigger and input edge.  DSOs with analog triggers do not suffer from this problem and sampled points align correctly.  If they did not, then equivalent time sampling would not work.

Trigger is indeed a good point. When I think about it, any sub-sample resolution triggering requires interpolation, (a) in order to determin the trigger point location between two adjacent sample points (in case of a digital trigger - as you say), and (b) in order to time-shift the captured samples by a fraction of the sampling interval. Even an analog trigger still requires (b), since the pre-trigger samples are already captured (with an arbitrary ADC clock pahse) when the trigger fires, and I guess even for post-trigger samples it were hard to enforce a different ADC clock phase momentarily, if I assume a PLL generated clock. On the other hand, if sub-sample resolution triggering is renounced, then the trigger point gets time-quantized, leading to trigger-jitter of +- 1/2 sampling interval (for single-shots, this does not matter of course, as subsequent frames don't neet to line-up on the time axis).

[gf]

I had initially objected to this, but in fact having thought about it, it makes very good sense. The overshoot tells you if you have significant signal above 50% of Nyquist. If that's the case, you do need a faster scope...

IMO this applies not only if the sampling theorem was violated, but also if the ringing was introduced by an AA filter which has a steep roll-off. On the other hand, an AA filter with a low cut-off frequency and a gentle roll-off, which degrades the signal softly, could make you believe that the signal does not contain high-frequency contents, although it does.

[nctnico]

Not really. There is some ringing in the signal and in linear/dot mode you just get a large smeared area. Even with the extra ringing sin x/x interpolation gives the most accurate representation given the circumstances.

Some DSOs do not allow sin(x)/x reconstruction to be disabled and even if they do, sin(x)/x reconstruction is still present for the digital trigger and *that* is what smears the results; the trigger point is changing depending on the relationship between the sample trigger and input edge.  DSOs with analog triggers do not suffer from this problem and sampled points align correctly.  If they did not, then equivalent time sampling would not work.

No (and that wasn't the effect I was describing; the trigger is rock solid in that case). Sin x/x only goes slightly wrong at sharp, undersampled edges, not at the zero crossing. Still it is likely the trigger system uses linear interpolation anyway. In fact you can get a far more stable trigger in the digital domain because you don't have the noise from the analog circuitry. The biggest trick however is to use the right threshold levels so the traces don't converge in a single dot at the trigger point (a typical effect on Siglent scopes).

[gf]

No (and that wasn't the effect I was describing; the trigger is rock solid in that case).

How else would you explain the "area" you see in points/lines mode? An area with a non-zero horizontal extent is usually an indication that subsequent frames do not line-up exactly in horizontal direction.

I'm in fact interested how it looks exactly. Could you please post a screen shot of point/line mode as well?

[nctnico]

No (and that wasn't the effect I was describing; the trigger is rock solid in that case).

How else would you explain the "area" you see in points/lines mode? An area with a non-zero horizontal extent is usually an indication that subsequent frames do not line-up exactly in horizontal direction.

I'm in fact interested how it looks exactly. Could you please post a screen shot of point/line mode as well?

The smearing only happens at the edges in non- sin x/x mode because the linear interpolation will draw lines between the sample points.

[SilverSolder]

I had initially objected to this, but in fact having thought about it, it makes very good sense. The overshoot tells you if you have significant signal above 50% of Nyquist. If that's the case, you do need a faster scope...

IMO this applies not only if the sampling theorem was violated, but also if the ringing was introduced by an AA filter which has a steep roll-off. On the other hand, an AA filter with a low cut-off frequency and a gentle roll-off, which degrades the signal softly, could make you believe that the signal does not contain high-frequency contents, although it does.

That is exactly the problem with AA filtering in photography -  a soft degradation which solves the problem of aliasing, but introduces a new issue of suppressing high frequency content. 

This is a trade-off between being able to see the high frequency, and accepting that you are going to see AA issues in some situations.

Perhaps a good question to ask is -   how important are the highest frequencies to getting the right idea of what the signal is doing?  In other words, how important is the performance at the limit of what a particular scope, rated at a particular bandwidth, can do? 

Are we basically talking about 'benign behaviour during extreme conditions' here?

[rhb]

I had initially objected to this, but in fact having thought about it, it makes very good sense. The overshoot tells you if you have significant signal above 50% of Nyquist. If that's the case, you do need a faster scope...

IMO this applies not only if the sampling theorem was violated, but also if the ringing was introduced by an AA filter which has a steep roll-off. On the other hand, an AA filter with a low cut-off frequency and a gentle roll-off, which degrades the signal softly, could make you believe that the signal does not contain high-frequency contents, although it does.

A <40 ps step has spectral contents up over 10 GHz.  The ringing you see on a slower scope *is* the AA filter shape.     In summary, if you see ringing on a fast step and the rise time is faster than .7/Nyquist you need a faster scope.  If the rise time is slower than that you've got a signal integrity issue.

It's an interesting subtlety to DSOs I'd not appreciated properly as it is not an issue that arises in seismic work or with an analog scope.

Have Fun!
Reg

[SilverSolder]

In summary, if you see ringing on a fast step and the rise time is faster than .7/Nyquist you need a faster scope.  If the rise time is slower than that you've got a signal integrity issue.

That is very concise,  and easily applicable in practice.   

[2N3055]

A <40 ps step has spectral contents up over 10 GHz.  The ringing you see on a slower scope *is* the AA filter shape.     In summary, if you see ringing on a fast step and the rise time is faster than .7/Nyquist you need a faster scope.  If the rise time is slower than that you've got a signal integrity issue.

It's an interesting subtlety to DSOs I'd not appreciated properly as it is not an issue that arises in seismic work or with an analog scope.

Have Fun!
Reg

Now THAT is very true , on topic and very well said....

[David Hess]

Not really. There is some ringing in the signal and in linear/dot mode you just get a large smeared area. Even with the extra ringing sin x/x interpolation gives the most accurate representation given the circumstances.

Some DSOs do not allow sin(x)/x reconstruction to be disabled and even if they do, sin(x)/x reconstruction is still present for the digital trigger and *that* is what smears the results; the trigger point is changing depending on the relationship between the sample trigger and input edge.  DSOs with analog triggers do not suffer from this problem and sampled points align correctly.  If they did not, then equivalent time sampling would not work.

No (and that wasn't the effect I was describing; the trigger is rock solid in that case). Sin x/x only goes slightly wrong at sharp, undersampled edges, not at the zero crossing. Still it is likely the trigger system uses linear interpolation anyway. In fact you can get a far more stable trigger in the digital domain because you don't have the noise from the analog circuitry. The biggest trick however is to use the right threshold levels so the traces don't converge in a single dot at the trigger point (a typical effect on Siglent scopes).

The trigger point looks rock solid because with digital triggering, the trigger point has zero jitter because by definition that is the point being aligned.  It *cannot* have any jitter because it is defined as the trigger point.

Go look at the old advertising videos LeCroy made touting how superior their digital triggering was compared to analog triggering and you can see exactly the problem I am describing.  The trigger point they show is exact, and the edge immediately above and below the trigger point is smeared.  They look pretty but really showed performance no better than an analog trigger and if aliasing is present, worse.

The other source of smearing is intermodulation between the sample clock and input which produces aliasing products and you might be seeing this, however this also smears the trigger point.

[nctnico]

Not really. There is some ringing in the signal and in linear/dot mode you just get a large smeared area. Even with the extra ringing sin x/x interpolation gives the most accurate representation given the circumstances.

Some DSOs do not allow sin(x)/x reconstruction to be disabled and even if they do, sin(x)/x reconstruction is still present for the digital trigger and *that* is what smears the results; the trigger point is changing depending on the relationship between the sample trigger and input edge.  DSOs with analog triggers do not suffer from this problem and sampled points align correctly.  If they did not, then equivalent time sampling would not work.

No (and that wasn't the effect I was describing; the trigger is rock solid in that case). Sin x/x only goes slightly wrong at sharp, undersampled edges, not at the zero crossing. Still it is likely the trigger system uses linear interpolation anyway. In fact you can get a far more stable trigger in the digital domain because you don't have the noise from the analog circuitry. The biggest trick however is to use the right threshold levels so the traces don't converge in a single dot at the trigger point (a typical effect on Siglent scopes).

The trigger point looks rock solid because with digital triggering, the trigger point has zero jitter because by definition that is the point being aligned.  It *cannot* have any jitter because it is defined as the trigger point.

Go look at the old advertising videos LeCroy made touting how superior their digital triggering was compared to analog triggering and you can see exactly the problem I am describing.  The trigger point they show is exact, and the edge immediately above and below the trigger point is smeared.  They look pretty but really showed performance no better than an analog trigger and if aliasing is present, worse.

That is exactly the effect that the Siglent DSOs show nowadays. However it is a matter of using the right thresholds to avoid smearing and thus showing a correct waveform; it is a software problem.

[Fungus]

That is exactly the effect that the Siglent DSOs show nowadays. However it is a matter of using the right thresholds to avoid smearing and thus showing a correct waveform; it is a software problem.

Ooooh! Let's have endless threads on this!

[rhb]

Could we please stick to something remotely relevant to a comparison of intro level DSOs?  I rather fear that no one except the very opinionated will ever read the actual test results.

The entire point is to apply *exactly* the same tests to multiple instruments with comparisons to higher end equipment so that a prospective buyer can make informed decisions and OEMs who conspicuously fail are motivated to fix the issues.

Reg

[Fungus]

Could we please stick to something remotely relevant to a comparison of intro level DSOs?  I rather fear that no one except the very opinionated will ever read the actual test results.

The type of tests to be performed probably shouldn't be used as a definitive buyer's guide for people who buy $350 'scopes anyway, so...