Author Topic: Oscilloscopes and harmonics  (Read 514 times)

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Offline sentry

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Oscilloscopes and harmonics
« on: August 15, 2018, 03:55:10 pm »
Hi,

I have some questions regarding scopes and harmonics. So I understand that any repetitive waveform is basically the sum of sinusoid waves of appropriate amplitude and frequency.

Does that mean that when we display any signal on a scope what we essentially are looking at is the scopes decomposition of the signal into the sum of the fundamental waveform and the harmonics up until the scopes bandwidth limit?

If I want to display a 20MHz square wave with my 50MHz Rigol 1054z scope, what I will essentially see was only the fundamental waveform and not any of the harmonics because the harmonics all have frequencies above my 50MHz limit? So a 20MHz square wave will not look like a square wave on my scope, but will look like a sine wave. Is that correct?
 

Online capt bullshot

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Re: Oscilloscopes and harmonics
« Reply #1 on: August 15, 2018, 04:13:07 pm »
This is correct, to some extent.
The oscilloscope won't cut the harmonics off at its maximum frequency, it will have a rather smooth decay in amplitude of the harmonics.
So with your exampe (20MHz square on a 50MHz scope) the third harmonic at 60MHz won't be cut off, but will be still there with a significant amount, the fifth (100MHz) will be there quite attenuated etc.
The result won't be neither a square wave nor a sine wave, but something "in between".
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Offline RoGeorge

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Re: Oscilloscopes and harmonics
« Reply #2 on: August 15, 2018, 05:13:49 pm »
In theory, yes. In practice, keep in mind that the frequency response of any oscilloscope does not look like a square ideal low pass filter.

However, if you care to do a simulation, then compare the results with what you really see on the screen of a Rigol DS1054Z, prepare to be surprised. The band of a "50MHz" Rigol DS1054Z is in fact much larger, about 120...150MHz. Now, if you enable the "BW Limit" (20MHz), the waveform on the screen will start to look like the simulated waveform. Otherwise, the 20MHz square wave will look more like a square than like a sinewave on a DS1054Z.

Offtopic: if you use Riglol tool to unlock the DS1054Z, you will see that the oscilloscope's band does not change when you unlock it to be a DS1104Z. I don't know why it was advertised as a 50MHz oscilloscope when, in fact, DS1054 is an 100MHz one. I guess that was just a marketing decision.
« Last Edit: August 15, 2018, 05:29:09 pm by RoGeorge »
 
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Online tggzzz

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Re: Oscilloscopes and harmonics
« Reply #3 on: August 15, 2018, 07:55:08 pm »
You are on the right track.

If you want to confirm your understanding in a way that will help you with real-world situations, see some measurements at https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/
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Offline Old Printer

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Re: Oscilloscopes and harmonics
« Reply #4 on: August 15, 2018, 10:40:45 pm »

Offtopic: if you use Riglol tool to unlock the DS1054Z, you will see that the oscilloscope's band does not change when you unlock it to be a DS1104Z. I don't know why it was advertised as a 50MHz oscilloscope when, in fact, DS1054 is an 100MHz one. I guess that was just a marketing decision.

Does this mean that if you order a new 1054z & a 1104z the only difference will be the label?
 

Offline RoGeorge

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Re: Oscilloscopes and harmonics
« Reply #5 on: August 15, 2018, 10:52:42 pm »
Does this mean that if you order a new 1054z & a 1104z the only difference will be the label?

Yes.

Online exe

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Re: Oscilloscopes and harmonics
« Reply #6 on: August 16, 2018, 12:31:41 am »
I don't know why it was advertised as a 50MHz oscilloscope when, in fact, DS1054 is an 100MHz one.

I think old 1054z did have a 50MHz bandwidth limit. They just removed the restriction recently: https://www.eevblog.com/forum/testgear/rigol-ds1000z-now-comes-unlocked/
 

Offline Old Printer

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Re: Oscilloscopes and harmonics
« Reply #7 on: August 16, 2018, 01:37:49 am »
Does this mean that if you order a new 1054z & a 1104z the only difference will be the label?

Yes.

While I don't doubt it for a second, if someone did a teardown of both and could show they are charging $270 for nothing more than a label change, that would be terrible publicity. Would love to see some corporate mouthpiece talk their way around that one :)
 

Offline rstofer

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Re: Oscilloscopes and harmonics
« Reply #8 on: August 16, 2018, 02:10:24 am »
Does this mean that if you order a new 1054z & a 1104z the only difference will be the label?

Yes.

While I don't doubt it for a second, if someone did a teardown of both and could show they are charging $270 for nothing more than a label change, that would be terrible publicity. Would love to see some corporate mouthpiece talk their way around that one :)

How is that different than having two filter capacitors in the DS1054Z that are FPGA selected for 50 MHz or 100 MHz?  Dave did that teardown!

The difference between the DS1104Z (100 MHz) and the DS1054Z (50 MHz) is a label and an output bit from the FPGA.
 

Offline David Hess

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Re: Oscilloscopes and harmonics
« Reply #9 on: August 16, 2018, 02:20:34 am »
I don't know why it was advertised as a 50MHz oscilloscope when, in fact, DS1054 is an 100MHz one. I guess that was just a marketing decision.

It was a measurement error or something was broken.  I think they were always broken.  Zike got good numbers but his analysis here is wrong because Dave's was wrong; the switched differential front end is not for bandwidth.  Switchable bandwidth is controlled by the 4 transistors shown in the lower right corner of Dave's schematic which you can calculate and come up with the 20/50/70 MHz numbers.  What Zike referred to as "dribble up" is saturation (or cutoff) of the amplifier where it becomes non-linear which neatly explains why the bandwidth and transition time vary with sensitivity and likely signal level.

Measure the transition time under different conditions both automatically and manually; I have seen everything from 3.7ns to 800ps reported.  This measurement (and Zike's measurements) I believe but they should be confirmed visually without automatic measurements.



Excluding some high bandwidth instruments, most oscilloscopes have a Gaussian response which is -3dB down at the bandwidth specification and -12dB one octave higher.  It is not really a Gaussian response but it is close enough to consider it so.  Attenuation of the square wave harmonics smooths the edges while the linear phase delay keeps them aligned preventing pulse distortion.  Modern high bandwidth oscilloscopes with "brick wall" response discard this sacrificing pulse response for rise/fall time and bandwidth.

While I don't doubt it for a second, if someone did a teardown of both and could show they are charging $270 for nothing more than a label change, that would be terrible publicity. Would love to see some corporate mouthpiece talk their way around that one.

The discussion thread says later that the bandwidth is not different.  I am not sure how much this matters if the bandwidth and pulse response vary with sensitivity and amplitude though.

While I don't doubt it for a second, if someone did a teardown of both and could show they are charging $270 for nothing more than a label change, that would be terrible publicity. Would love to see some corporate mouthpiece talk their way around that one :)

How is that different than having two filter capacitors in the DS1054Z that are FPGA selected for 50 MHz or 100 MHz?  Dave did that teardown!

There may be other differences that a teardown or even complete reverse engineering does not show.  Rigol could be grading these oscilloscopes for good pulse response at 100 MHz and selling the ones which fail as 50 and 70 MHz models.  Or they could be spending extra time time to calibrate the 100 MHz models.

Quote
The difference between the DS1104Z (100 MHz) and the DS1054Z (50 MHz) is a label and an output bit from the FPGA.

It is two output bits.  See my discussion above in the first paragraph about the bandwidth filter.
 

Offline Old Printer

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Re: Oscilloscopes and harmonics
« Reply #10 on: August 17, 2018, 02:58:45 am »
Does this mean that if you order a new 1054z & a 1104z the only difference will be the label?

Yes.

While I don't doubt it for a second, if someone did a teardown of both and could show they are charging $270 for nothing more than a label change, that would be terrible publicity. Would love to see some corporate mouthpiece talk their way around that one :)

How is that different than having two filter capacitors in the DS1054Z that are FPGA selected for 50 MHz or 100 MHz?  Dave did that teardown!

The difference between the DS1104Z (100 MHz) and the DS1054Z (50 MHz) is a label and an output bit from the FPGA.

In reality, not much. I imagine this sort of thing has been going on for years, but only recently have I been around people knowledgeable enough to call them on it.
 

Offline David Hess

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Re: Oscilloscopes and harmonics
« Reply #11 on: August 17, 2018, 02:55:29 pm »
In reality, not much. I imagine this sort of thing has been going on for years, but only recently have I been around people knowledgeable enough to call them on it.

Although I disapprove of that sort of price discrimination when it is taken advantage of, which is easy enough for a manufacturer to do, there is another way to think about it.

The additional costs of manufacturing several different versions may make them less economical than manufacturing a uniform design.  By offering price reduced models with identical hardware, the manufacturer provides a lower performance instrument to those who would not otherwise be able to afford it.  This applies even more now to programmed capabilities like decoding which have considerable development costs; when you buy the option, this is what you are paying for and it might be considered unfair to charge everybody for it even at a lower cost to everybody when they do not actually desire it.

Do not discount the testing, calibration, and grading issues.

Testing and calibration can take significant time adding to the cost.  For an example of this consider the LMC6081 operational amplifier and the identical LMC6001 operational amplifier. The only difference between them is that the LMC6001 is tested for a much lower maximium input bias current which takes an extra 10 seconds resulting in a part which costs several time more only because time testing is expensive.  Many of the old Tektronix TDS series of oscilloscopes had 500MHz and 1GHz models and the only difference between them was calibration, which even while automatic, took considerable time.  If you hack the Tektronix TDS 500MHz models to 1GHz, they show poor (useless) transient response unless the time consuming 1GHz calibration procedure is done.

Tektronix used to grade 2N3904 transistors for collector-base time constant (rb'Cc) for use in their 100MHz oscilloscopes which is why I found it interesting that Rigol used 2N3904s in their vertical amplifiers which Tektronix never did; they used their fast 2N3904s in other areas of their 100MHz instruments.  Is Rigol grading the transistors or finished instruments for 100MHz performance?  It might explain the non-linearity I pointed out above in the bandwidth hacked instruments but I have not seen the same test on done an official 100MHz DS1104Z.

Below is a photograph from an earlier discussion where I marked the problem and you can see the same thing in the previously linked discussions.  That unusually linear feature in the settling time (1) should not be there; it may not look like much, but it even exceeds the transition time of the oscilloscope!  That is *not* 100MHz performance no matter what the measured transition time and bandwidth are.

Take another look at the reverse engineered schematics of the DS1000Z series that Dave made.  That switched differential amplifier concept was used in many analog oscilloscopes as an additional switched gain stage but not here; the same emitter and collector loads are used for both differential amplifiers producing the same gain.  But the emitter networks for equalization are different.  (2) Take a closer look and there is something even more odd; the tail current of each switched stage is adjustable!  That is not a simple selection between two different equalization configurations; it is more like some kind of calibrated peaking adjustment which is exactly what I would expect if the 50, 70, and 100 MHz models have separate calibration requirements.  (3)  And if the differential tail current was set low, it could result in the waveform problem mentioned above.  Low tail current is actually something I would specifically look for if I saw that.

(1) If you refurbish and calibrate enough oscilloscopes, you start looking for things like that which indicate a real albeit obscure problem.  Another problem would be bandwidth which varies with vertical sensitivity.

(2) Unfortunately Dave did not record the capacitances.

(3) Analog oscilloscopes make this adjustment with capacitive and resistive trimmers where those series RC emitter networks are (4) but if you want automated calibration, then you have to do something else like what apparently Rigol did.  Adjusting the tail current does affect the -3dB bandwidth but it affects the transient response even more and I have never seen it used to limit bandwidth and I do not think that is what is going on here; some Tektronix oscilloscopes adjusted tail current for maximum bandwidth of large signal stages which is a completely different situation.

(4) The Tektronix 2235 series of oscilloscopes came in 100MHz and 60MHz models.  One of the major differences between them was the existence of those equalization networks; they are completely missing on the 60MHz models although the boards have the spots for them.  You can remove the bandwidth limiting network but the transient response will never be correct.
 
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