Old analogue scopes beat new digital scopes, for bit-bashed digital outputs

[tggzzz]

Comparing a 50 MHz Rigol against a 300 MHz Tektronix doesn't prove a lot.  Of course the Tek has more bandwidth.

I didn't toss my 350 MHz Tek 485 just because I bought the 100 MHz Rigol DS1054X.  Different tools for different jobs.

I see no point in comparing the scopes.  And, yes, I can get a 350 MHz 485 for a couple of hundred dollars.  But it still won't decode SPI or provide measurements or do single shot or FFT or any of the other things a modern DSO can do.  It does have the bandwidth advantage.

Pretty meaningless comparison...

Oh, I agree with all of that - but that wasn't the point of the post!

The point was to highlight the consequences of typical statements made in this forum.

[tggzzz]

Also, who the fuck pays 275 pounds for a 1052E?? 

The usual advice in this forum is "buy new and get the guarantee"; so I just lifted the price from the Rigol website.

[tggzzz]

I disagree completely, and I'll give you an example of why.  Just a few days ago, I was unexpectedly debugging a 16 bit, bit banged SPI transfer.  Since my DSO is in the mail (or possibly hasn't been manufactured yet) I had to use my analog CRO, with the intensity set way up, the lights turned out and a bunch of shots with my cell phone camera hoping for a lucky strike. 
If you look at the trace, you'll see the upper is the SCLK and the bottom is the data.  Due to the bit banging, it hangs a bit between bytes.  However the data is completely wrong:  the first few bits should be 0.
After too much time I found the problem.  My 3 almost 4 year old nephew had previously pressed practically every button on the 'scope.  While I had reset most, I neglected the 2nd channel invert button.  Therefore any DSO is superior to an old analog CRO because the invert channel 2 button doesn't exist.

Out of curiosity, what was the bit rate? I might try to duplicate that on an analogue storage scope, for purely masochistic fun, of course.

[tggzzz]

Let's see if I've got this straight:

We're looking at a 150MHz signal on a 50MHz DSO and a 300MHz Analog 'scope and concluding that Analog 'scopes are "better"?

Not quite Look at the first post, and see why I've shown the two sets of results.

This is a simple 10kHz waveform output.

No it isn't. It's a program that generates 10,000 very narrow pulses per second.

That is a very simple waveform. It is also a very common waveform when you are blipping a bit to see

• how fast/slow a loop is occurring
• latency between two events, typically a stimulus and a response

Hence I observe such waveforms most days when I am debugging and performance testing hard realtime systems.

Code: [Select]

        outPort <: 1;
        outPort <: 1;
        outPort <: 1;
        outPort <: 1;
        outPort <: 0;
Is there a reason for having 4 ones and 99996 zeros?

(I mean apart from when you manually tuned the program to look bad on the 50MHz DSO?)

I was wondering if someone would ask that! The results would be exactly the same if the program was
configure_clock_rate(clk0, 250, 4);
...
outPort <: 1;
outPort <: 0;
...

And, as I said above, such waveforms are probably the most common ones I generate and observe. I should have made that explicit in the first post; mea culpa.

[tggzzz]

Let's see how many "immune responses" are triggered and replies made without reading the qualifications stated in the OP. 

Too many, but that was anticipated

Some people will also jump on obscure points whether or not they are correct and/or support their contention and/or whether or not they are relevant to the reason for the first posts.

[Fungus]

Let's see if I've got this straight:

We're looking at a 150MHz signal on a 50MHz DSO and a 300MHz Analog 'scope and concluding that Analog 'scopes are "better"?

Not quite Look at the first post, and see why I've shown the two sets of results.

I already know why you've done it.

This is a simple 10kHz waveform output.

No it isn't. It's a program that generates 10,000 very narrow pulses per second.

That is a very simple waveform. It is also a very common waveform when you are blipping a bit to see

• how fast/slow a loop is occurring
• latency between two events, typically a stimulus and a response

Hence I observe such waveforms most days when I am debugging and performance testing hard realtime systems.

You're doing it wrong.

Clue: Toggle the pin instead of blipping it then you can use the DSO's pulse width measurement to get the elapsed time instead of trying to count fractions of squares on a a green screen.

(You do have pulse width measurement on the Analog 'scope, right?)

You'll also save three clock cycles.

[Fungus]

Is there a reason for having 4 ones and 99996 zeros?

(I mean apart from when you manually tuned the program to look bad on the 50MHz DSO?)

I was wondering if someone would ask that! The results would be exactly the same if the program was
configure_clock_rate(clk0, 250, 4);
...
outPort <: 1;
outPort <: 0;

Math. It works, bitches!

But the reason you can't do six one was...?

[nctnico]

In this test the program sits in a tight loop outputting a constant pattern (0x005555ff) using an I/O port's SERDES output running as fast as possible.

While the old Tek 2465 displays a sensible waveform, the new Rigol is clearly struggling; it looks like the logic signals are invalid.

All you have proven here is that a 50MHz scope isn't up to the job (assuming it is setup properly). This is a GDS2204E 200MHz DSO decoding a 125MHz SPI bus:

[nctnico]

In this test the program sits in a tight loop outputting a constant pattern (0x005555ff) using an I/O port's SERDES output running as fast as possible.

While the old Tek 2465 displays a sensible waveform, the new Rigol is clearly struggling; it looks like the logic signals are invalid.

All you have proven here is that a 50MHz scope isn't up to the job (assuming it is setup properly). This is a GDS2204E 200MHz DSO decoding a 125MHz SPI bus:

BTW that Rigol isn't new but very old. Turning on peak detect will help a lot to see more.

If you are trying to prove analog scopes work easier then I think you are wrong. Sometimes training wheels cause more problems than they solve.

[tggzzz]

Let's see if I've got this straight:

We're looking at a 150MHz signal on a 50MHz DSO and a 300MHz Analog 'scope and concluding that Analog 'scopes are "better"?

Not quite Look at the first post, and see why I've shown the two sets of results.

I already know why you've done it.

This is a simple 10kHz waveform output.

No it isn't. It's a program that generates 10,000 very narrow pulses per second.

That is a very simple waveform. It is also a very common waveform when you are blipping a bit to see

• how fast/slow a loop is occurring
• latency between two events, typically a stimulus and a response

Hence I observe such waveforms most days when I am debugging and performance testing hard realtime systems.

You're doing it wrong.

Clue: Toggle the pin instead of blipping it then you can use the DSO's pulse width measurement to get the elapsed time instead of trying to count fractions of squares on a a green screen.

(You do have pulse width measurement on the Analog 'scope, right?)

You'll also save three clock cycles.

Time accuracy is indeed one of the significant advantages of digital scopes; no surprises there. But not the point being made.

Usefully, with XMOS processors and xC it is trivial to have a specified pulsewidth with almost zero software impact. There are language and hardware constructs stating "change the output to X at clock tick Y". But not many people are using XMOS processors.

[tggzzz]

In this test the program sits in a tight loop outputting a constant pattern (0x005555ff) using an I/O port's SERDES output running as fast as possible.

While the old Tek 2465 displays a sensible waveform, the new Rigol is clearly struggling; it looks like the logic signals are invalid.

All you have proven here is that a 50MHz scope isn't up to the job (assuming it is setup properly). This is a GDS2204E 200MHz DSO decoding a 125MHz SPI bus:

Of course; no surprises there.

But read the first post to see that I deliberately chose the tests to show the consequences of typical advice on this forum.

[tggzzz]

BTW that Rigol isn't new but very old. Turning on peak detect will help a lot to see more.

Of course. As I said, I know which 5 buttons to press to change from the "normal" (that's what is it called) setting to "peak detect". But beginners might not even notice the choice exists.

(I've skipped over the point that not all digitising scopes have peak detect)

If you are trying to prove analog scopes work easier then I think you are wrong. Sometimes training wheels cause more problems than they solve.

That point can legitimately be argued in either direction, but simplicity is usually a virtue.

[Fungus]

Usefully, with XMOS processors and xC it is trivial to have a specified pulsewidth with almost zero software impact.

So, ummmm ... your argument is: It wouldn't cost you anything to make it wide enough to see on your DSO.

Is that right?

[Fungus]

That point can legitimately be argued in either direction, but simplicity is usually a virtue.

Hammers are simple! (but not much use for driving screws)

[tggzzz]

Usefully, with XMOS processors and xC it is trivial to have a specified pulsewidth with almost zero software impact.

So, ummmm ... your argument is: It wouldn't cost you anything to make it wide enough to see on your DSO.

Is that right?

For people wise enough to use XMOS processors, it wouldn't cost anything. But this thread is about unthinking advice given to beginners; how many beginners use XMOS processors?

[tggzzz]

That point can legitimately be argued in either direction, but simplicity is usually a virtue.

Hammers are simple! (but not much use for driving screws)

I've said that too many times. (I've also noted that professional sometimes hammer screws into wood, except for the last turn).

But let's keep away from homilies; they obscure as much as they reveal.

[Fungus]

I've also noted that professional sometimes hammer screws into wood, except for the last turn.

That doesn't mean that amateurs can.

Hammering in a screw takes years of experience. 

[nctnico]

BTW that Rigol isn't new but very old. Turning on peak detect will help a lot to see more.

Of course. As I said, I know which 5 buttons to press to change from the "normal" (that's what is it called) setting to "peak detect". But beginners might not even notice the choice exists.

Then they'll learn an important lesson: RTFM!
The way you are reasoning everyone should crawl because it takes effort to learn to walk, ride a bike and/or how to drive a car.

[tggzzz]

BTW that Rigol isn't new but very old. Turning on peak detect will help a lot to see more.

Of course. As I said, I know which 5 buttons to press to change from the "normal" (that's what is it called) setting to "peak detect". But beginners might not even notice the choice exists.

Then they'll learn an important lesson: RTFM!
The way you are reasoning everyone should crawl because it takes effort to learn to walk, ride a bike and/or how to drive a car.

RTFM is necessary but insufficient; it should be R&UTFM. The "& Understand" is a chicken-and-egg problem with beginners given unthinking overly dogmatic advice.

Everybody does learn to crawl before they learn to walk - so your statement adds nothing to the conversation.

[mikeselectricstuff]

The standard statement seen all too often on this forum is that an entry-level Rigol scope is better than an old analogue scope. But is that really true for a common basic use-case: a beginner looking at bit-bashed outputs from a microprocessor?

Yes, absolutely.
The reason the beginner is looking at the output is to see if their code is doing the right thing. They need to capture the output.  zoom in, make timing measurements and tweak code until it's right.
 To suggest that an analogue scope is better for this is just ridiculous.
 There are probably some areas where an analogue scope beats an old or lowest-end digital scope, (but this isn't one of them).
Any digital scope with reasonable memory and intensity display beats an analogue scope in pretty much all respects.

[ebastler]

That statement should provoke the immune response

I assume there was some Freudian slip at work when you typed the "bit-bashing" title? 
Well; nothing wrong with some bit-banging, scope-bashing fun on a slow day...

[Fungus]

this thread is about unthinking advice given to beginners

No it isn't, it's a troll.

[Jwalling]

this thread is about unthinking advice given to beginners

No it isn't, it's a troll.

And, as always when this subject comes up, successful!

[tggzzz]

That statement should provoke the immune response

I assume there was some Freudian slip at work when you typed the "bit-bashing" title? 
Well; nothing wrong with some bit-banging, scope-bashing fun on a slow day...

Possibly But I've heard both terms used.

Strictly speaking my point isn't scope bashing, since the scopes are performing to specification and as expected. Know your tools, understand their limitations. And make explicit the limitations of advice given to beginners.

[tggzzz]

this thread is about unthinking advice given to beginners

No it isn't, it's a troll.

And, as always when this subject comes up, successful!

It is a shame people ignore the multiple explicit caveats in the first post.