Author Topic: Buying a scope  (Read 3294 times)

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Offline carbon dude oxide

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Buying a scope
« on: May 03, 2013, 08:03:20 am »
Hello, i am planning on getting a scope for myself but i am tied between either getting a ds1000E or a ds1000B

Reason being is that the ds1000e has quite high reviews here but i do alot of 4-bit binary debugging so having 4 chanels would be very usefull as currently i use a multimeter to do the job :)

What are other peoples thoughs?
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Offline mazurov

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Re: Buying a scope
« Reply #1 on: May 03, 2013, 09:44:24 am »
A dedicated PC-based logic analyzer is many times more useful for digital debugging and often much cheaper too. This is what I'm using -> http://dangerousprototypes.com/docs/Open_Bench_Logic_Sniffer
 

Offline Hydrawerk

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Re: Buying a scope
« Reply #2 on: May 03, 2013, 10:03:41 am »
I would choose the DS1000B because it has a better front panel. It has knob for each channel. On the other hand, it has short memory, probably only 8kpoints per channel.
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Offline carbon dude oxide

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Re: Buying a scope
« Reply #3 on: May 04, 2013, 01:20:35 am »
Could you explain kpoints per second please
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Offline Wuerstchenhund

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Re: Buying a scope
« Reply #4 on: May 04, 2013, 07:47:36 pm »
Could you explain kpoints per second please

k = kilo = 1000;

kpoints = kilopoints = 1000 points;

'points' It's a measure for a scope's sample memory.
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Offline carbon dude oxide

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Re: Buying a scope
« Reply #5 on: May 04, 2013, 08:20:49 pm »
Is a point 1 measurment of the channel by chance?
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Offline jpb

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Re: Buying a scope
« Reply #6 on: May 04, 2013, 11:09:04 pm »
Is a point 1 measurment of the channel by chance?
Yes, it is a single sample point. The kpoints refers to memory above so it is not kpoints/sec which would be a (very slow) sample rate.

A typical oscilloscope display can show 500 to 700 points (about 50 points per div and normally there are 10 divisions but some scopes have extended this to 7 or more).
On digital scopes with VGA displays there are 640x480 pixels but some of these are typically dedicated to menus so the display area is reduced to 500x400 pixels.
Older digital scopes only had 320x240 displays which presumably meant they could only display something like 250 points.

A 8kpoint of memory thus allows a zoom of x16 before gaps start to appear. Of course at faster time bases the number of points per screen will be limited by the sample rate.

More memory also allows you to sample at a higher rate at lower time bases.

8 kpoints of memory is not very much by modern standards with some Rigol scopes having 140Mpoint memories but if you're happy to re-sample at a new time base rather than zoom then you don't need the memory. So if you simply want to look at repetitive waveforms it is not so much of a problem. If you want to be able to study a high frequency glitch on a low frequency waveform then memory is very useful.
« Last Edit: May 04, 2013, 11:13:15 pm by jpb »
 

Offline carbon dude oxide

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Re: Buying a scope
« Reply #7 on: May 05, 2013, 06:30:02 pm »
Ok thanks :) that is very helpfull to know :)

How high does my memory/ sample rate need to be to see I2C and SPI at work?
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Offline jpb

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Re: Buying a scope
« Reply #8 on: May 05, 2013, 09:47:55 pm »
Ok thanks :) that is very helpfull to know :)

How high does my memory/ sample rate need to be to see I2C and SPI at work?

It depends what you mean by "see". Some scopes provide decoding in hardware for these.

The more memory you have the more bits that you can capture and then zoom in on.

At the lowest level I'd say you want to capture data at a rate that gives you 10 points per clock tick (so you can see the shape as well as the level).
At say I2C at 100 kbits a second this would be only 1MS/sec which is very low. Most scopes should be able to go at 100 times this at least.

SPI according to Wikipedia typically talks to peripherals at up to 100MHz. To accurately observe a 100MHz clock and get accurate rise times and so on
might require a very expensive 1GHz bandwidth scope.

For hobby use you're probably going to be working at much lower clock rates.

If you want to look at the shape of clocks that are around 30MHz then you probably want a bandwidth that doesn't attenuate much up to the 5th
harmonic so a 200MHz bandwidth scope. The Nyquist sampling rate needed would be at least twice this or 400MS/s but in practice you really
need to have 1GS/s

More advanced scopes add features like special triggers so you can trigger off particular bit patterns and also decode the data on screen.

The main requirement you may have for SPI is the number of channels on the scope. If you want to look at MISO MOSI and SCLK then you need a 4 channel scope
rather than a 2 channel scope.

I think though that a 4 channel scope with 100MHz bandwidth would let you check the logic levels and timings in most circumstances. The memory question is
difficult to answer because it depends how many bits you want to capture at a time.
 

Offline Bored@Work

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Re: Buying a scope
« Reply #9 on: May 05, 2013, 10:00:34 pm »
In the extreme case you only need > 2x clock frequency. This guarantees that you won't miss a regular signal transition. It does not guarantee that you catch glitches, it doesn't give you the exact timing of the transitions, it does not show you rise-/fall-times.
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Offline MasterOfNone

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Re: Buying a scope
« Reply #10 on: May 05, 2013, 11:10:51 pm »
A question for anyone:
Is there any reason you wouldn’t/shouldn’t use a USB Logic Analyzer with free I2C/SPI protocol Analyzer plugin for this type of work?
‘Carbon Dude’ it sounds like you will need a Scope anyway (if you are using a Multimeter), but have you looked into buying a scope and a Logic Analyser?
 

Offline carbon dude oxide

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Re: Buying a scope
« Reply #11 on: May 06, 2013, 04:20:33 am »
yes logic analyser is one of the next things on my list of equipment to get ^.^

one of the reasons for getting a scope first is that i need to learn how to use it for an up coming job where i will be using them alot :) so i need to get to know them. Im also planning on going into some RF projects so the scope will be a definite help :) but current usage would be to check logic levels on multiple pins at the same time, making sure my I2C is working and reading 4 bit BCD's.
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