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Old 3rd July 2013, 05:24 PM   #21
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Re: oscilloscope suggestions?

John, when wrapping the foil do it consistently so that the signal 'pick-up' is equal. K
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Old 3rd July 2013, 11:26 PM   #22
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Re: oscilloscope suggestions?

How much bandwidth do I need for automotive use? The Hantek I was thinking of getting has 70mhz, but its coming all the way form hong kong. I'd rather get something that's already on our shores, and I found a few on amazon that are in the price range I'm looking at but they have a lot lower bandwith, like 25mhz...
Owon PDS5022T Series PDS Portable Digital Storage Oscilloscope, 2 Channels, 25MHz, 100MS/s Sample Rate: Amazon.com: Industrial & Scientific Owon PDS5022T Series PDS Portable Digital Storage Oscilloscope, 2 Channels, 25MHz, 100MS/s Sample Rate: Amazon.com: Industrial & Scientific
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Old 4th July 2013, 02:17 AM   #23
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Re: oscilloscope suggestions?

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How much bandwidth do I need for automotive use? .. they have a lot lower bandwith, like 25mhz...
If you we're trying to observe the contact points waveform for a 12 cyl. 2-stroke at 12,000 rpm you'd have a pulse rate of 2,400 Hz. This is 10,000 times slower than the frequency rating of a 25 MHz scope.

The only interesting thing going on in a car anywhere near the limits of that scope would be CAN bus signalling or similar and a scope would not be particularly useful debugging that other than for seeing presence and shape of the physical layer signal.
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Old 4th July 2013, 06:28 AM   #24
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Re: oscilloscope suggestions?

John, if she's only missing the odd firing, (which I guess is the case) then you may want a scope with storage, so you can look at a prolonged period of time, then analyse the sequences. We have one of the Owon scopes, the DS7102V, which is fine. K
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Old 5th July 2013, 12:12 AM   #25
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Re: oscilloscope suggestions?

A scope bandwidth is usually thought of as how high a frequency you can measure before your reading drops 3 db (about 30%). And that works, sort of, if you're talking about measuring waveforms that are sinusoidal. But bandwidth is also the limiting factor in measuring signal rise time, which is very important in measuring or even observing pulses and other non-sinusoids like square waves. Unless you're diagnosing your sound system, most automotive signals are not going to be sine waves, and you need to know their rise times, durations, whether there is "ringing", if the waveform is flat-topped or droops- and a 0-20KHz scope isn't going to do a good job at that.
Figure out what you need to measure, how accurate your measurement or observation needs to be, and then buy the highest bandwidth you can afford. A couple of points to keep in mind: you'll spend most of your time evaluating your waveform shape, not it's amplitude, if you want to capture the occasional glitch or unexpected event you'll want a storage scope, don't buy a used scope from a seller who doesn't make their living selling used lab equipment, and finally, the best scope in the world is no better than the quality of the leads you use.
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Old 5th July 2013, 11:39 AM   #26
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Re: oscilloscope suggestions?

I ordered a new OWON 25mhz scope from amazon, this one:
Owon PDS5022T Series PDS Portable Digital Storage Oscilloscope, 2 Channels, 25MHz, 100MS/s Sample Rate: Amazon.com: Industrial & Scientific Owon PDS5022T Series PDS Portable Digital Storage Oscilloscope, 2 Channels, 25MHz, 100MS/s Sample Rate: Amazon.com: Industrial & Scientific

John, in your above post, you say 20KHz, do you mean MHz? 20khz is quite low... but if 20mhz is too low, i will have to return this one.
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Old 5th July 2013, 02:39 PM   #27
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Re: oscilloscope suggestions?

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I ordered a new OWON 25mhz scope from amazon, this one: Owon PDS5022T Series PDS Portable Digital Storage Oscilloscope, 2 Channels, 25MHz, 100MS/s Sample Rate: Amazon.com: Industrial & Scientific

John, in your above post, you say 20KHz, do you mean MHz? 20khz is quite low... but if 20mhz is too low, i will have to return this one.
I think he was referring to audio bandwidth which by convention stops at 20 kHz. And in fact, if there were a scope who's bandwidth limit were that low it still would work for the problem you are trying to solve right now. A V8 at 6,5000 RPM is firing each cylinder at 54 Hz.

In any event, per my earlier post, 25 MHz is way more than you are ever going to need diagnosing an automotive signal. A scope like that has a rise time measured in tens of nano-seconds; high performance ignition systems deal in rise times in micro-seconds, and there is no situation with a car that requires your evaluating a signal in nano-seconds, period. Bandwidth in a scope is not like horsepower in a car. Buying more than you need is a total waste of money that would be better used for some other capability (like more channels or wider selection of probes).

There's a whole world of electrical engineering that involves signals in the millions and billions of Hz and above, but none of it's relevant to getting American V-8s running properly.

If someone here thinks there is a situation with fixing a V-8 where a 70 MHz scope would be fine but a 25 MHz scope would not, I really want to hear the details of that measurement scenario.

Last edited by awatkins; 5th July 2013 at 02:58 PM.
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Old 5th July 2013, 03:23 PM   #28
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Re: oscilloscope suggestions?

Thanks Alan,

Just was a little confused. I cant wait to get the scope!
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Old 5th July 2013, 05:38 PM   #29
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Re: oscilloscope suggestions?

Reading the scope can be a challenge unless you know what your looking for and how to adjust it to lock the waveform. One good thing about modern scopes is they have an auto adjustment button. My first scope in the mid 1970s was 20 meg triggered sweep and it weighed about 50 lbs. I stayed up all night looking at different signals being amplified stage after stage, I was so excited to have a tool that I could trace RF with, I also had to make payments to the bank for 2 years to own it! My how things have changed....
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Old 5th July 2013, 09:58 PM   #30
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Re: oscilloscope suggestions?

I'm not trying to tell you what bandwidth, or sampling rate, analog/digital/usb, storage type, scope maker or seller to pick. I don't know if this is a one-time buy for a one-time use or if later on you want to delve into the deeper workings of the digital automotive controls.
Just don't be fooled into thinking a 250MHz scope is overkill unless you're into Ham radio.

The excerpt below was written for college EE students at KU and is an excellent quick overview of the subject of bandwidth, which is how scopes are advertised, and rise times, which is what you see. If you don't want to read the whole PDF, look over the excerpt below which shows how your equipment determines what you see.


Rise time Vs. Signal Bandwidth

tr=3.5/f-3db



"One very useful application of this equation is when using an oscilloscope. The risetime
observed on the display will be a combination of the risetimes of the signals being
measured, the oscilloscope probe, and the oscilloscope as given by:
trise Sqrt [ trise (scope)2 +trise (probe)2+ trise (signal)2 ]
To examine the influence of each of these quantities, let’s look at a digital system. The
question to be answered is “When does digital become analog?” Is it for a 1 MHz clock, 10
MHz clock, or 100 MHz clock? In reality, clock speed is less important than rise time!
Modern logic families have risetimes on the order of 1 ns. This yields a signal bandwidth of
f3dB= 0.35/1ns =350MHz
Assume that a 50.0 MHz oscilloscope and a 10.0 MHz probe are available for making this
measurement. The risetime observed on the display will be closer to 1.55 ns, a ~50% error.
For this reason, it is very useful to compute the best-case risetime for the scope/probe
combination you are using and keep this in mind as you are making measurements. If the
risetime of the signal you are viewing approaches the best-case value, your scope is limiting
the measurement and there may be features in the signal that are hidden from you."

The best advice I can give you having spent five years in oil field data transmission R&D is to match your needs to the capability of the equipment you buy. You're better off borrowing or leasing good stuff than buying cheap.
Attached Files
File Type: pdf Technote 2 - Bandwidth and Risetime.pdf (89.3 KB, 514 views)
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Old 12th July 2013, 09:40 PM   #31
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Re: oscilloscope suggestions?

Quote:
Originally Posted by Stavros View Post
John, here's a mock-up with a copper foil, try this and see if you get a good pulse, try the probe on x10 first and see what you get, then move to x1. You can get quite a thump off the leads when she's running, so keep the HT lead on the plug. K
I just got my scope in and tried this foil setup. I used aluminum foil since thats all I had. With the probe set to 10x, the read was so strong that it was off the charts. I think i'm going to need a 100x probe.
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Old 13th July 2013, 04:11 AM   #32
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Re: oscilloscope suggestions?

John, you should not need x100,.. the magnitude of the signal is proportional to the area of foil enclosing the lead, use less length of foil. e.g. use a wedding ring size, if it is still too much, rather then foil use a piece of wire, wind it once around the lead and clip to the end of it. K
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Old 2nd August 2013, 04:51 PM   #33
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Re: oscilloscope suggestions?

Hey guys, finally got a chance to test more things out with the scope. I'm getting a strange reading that I don't really understand. I made a post about it in the 'race track' section - cylinders 1 and 7 are cold

Anyone have any ideas?
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