Automotive Labscope Specifications

February 17, 2010

By Barry Gersten. The problem most automotive technicians face when trying to select an appropriate labscope is reading and understanding labscope specifications and then matching those specifications to the test requirements. Our objective is to explain bandwidth and sampling specs as they apply to automotive testing. Once you’ve read the labscope spec data and compared that data to your requirements you can make a better decision.

A Method to Evaluate an Automotive Digital Storage Oscilloscope

Typical automotive ignition scopes and analyzers show the secondary ignition firing line looking like this.

However, a really good scope will reveal the details concealed within the firing line. Because the duration of time that the firing line lasts is very brief, few scopes can show the details that can make a big difference in driveability diagnosis.

Automotive Labscope Specifications

Automotive service technicians recognize that traditional testing of electronic circuits using a DMM or scan tool will not always give the fast reliable results they need. The Digital Storage Oscilloscope (DSO) is a necessary tool for troubleshooting sensors, solenoids, relays, secondary & primary ignition, and CAN-C data stream. The problem most automotive technicians face when trying to select an appropriate labscope is reading and understanding labscope specifications and then matching those specifications to the test requirements. Our objective is to explain bandwidth and sampling specs as they apply to automotive testing. Once you’ve read the labscope spec data and compared that data to your requirements you can make a better decision.

There is one other issue that should be considered when shopping for a DSO. Training. Some of the better labscopes with the most sophisticated testing capabilities require long learning time, or coaching from an experienced properly trained expert. Avoid the scopes originally intended for scientific, industrial and telecommunication applications, unless you are prepared to spend lots of time and money on training.

BANDWIDTH – What is Bandwidth?

Bandwidth is a specification that defines the highest frequency electrical signal that the scope can display.

How much Bandwidth is necessary?

CAN-C Data Systems communicate using digital pulses lasting only one millionth of a second (1 microsecond), which is the fastest measurable signal that an automotive technician need be concerned with. Labscope design engineers recommend that bandwidth should be five times the fastest signal speed. This means that a labscope with a Bandwidth of 5 MHz is appropriate for testing today’s automotive engine management systems. A little more is okay. Much more is not okay.

Is too much bandwidth a problem?

Unnecessarily high bandwidth will result in noise or unwanted signals showing up on your labscope screen. The problem of waveform interpretation, already challenging, is made more difficult when the labscope bandwidth is not matched to the measurement task.

SAMPLE RATE – What is Sample Rate?

Sample rate is the number of times per second that the DSO samples the circuit under test.

How much sampling is necessary?

Since automotive high-speed communication pulses (CAN-C) last only 1 microsecond (1μS), at the minimum sample rate of ten samples per microsecond, 10,000,000 samples per/second is required to be minimally sufficient. This is expressed as 10MS/s, and verbalized as “ten megasamples per second”. To be current with today’s automotive technology a Labscope specification requirement must include a sample rate of at least 10MS/s on each channel. Less sampling will NOT be enough. More is better.

How much is necessary?

Because today’s on-board computer can recognize and respond to voltage pulses lasting just a single microsecond (millionth of a second) voltage spikes (because of drivability issues) of only 1 microsecond duration can be a problem. This is a serious because automotive alternators and switches can produce very fast voltage spikes. Sampling a typical normal automotive waveform is usually far less demanding than sampling fast enough to find a 1μS spike or see individual CAN-C pulses.

To capture a spike of 1μS duration the scope should sample at 10 times per microsecond to get 10 samples of the pulse. 5 samples on the rising edge and 5 samples on the falling edge – this means a 10MS/s sample rate is necessary. Automotive labscopes using a lower sample rate can not accurately reveal this type of problem – or properly display a CAN-C Data Pulse Signal.

An insufficient sample rate (under-sampling) is not reliable to reveal the true amplitude and duration of any given pulse. Some scopes with a low sample rate use a process called repetitive sampling to enable the instrument to display a signal that is really too fast for it. However, the signal (or glitch) must be repetitive for this process to work. The situation where the signal/problem is random leaves the user of a low sample rate instrument looking at a waveform that appears normal but is in fact abnormal.

Is the advertised sample rate the actual rate you are using?

Many automotive labscopes are sampling so slowly they must sample a signal many times to finally display a composite waveform on the screen – spikes or faults can get averaged out and the picture you see shows no problem. Be sure your labscope does what you want a labscope to do.

A standard engineering practice in Labscope design is to enable the peak advertised sample rate only when the fastest time per division setting is engaged. With most Automotive Scopes each increase in time displayed on the screen reduces the sample rate by half. When checking typical sensors and solenoids the sample rate is usually only in the thousands. A well designed scope will allow the user to manually adjust the sample rate upwards to get the full benefit of the instrument and get a sample rate in the millions. Lesser scopes can not do this. What’s in your tool box.

Our Recommendations

A good choice would be a 25 Million Samples Per Second Scope when set at 1μS per division… or all the time. You make the selection. Few Automotive Scopes have this feature.

Look for a Scope that is designed to properly display fast Alternator Ripple Spikes and CAN-C signals. A Scope that will properly handle Secondary Ignition firing line components on a Distributor Ignition System with the cap-rotor gap and spark plug voltages individually seen on the display along with any faults such as a very tiny extra gap caused by a poor ignition wire connection at the spark plug is a very good scope and is doing the hard work. Everything else will be easy for that kind of scope.