eSCOPE Article – Automotive Oscilloscopes

eSCOPE Article – Automotive Oscilloscopes
by Bernie Thompson December 2017

There are many things one needs when using an automotive oscilloscope. Have you ever used an oscilloscope with a speed such as 5 Million samples per sec (5Ms/sec), but find it does not work very well? Many scopes have sufficient speed yet work poorly. Speed is but one aspect of a scope. A scope collects voltage samples then time stamps these samples so they can be displayed. Each sample is a point on the display screen that is then connected together with lines which are referred to as linear interpolation. The greater the number of points that can be displayed on the screen the better the waveform. Some scopes can allow more points to be displayed on the screen than other scopes can, and the more points the better representation the scope can produce of the acquired signal. Additionally if sample points are missing then the waveform may not be represented correctly. Analog-to-digital converters (ADC) are devices that sample continuous analog signals and convert them into digital words. Oscilloscopes use high speed ADCs to acquire data. The bit resolution and clock speed of the ADC are very important for the design of the oscilloscope. The 5Ms/sec that was referred to is the clock speed of the ADC, this is the horizontal axis or X axis. There are also data points that are put vertically and are referred to as the Y axis. These vertical points are also part of the ADC and define the voltage level that can be read. ADC resolution is the conversion of an analog voltage to a digital value in a computer. A computer is a digital machine and thus stores a number as a series of ones and zeroes (binary code). If you are storing a digital 2-bit number you can store 4 different values: 00, 01, 10, or 11. Now, say you have a device which converts an analog voltage between 0 and 40 volts into a 2-bit digital value for storage in a computer. This device will give digital values as follows:

2- Bit Digital Representation
00 = 0V to 10V
01 = 10V to 20V
10 = 20V to 30V
11 = 30V to 40V
So in this example the 2-bit digital value can represent 4 different numbers and the voltage input range of 0 to 40 volts is divided into 4 pieces giving a voltage resolution of 10 volts per bit.

3- Bit Digital Representation
000 = 0V to 5V
001 = 5V to 10V
010 = 10V to 15V
011 = 15V to 20V
100 = 20V to 25V
101 = 25V to 30V
110 = 30V to 35V
111 = 35V to 40V
So in this example the 3-bit digital value can represent 8 different numbers and the voltage input range of 0 to 40 volts is divided into 8 pieces giving a voltage resolution of 5 volts per bit.

4- Bit Digital Representation
1 1 1 1 = 0V – 2.5V
1 1 1 0 = 2.5V – 5.0V
1 1 0 1 = 5.0V – 7.5V
1 1 0 0 = 7.5V – 10V
1 0 1 1 = 10V -12.5V
1 0 1 0 = 12.5V – 15V
1 0 0 1 = 15V – 17.5V
1 0 0 0 = 17.5 – 20V
0 1 1 1 = 20V – 22.5V
0 1 1 0 = 22.5V – 25V
0 1 0 1 = 25V – 27.5V
0 1 0 0 = 27.5V – 30V
0 0 1 1 = 30V – 32.5V
0 0 1 0 = 32.5V – 35V
0 0 0 1 = 35V – 37.5V
0 0 0 0 = 37.5V – 40V
So in this example the 4-bit digital value can represent 16 different numbers and the voltage input range of 0 to 40 volts is divided into 16 pieces giving a voltage resolution of 2.5 volts per bit.

The bit resolution for the ADC referrers to the vertical or Y axis. Automotive oscilloscopes typically uses 8 bit resolution but can be; 8 bit, 10 bit, 12 bit, 14 bit, and 16 bit. These bits or points of resolution, refer to how many parts the maximum signal can be divided into. An ADC takes an analog signal and turns it into a binary number. Thus, each binary number from the ADC represents a certain voltage level. Resolution is the smallest input voltage change a digitizer can capture.

1) 8 bit = 256 discrete levels, @ 40V = 156.25mV/bit
2) 10 bit = 1,096 discrete levels, @ 40V = 36.49mV/bit
3) 12 bit = 4,096 discrete levels, @ 40V = 9.76mV/bit
4) 14 bit = 16,384 discrete levels, @ 40V = 2.44mV/bit
5) 16 bit = 65,536 discrete levels, @ 40V = .061mV/bit
40 volt span = -20V to +20V, 20V being typical of automotive scope use.

As you can see the data point exponentially increases with the bit number increasing. The more vertical points that are assigned to the waveform the better they represent the waveform. This is true of the horizontal points as well. The more points that you have horizontally the better they represent the waveform. However, the horizontal points are a product of time and must match the signal that is acquired. This means if you are acquiring a slow signal then the time will have to be set to a slower time as well. If the frequency of the acquired signal is high then the time will have to be set to a faster time. The time on the scope represents the frequency and, in order to have the waveform correctly represented, the speed that the scope is running must match the speed of the acquired signal. Any time the frequency of the signal that is being acquired by the scope is not close enough to the frequency set within the scope, an inaccurate representation of the waveform will be displayed. This occurs when the scope is set to fast for the signal or when the scope is set to slow for the signal.

On vehicles the voltage that is being read by the oscilloscope is produced by the vehicle sensors and microprocessors outputs. Sensors take a physical quantity and convert this to an electrical output (Voltage). The sensor produces a signal that is proportionate to the physical quantity that is being read. Since the physical quantity is based on the speed of the parts on a vehicle, the frequencies are quite slow. The sensors produce data from the vehicle that is read with a microprocessor. The microprocessor then analyzes this data and produces outputs to control the operation of the vehicle’s systems. Since the outputs are controlling the vehicle’s systems they must work at the same frequencies of the vehicle. This means that the outputs will be slow as well.

In order for the scope to produce a waveform that correctly represents the signal that is being acquired the scope speed will be set at a lower level. The ADC clock speed is variable so when you change the oscilloscope’s time base it changes the ADC clock speed. This means if you have a 5Ms/sec scope, this is the maximum speed the scope can run. However, the scope can run at many different slower clock speeds. When you are viewing the waveform on your scope display you will need to set the scope speed at a level where the waveform is best displayed. On a vehicle this means you will being running the scope at levels far below the scope maximum speed for 99% of the time the scope is in use.

To properly construct a waveform the Nyquist-Shannon sampling theorem states that for a true representation of a waveform, greater than two samples per period are required. For an ADC, this means for a true representation of the analog input signal, the clock frequency must be two times greater than the analog input frequency. However for the waveform to be correctly constructed you will need a minimum of 5 sample points.
An example of this would be if the engine is turning at 6000 Revolution Per Minute (RPM); With a 360 degree Crankshaft (CKP) trigger wheel the Frequency would be:
6000 X 360 = 2,160,000 pulse per minute
2,160,000/60 seconds = 36,000Hz
The scope speed minimum would be
36,000Hz x 2 = 72,000Hz or 2 samples per trigger event
To construct a good waveform at least 5 samples will be needed
36,000hz x 5 = 180,000Hz or 5 samples per trigger event
As you can see 180,000 samples/sec needed to produce a waveform is not close to the 5,000,000 samples/sec maximum scope speed. This would indicate that the scope speed needed to view a good representation of this signal would be set under the scope maximum speed.
At 1Ms/sec maximum scope sample rate would be:
6000 X 360 = 2,160,000 pulse per minute
2,160,000/60 seconds = 36,000 pulse per second
1,000,000 / 36,000Hz = 27.77 samples per trigger event
At 5Ms/sec maximum scope sample rate would be:
6000 X 360 = 2,160,000 pulse per minute
2,160,000/60 seconds = 36,000Hz
5,000,000 / 36,000Hz = 138.88 samples per trigger event.
. (NOTE: The 360 degree CKP target wheel is only used as an example, a 360 degree CKP target wheel is not used on vehicles as this would use too much of the engines microprocessor. Most vehicles use a 36 tooth or a 64 tooth target wheel.)
The problem that occurs when the scope is set at a faster time base than needed is that the waveform will be spread out over the scope display for the time selected as shown in Figure 1. This means rather than having a good representation of the waveform only a small portion of the waveform will be displayed. If the scope is set to slow then the waveform will not be displayed correctly due to compression or aliasing, as shown in Figure 2. The only way to view the signal correctly is to have the time base of the scope match the frequency of the signal as shown in Figure 3.

Additionally as the scope speed is increased most scope buffers will be overdriven. When the scope is using high speed acquisition the data buffers are over driven quickly causing data losses. A scope can only display what was captured within the memory system. This means that if the drivability problem you are looking for causes the vehicle to fail when the scope buffer drops the data then no failure will be shown on the scope display.