Vertical Resolution

March 23, 2011

By Matt Fanslow. As a Pico user, you should be very familiar and spoiled by the power provided to you by your PicoScope. But, how do you apply and make use of all that power? Starting in Version 6.0.11.2 PicoScope 6 allows you to apply “vertical resolution enhancement” up to 16 bits. In this tutorial you will see how the enhancement works, when to us it and when not too.

If you’ve been looking at purchasing a DSO (Digital Storage Oscilloscope) in the last few years, undoubtedly you’ve seen much attention paid to Vertical Resolution. To the scope savvy, it is the new “Sample Rate Rage”. For years, Sample Rate was advertised as the most important of scope specifications. The higher the sample rate, the better the scope. Thanks to Pico Technology, and those few individuals that embraced its use in the auto repair industry, we have been re-educated. Sample Rate, it turns out, while important; hinges on other factors. Vertical Resolution, you will find, is also one of many factors that determine the true “power” of a DSO.

[sws_2_column title=””] To accurately explain Vertical Resolution, we have to discuss a little about how a DSO functions. An oscilloscope simply plots voltage over time (Figure_1). Voltage is displayed vertically, while time is represented horizontally. Simply put, a steady voltage will show up on a scope as a straight, flat line across the screen (Figure_2). Now, in reality, our “line” is made up of many, many sample points or “dots” (Figure_3). The DSO then connects the dots to give us our recognized pattern or waveform. Where this is going is that when plotting points, two points CANNOT occur at the same point in time (Figure_4). Even if our scope screen depicts a “textbook” square wave, the vertical rise and fall of the square wave are NOT straight up and down (Figure_5). In our example (Figure_6), we have a typical looking square wave off of a Chrysler CMP (Cam Position Sensor). The rise and fall are straight up-and-down, and our corners are square. Now look at our next example (Figure_7). I’ve “zoomed in” on the pattern. Notice now that the vertical rise is sloping, as is our falling edge. This happens on ANY vertically changing pattern.

Vertical Resolution is often defined as the number of “steps” (Figure_8) that can be vertically displayed. Put very plainly, Vertical Resolution is voltage accuracy (how accurately placed is that sample point on our screen). Said another way, it’s the amount of vertical lines available (vertically) for a “dot” to be placed. See Figure_9.

Specifications for resolution are given by “Bits”. A bit, as I’m sure you are aware of, is one part of a byte. For instance, a popular resolution spec is 8 Bit. Broken down, this means 28. The 2 comes from the fact that the ADC (Analog-to-Digital Converter) converts the measured voltage (what you’re testing) from an analog input (which the processor cannot decipher) to a digital word in binary language. Binary actually means “base two”. There are only two “states” to which binary “bits” can communicate: 0 and 1. One bit has two states. Our example is 8 bits, so we get 28.

So, what is that telling us? With an 8 bit Vertical Resolution, our voltage range will be divided up into 256 steps or levels if you will. To think of it a different way, think of a DSO “screen” (a factor that has to be acknowledged here is that the actual screen you are viewing may have better or worse resolution than the scope. Meaning, the scope may be able to see a certain amount of points across a given time frame, but the screen itself can’t show it because it doesn’t have enough places on it to display it and vice versa) as being a peg board. Yes, the board you use in the garage or your closets to hang stuff. Each hole could be defined as a plot point. We have holes going vertically and horizontally. They are fixed, they do no move. The gap between two vertical running holes is the same as the gap between vertical points on a DSO. So, if we have an 8-bit scope, we have 256 holes going from top to bottom. On a 12-bit scope, we have 4096 holes. To put this in perspective, with a voltage setting of -1 volt to 1 volt our sample points can be no closer than 8mV (vertically) away from each other. To use a more common setting for automotive, -20 volts to 20 volts, our calculations would show us that our sample points could not be closer than 156mV (vertically) from each other!

Let’s compare that to a 12 bit Vertical Resolution scope. Instead of 28 we will now use 212. This gives us 4096 vertical lines of resolution. Now back to our -1 volt to 1 volt vertical scale. As you can see, we went from a minimum 8mV separation down to 0.5mV! Let’s run our -20 volts to 20 volts calculation. It comes out to about 10mV minimum separation. That’s 16 time more accurate. 16-bit scopes display voltage extremely accurately, but often suffer from slower sample rates (thus fewer sample points).

Vertical Resolution, teamed with the number of sample points, dictates the accuracy and detail. The number of sample points is determined by memory depth (buffer size) and sample rate. To put it bluntly: you can have a high amount of Vertical Resolution, but if there is not enough sample points you will not have an accurate and detailed representation of the waveform. Also, on the other hand, if you have a great number of sample points, but low resolution, you still will not have an accurate and detailed pattern.

Where does this come into play when diagnosing automotive faults? Few things are faster in the automotive world than the firing line of a primary or secondary ignition event. If a firing line is “opened” up to be viewed in detail, on a DI ignition system you can (with a capable DSO) see the “rotor notch”. This is the dip in the pattern where the rotor to distributor air gap is ionized and then current flow is established before the spark plug is ionized and then “fired”. The diagnostic value here is this if you are looking for a rotor notch and cannot see one where there should be, the rotor air gap has become the “greatest gap” and is represented by the peak of the firing line. Also, on EI (waste-spark) ignition we still have two gaps (instead of the rotor to distributor air gap and sparkplug gap we have two sparkplug air gaps), if there isn’t a “notch” (which now represents the sparkplug on the exhaust stroke we have a secondary resistance issue. Another instance where vertical resolution can show up as an issue is when “zooming” in on a capture at lower voltage scales the pattern can take on a block like appearance.

Rotor “notch” with 8-bit DSO (Figure_10, Figure_11 and Figure_12)
Rotor “notch” with 12-bit DSO (Figure_13, Figure_14 and Figure_15)
Rotor ‘notch” with 16-bit DSO (Figure_16 and Figure_17)

The key to all of this is to not be misled by the overwhelming number of specifications that we get bombarded with, and also those specs that are not given to us. Learning about how a DSO truly works and how the specifications work together to provide a waveform is the key to choosing a scope that is suited to our skills and needs. One could make a comparison to exhaust gas analysis. High hydrocarbons in the exhaust could mean that the engine is running rich, but it could also signify an ignition misfire. Other information provided to us must be taken into consideration before a decision can be made (CO, O2, Lambda, etc). The same can be said of DSO choices. Take in as much information as you can, then make your decision.[/sws_2_column]

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