Measuring Top, Base and Amplitude in PicoScope 7

What are Top and Base?

Top and Base are measures of the high and low levels of a signal, excluding any noise or ringing present on those levels. These differ from the Maximum and Minimum which do include any noise or ringing on those levels.

An example can be seen here where we measure the Maximum, Minimum, Top, Base and Amplitude. There is a very slight overshoot at the top of the rising edge and bottom of the falling edge which the Top and Base measurements will ignore.

When zoomed into the top of the rising edge we can see how the Top measurement differs from the Maximum measurement by excluding the Positive overshoot (left). 
Similarly, when zoomed into the bottom of the falling edge, we can see how the Base measurement differs from the Minimum measurement by excluding the Negative overshoot (right).

What is Amplitude?

Amplitude refers to the magnitude or strength of an electronic signal, it is usually measured in volts or current in the vertical axis for AC (alternating current) signals.

The Amplitude automated measurement differs from peak to peak (another measure of amplitude), by measuring the difference between the top and base reference levels (also known as steady states), instead of the minimum and maximum signal values. The top and base reference levels are attained via the same algorithms, used in the Top and Base automated measurements. These reference levels help avoid noise (potentially overshoot), influencing the result. 

Automated measurements - Top, Base & Amplitude

Setup and configuration

The Top, Base and Amplitude measurements can all be found in the Amplitude category of measurements. After adding the measurement, it will default to measure the whole trace but can be further constrained to only measure between the time rulers, the cycle at ruler 1 or the cycle at ruler 2 using the settings popup.

These screenshots show where these measurements can be found within the application and the settings popup window used to configure the area to measure:

Interpreting results

Top, Base and Amplitude measurements results are shown in their own respective measurement lozenge in the Measurements viewport. By default the measurement is calculated over the whole trace but this can be changed to other options selected in the settings popup which is accessed from the measurement lozenge itself. The Measurement lozenge also provides other statistical calculations about the measurement, e.g. maximum, minimum, standard deviation, mean and number of measurements taken.

Calculation and algorithm - Top and base

Top and Base measurements are calculated using the histogram method where the histogram contains a number of bins which span across the measured signal range, e.g. from -5V to +5V. Each bin therefore has a width which represents a range of sample values, in this example 10V / 512 each bin is roughly 20mV wide. Each bin contains a count of how many samples fall into its particular range. Therefore if a bin contains a large number of samples then those samples are likely to occupy a relatively flat portion of the waveform, whereas if a bin contains very few samples then those samples are likely to occupy a rapidly changing portion of the waveform, e.g. an edge.

To obtain Top and Base measurements we target those bins that are at the top and bottom portions of the histogram. From those portions we then take the bins which have the highest counts. Once we have those bins we take the mean value from each bin and take the mean of those to get the actual value.

Calculation and algorithm - Amplitude

The Amplitude measurement is calculated as the difference in the Top measurement and the Base measurement, i.e. Top - Base.

Math channel - Top and Base

Setup and configuration

The Top and Base Math channels use the same algorithm as the measurements do, i.e. use of the histogram method to split the signal levels into a number of bins spread across the signal voltage range. The difference for Math channels is that where a measurement calculates its result over the whole waveform, a Math channel calculates its result per cycle* of the main waveform, so the Math channel waveform will move up and down to follow any changes in the main waveform.

To configure a Top or Base Math channel we use the Math channel creation wizard, available from the Math channels options panel. Selecting the + button on the Math channels panel will open the wizard to add a new Math channel.

The Top and Base Math channel options are found in the Scientific functions section of the Formula page (first page) of the wizard. Simply select the Top or Base channel as required, then select the source channel from the available Channels, e.g. A, B, C and D as shown in the screenshot (right).

An example of a Top and a Base Math channel waveform is shown below. Top is showing 0.8V and Base is showing -0.8V:

Math Channel - Amplitude

Setup and configuration

The Amplitude Math channel uses the same calculation algorithm as the Amplitude measurement and so the calculation is merely Top - Base. Like other Math channels though, the result is calculated per cycle* rather than for the whole trace.

Configuring an Amplitude Math channel is done in the same way as for the Top and Base Math channels, by using the wizard creation tool. The Amplitude Math channel is also found in the Scientific functions section on the first page of the wizard. Select the Amplitude channel, then select the source channel from the available channels.

As the Amplitude measures Top - Base, it may be necessary to adjust the Math channel range so it can show the correct value. By default the range is set to the source channel range which may not be enough for some Math channels. Change the range on the next page of the wizard as shown in the screenshot (right) where we need to double the default range to accommodate the Top - Base calculation.

An example of an Amplitude Math channel waveform is shown below. Note the Math channel looks like it’s showing the Amplitude to be the same as the peak of the Channel A which would be wrong. However, notice that the Math channel scale is double that of the main waveform so the amplitude is actually showing as 1.6V, which is Top (0.8V) - Base (-0.8V).

Download PicoScope 7 here and test the top, base and amplitude automated measurements with our demo oscilloscope! 

*Note that the Math channel waveform may not start from the very start of the graph if it can’t find a whole cycle at the start. Likewise for the end of the graph where the Math channel trace may fall short if it can’t find a full cycle at the end.