Trend Plots are plots of measured parameter values displayed in the order they were taken. The time between measurements can be set using the internal trigger hold off timer in the trigger setup. An example of just such a setup is shown in figure 1. The internal temperature of an oscillator is measured with a thermal probe with a sensitivity of 39 μV / °C. Simultaneously, the average frequency, taken over a single cycle, is acquired. The 100 measurements in each trend are acquired over 100 acquisitions. The time between acquisitions is set to 10 seconds using trigger hold off.

Figure 1 shows typical acquisition of the oscillator output in the top trace (Channel 1). The temperature is read using Channel 2. Note that bandwidth limiting and the noise filter have been applied to channel 2 to reduce high frequency pickup from the thermocouple as we are only interested in the DC component.

Figure 1:

The trend plots of internal temperature (trace A) and oscillator output frequency (trace B) taken over 1000 seconds show its thermal response characteristics

The frequency of the oscillator output is read using parameter P1.

Parameter gates are used to select the single cycle at the trigger point. This is done so that the temperature and frequency measurements are synchronous.

Similarly the mean value of the temperature is read in parameter P2. This readout is in millivolts. Parameter math is used to rescale the data by dividing the voltage reading of the thermocouple by its sensitivity. The same operation adds an offset (cold reference) to provide an absolute scale to the temperature reading. The units are also changed to read out in C.

The trend of the frequency measurement is shown in trace F1 and the trend of the temperature is read in trace F2. Both of these trends are also filtered, using enhanced resolution, to display the mean values of frequency and temperature on an acquisition by acquisition basis.

The trend plot maps the measured parameter value versus the measurement number. That is the measurements, one per acquisition, are inserted into the trend in the order they were captured. The samples in the trend plots are values of the respective parameters taken at ten second interval set by the trigger holdoff.

Once the trend data has been acquired cross plots, using X-Y display mode, can verify functional relationships between the trended parameters as shown in Figure 2.

Figure 2:

A cross plot of frequency as a function of oscillator temperature showing the functional relationship

Trend plots provide a simple means of datalogging parameters. The number of parameters you can trend is limited by the number of math traces available in your oscilloscope. This plot, like any waveform, can be measured and applied to other math operations giving you a full range of analysis capabilities.