Do you want to make sure your oscilloscope measurements are the best they can possibly be? Don’t settle for just an average measurement; simply scaling your signal properly can dramatically improve measurement quality. Why? Because both sample rate and the bits of resolution of your oscilloscope play a part in your scope’s measurements.
Sample rate is affected by the oscilloscope’s horizontal scaling. The equation to remember is:
Sample Rate = Memory Depth/Acquisition Length
Memory depth is a constant value, and the acquisition length (or trace length) is a variable dependent on your time-per-division settings. As the time/division setting increases, the acquisition length increases. Since all of this must fit into the scope’s memory, at a certain point the oscilloscope’s ADC will have to decrease its sample rate. What does this mean practically? Let’s look at a frequency measurement on a 100 kHz square wave. We know the frequency is precisely 100 kHz and is very stable, so we can use the standard deviation of our measurement to judge the quality of the measurement. Figure 1 has our 100 kHz square wave scaled to be viewed over 20 ms of time. And, the scope’s sample rate has been automatically decreased from 5 GSa/s down to 100 MSa/s in order to fit the entire trace into the oscilloscope’s memory. And, the standard deviation of our measurement is 1.49 kHz (about 1.5%) after around 1,500 measurements.
But, look at what happens when we choose a much smaller time/div setting, effectively shortening the acquisition length and increasing the sample rate. Figure 2 has the same signal, but horizontally scaled to 1.2 us/div. The standard deviation is now 1.5 Hz, which is one thousand times smaller than our previous measurement.
All that changed was the horizontal scaling of the signal, and in turn the sample rate of the oscilloscope. So, proper horizontal scaling of your oscilloscope can have a dramatic effect on the quality of your time dependent measurement.
Just as horizontal scaling effects your time dependent measurements, vertical scaling effects your vertically dependent measurements (peak to peak voltage, RMS, etc.). Again, let’s take the same 100 kHz square wave, but instead look at peak to peak voltage. Figure 3 has the signal scaled to 770 mV/div. And, the standard deviation of the peak to peak measurement is 18 mV. By decreasing the V/div settings on the scope to 66 mV/div the measurement’s standard deviation becomes 1.22 mV. This is almost a 15x improvement!
Why does the vertical scaling make a difference? By scaling the signal to fill as much of the screen as possible, we are able to take advantage of the oscilloscope’s full bits of resolution. Bits of resolution is essentially a signifier of how precise an ADC is capable of being. The higher the bits of resolution, the more vertical levels the ADC is able to detect. For example, the image below shows a two bit ADC. The red sine wave is the analog input to the ADC, and the blue waveform is the digitized version. As you can see, there are four different quantization levels possible.
This image shows a three bit ADC digitizing the same analog waveform. By having more quantization levels, the ADC’s digital output is able to more closely approximate the analog input.
When vertically scaling a signal to fill only a portion of the oscilloscope screen, you are not utilizing the ADC’s bits to the full potential. For example, if you scaled a signal to half of the 3 bit ADC’s screen, you would leave two quantization levels unused above your signal and two levels below. This would mean that your three bit ADC would only be able to use four quantization levels, rendering it just as precise as a fully utilized two bit ADC.
Knowing how to properly scale signals on your oscilloscope can make a dramatic difference in the quality of your measurements. Proper horizontal scaling significantly effects your time dependent measurement, and proper vertical scaling effects your vertically dependent measurements. Next time you are in front of your scope, remember: good signal scaling makes great measurements!