Understanding Oscilloscope specifications - Part 1

A very important test and measurement equipment in the hands of electronics engineer is Oscilloscope. An oscilloscope gives a real time visual inspection of the signals on board. In other words, it captures and graphically represents an electrical signal on display. Choosing an oscilloscope for a specific application is always challenging. There are several versions of oscilloscopes in market and to name some of them are analog oscilloscope, Digital storage oscilloscope, hand held oscilloscope, PC based oscilloscope, etc.

You may not be able to use oscilloscope in every case. For example, let us assume you want to measure a RF signal of frequency 20GHz. Then oscilloscope doesn’t fit your need. You might need a spectrum analyzer for your measurements. Spectrum analyzer does measurements in frequency domain (Amplitude vs. Frequency) where you get a frequency spectrum of the signals. Oscilloscope does measurements in time domain (Amplitude vs. Time). The main reason for oscilloscopes not having that range for measurements is the sampling rate. For 20GHz, sampling rate must be too high which can’t be achieved with scope.

The main criteria on which we select an oscilloscope are:

• Analog bandwidth
• Sample rate
• Memory depth
• Resolution
• Triggering capability
• Channel count
• Cost
• Reliability
• Accuracy

Analog bandwidth: Analog bandwidth applies to all types of scopes. Every scope has a front-end amplifier and maximum frequency that can be passed through this amplifier determines the scope band-width. I other words, analog bandwidth determines the maximum signal frequency that can be measured with a given scope. For example, if a scope says a bandwidth of 50MHz, signals up to 50MHz can be measured. So, as you go higher in frequencies to measure signals like USB, PCIe, SATA, etc, you need more bandwidth.

Generally, if the measured frequency is of ‘x’, scope bandwidth preferred is 5 times the frequency to be measured. This is for correct reproduction of the signal. So, one might have a question regarding the maximum frequency a scope can measure. This maximum bandwidth is of the same terminology we use in filters, a 3-db bandwidth. The maximum bandwidth is the point at which the signal input diminishes by 3-dB. So, a signal is shown diminished in voltage if it goes beyond this frequency.

As Analog bandwidth rating increases, the scope gets costly. So, while choosing an oscilloscope, study your requirements and come up with a optimal decision.

The main challenge of the scope designers is to maintain perfect signal characteristics (undershoot, overshoot, ringing) as it passes through various input stages of the scope. In other words, maintaining signal fidelity is very important.

Sample Rate: One of the important criteria for DSO is sample rate. In accordance to Nyquist rate, for good reproduction of the signal, the sampling must be greater than twice the signal frequency. But for good reproduction, in scopes you need minimum of 10 samples for good reproduction of the signal. Sampling in a scope can be in Mega samples per Second (MS/s) or Giga samples per second (GS/s). The more the sampling rate, good the measurement.

Let us assume we are measuring a 20MHz signal with an oscilloscope of 1GS/s sampling rate. Then, the scope samples the given input signal at 50 times in a given cycle.

Memory Depth: The samples captured are stored in an internal buffer before signal reproduction. The amount of memory is indicated by a factor called memory depth. Memory depth is an important specification which affects the performance of oscilloscope. Memory depth and sample rate are inter related. Let us assume you have a scope with good sampling rate but with less memory depth, then you may not be able to utilize the maximum sampling rate of the scope. So, while purchasing a scope, checking the memory depth is as important as checking the sampling rate.

Let us assume a scope with 5K buffer size (memory depth) and 1GS/s sampling rate measuring a signal of 50us. Then the scope can sample at 5K/50us = 100MS/s which is well below capability of scope sampling.

Triggering capability: The triggering capability mainly determines the oscilloscopes capability to measure one shot signals. In a real time scenario let us assume a signal expected to be of max. 1V amplitude is having some distortion at a specific point causing the signal to go above 1V. In such scenario, the point at which it goes above 1V can be captured using triggering functionality. 

What are the advantages of Digital storage oscilloscopes (DSO) when compared to age-old analog oscilloscopes?

• Storage facility
• Remote connectivity (Ethernet)
• High band-width
• Smaller in size
• Display measurements on screen
• Single shot as well as repeated signals can be measured

How to select a scope if I am measuring signals in milli volts and micro volts range and signals of high voltage level?

To measure low voltage ranges consistently, the scope must have high resolution ADC. Generally, ADC will be in the range 12 to 16-bit for a given scope. If signal to be measured are of high voltage, use attenuating scope probes. When measuring high voltages, use 10:1 attenuation factor. This helps scope protect against accidental high voltages. Also, 10:1 probe setting minimizes the overload condition.

How to determine a matching probe for a given oscilloscope?

You cannot use every possible probe with a given scope. The scope vendor generally gives the matching probe specifications for a given scope model. The probe capacitance mainly determines the usage. The probes used should match with the band-width of the scope.

How to select the oscilloscope in terms of rise time requirements?

The oscilloscope rise time must be one-fifth of the fastest rise time of the signal to be measured. If this is not met, a distorted waveform appears on scope which in real time may be perfect.

What probes need to be chosen for measuring high frequency signals?

The capacitance of the probe determines the frequency at which probe can be used. As the measured frequency increases, the normal passive probes may not be suitable. For measurement of high frequency signals, it is preferable to use active probes. Active probes have a FET amplifier inside. These active probe have a good matching capability.

Who are the vendors of oscilloscopes in the market?

• Agilent technologies
• Tektronix
• Lecroy
• Fluke
• Gwinstek
• Oscium
• Hameg
• BKPrecision
• Promax
• EZ Digital