Oscilloscope Primer

Oscilloscopes are very useful tools in the electronics lab. There have been many varieties over the years, with a wide range of capabilities. Through the narrative below, we will explore some of the differences, capabilities, and short comings of oscilloscopes.

Basic Oscilloscope Types:

There have been three basic types oscilloscopes available over the years.

1. Analog: Analog oscilloscopes use the phosphor on the screen as memory, to hold the image long enough so that the user can obtain data. This usually means multiple sweeps and/or continuous signal to trigger the scope. This works well enough for continuous signals, but, when trying to view short duration or intermittent glitches it becomes problematic. Another downside is that the faster the sweep, the dimmer the display. Also, it is difficult to save data and share it. With the earlier analog scopes the only way to save was to photograph the screen with a camera.
2. Digital: Digital storage scopes provided answers to the downsides of analog scopes. DSOs allow the displayed image to remain on the screen, and for measurements to be taken automatically, and be stored and shared easily. It wasn’t all roses, early digital scopes suffered from slow DACs, slow processing, and limited memory. This made for poorly displayed data. There were also some early attempts at reducing the knob count on oscilloscopes from many knobs to one knob for all functions and for all channels. Making multiple adjustments was a very frustrating process. On the plus side, the earlier DSOs could save data and setups.
3. Mixed Signal: Mixed Signal oscilloscopes brought in another improvement in oscilloscopes, the ability to see both multiple digital and analog signal at the same time, with the ability to trigger off either, or a combination of both, signals. Some MSOs have the capability of a spectrum analyzer as a third type of input.

Important Characteristics:

Bandwidth: Bandwidth is one of the more import specifications when considering an oscilloscope. It is measured in Hz, and it is defined as the lowest frequency that the signal is attenuated by 3db.

As a rule, when measuring analog signals, you need to have 3x the bandwidth of your highest frequency signal being tested.

Also, when measuring digital signals you need to have 5x the bandwidth of your fastest clock in the system being tested, or more accurately, the fastest edge speeds (rise and fall times).

Memory Depth: Memory depth is another important specification when considering an oscilloscope. This equates to how many data points can be stored at a time. Keep in mind that even scopes with deep memory can be compromised if the time per division setting is too long. This much processing can slow your update rate considerably.

Sample Rate: Sample Rate is defined as the number of samples acquired per second. The sample rate should be, at minimum, twice the bandwidth. The more samples, the more accurate the waveform will be portrayed.

Types of sampling: Real-time sampling captures the complete image with each sweep.

Equivalent-time sampling creates a composite out of multiple sweeps.  This may be necessary for high frequency work, but it can introduce inaccuracies.

Update Rate: Update Rate is defined as the speed of acquiring and displaying data. No matter how fast this occurs, there is always down time where data can be missed. The faster the update rate the less down time and the less data that will be not be displayed. This is very important for catching infrequent events.

Triggering:

The trigger control makes it possible to capture the portion of the signal you want and stabilize the display at the same time. You have a lot of choices when it comes to triggering, here are a couple of examples.

Edge Triggering: When the voltage reaches a preset level, the scope will trigger. You can set the scope to trigger on ether the rising or falling side of the signal.

Glitch Triggering: When the length of an event reaches a preset width, the scope will trigger. Useful when looking for intermittent or infrequent events.

Probes:

The oscilloscope is part of a system that includes your probes. To get the most out of a scope it is very important to match your probe and scope’s capabilities carefully. When choosing a probe, pick one with at least 5x the band width of your signal, 10x the resistance of the circuit to be measured, and keep your ground lead as short as possible. Be sure to choose a probe with a voltage rating sufficient for your setup. Most name brand probes are good too 300v.

Types of probes:

Passive probes do not contain any active components and are good for frequencies up to 500MHz.

Active probes contain active components and are good for frequencies up to 1.5GHz. They have very high impedance.

Tips:

• Fourier transforms mathematically produce the frequency components of your wave form. If the waveform is distorted, you will see unexpected harmonics. Also, if you suspect your scope is at its limits, then you may see increased noise or harmonics.
• Check your probe grounds, keep them short.
• Keep the voltage at the probe below its maximum rating.
• Be sure your oscilloscope’s cord ground is in good condition. I have seen many cords over the years with broken ground prongs connections.
• If your device under test is connected to a bench power supply, then be sure the appropriate line to the power supply is grounded. Most supplies come with both the negative and positive terminals floating, neither connected to ground.

References:

• Basic Oscilloscope fundamentals (Keysight) 5989-8064.
• 5 common Mistakes to Avoid when Purchasing a low-cost Oscilloscope (Keysight) 5992-2115.
• XYZs of Oscilloscopes (Tektronix) 03W-8605-7.
• Can your Oscilloscope Capture Elusive Events (Keysight) 5989-7885.
• Evaluating Oscilloscope Bandwidth for your application (Keysight) 5989-5733.
• Evaluating Oscilloscope Sample Rates versus Sampling fidelity (Keysight) 5989-5732.
• Using an Oscilloscope with Time gated Fast Fourier Transforms for time correlated Mixed Domain analysis (Keysight) 5992-0244.
• FFT and pulsed RF measurements with 3000 (Keysight) 5992-1094.
• Measuring with Oscilloscopes, Educational Note (Rohde & Schwarz) 1MA265-2e.
• Oscilloscope, Why Deep Memory Matters (Rhode & Schwarz) 3607-7992-92.