- Sampling Oscilloscope Definition: A sampling oscilloscope is defined as an advanced type of digital oscilloscope designed to sample high-frequency waveforms by capturing multiple data points.
- Functioning of Sampling Oscilloscope: It operates by collecting samples from successive waveforms and reconstructing the complete waveform for display, useful for observing fast electrical signals.
- Sampling Methods: There are two primary sampling methods: real-time sampling, which captures transient events, and equivalent sampling, which works with repetitive waveforms.
- Real-Time Sample Method: This method captures high-frequency transient events in one sweep, requiring high-speed memory to store the data.
- Equivalent Sample Method: This method relies on repetitive waveforms, using either random or sequential sampling to improve accuracy in signal capture.
Before discussing sampling oscilloscope, we must know the basic principle and functioning of an ordinary oscilloscope. It is an instrument which receives one or more electrical signals and then produces the waveform on the screen simultaneously. The sampling oscilloscope is an advanced version of the digital oscilloscope with some added features and uses for the special purpose.
It is designed to handle very high-frequency functions by sampling multiple waveforms successively. The sampling oscilloscope uses the sampling theorem to construct a waveform from several input signals. Like a strobe light capturing fast motion, it collects many images to show fast electrical signals. About 1000 points are needed to create a waveform.
Functioning of Sampling Oscilloscope
As its name suggests, it collects samples from successive waveforms to construct a complete picture from the data. The resulting waveform is amplified with a low band pass filter and displayed on the screen. This waveform is made by connecting many dots to form the complete shape.
Each dot of the wave is the vertical deflection of the point of the progressive layer in each successive cycle of a staircase waveform. They are used to monitor high-frequency signals up to 50 GHz or more. The displayed waveform’s frequency is higher than the scope’s sample rate, showing about 10 pieces per division or more. It has a large amplifier bandwidth of about 15 GHz. During sampling, signals have low frequency, and to achieve a large bandwidth, they combine with an attenuator.
However, this reduces the instrument’s dynamic range. Sampling oscilloscopes are limited to repetitive signals and do not respond to transient events. They only display high frequencies within their range limit.
Sampling Method
Before each sampling cycle, the trigger pulse activates an oscillator and liner voltage is generated. When the amplitude of two voltages is equal, the staircase move one step and a sampling pulse is generated and it opens the sampling gate for a sample of the input voltage. The resolution of the waveform depends upon the dimension of the steps of the staircase generator. There are different ways of sample taking but two are commonly used. One is real-time sample and other is equivalent sample method.
Real Time Sample Method
In real-time method digitizer works at high-speed so it can register maximum points in one sweep. It’s main purpose to capture high-frequency transient events with accuracy. The transient waveform is so unique that its voltage or current level at any instant of time cannot be associated with its nearest ones. These events do not repeat themselves, so it must be registered in the same time frame as they occur. The frequency of samples is very high about 500 MHz and sample rate is about 100 samples per second. To store such a high-frequency waveform, a high-speed memory is required.
Equivalent Sample Method
Equivalent sampling relies on predicting and estimating, which works with repetitive waveforms. The digitizer collects samples from many signal repetitions, taking one or more samples per repetition. This increases the accuracy of signal capture. The resultant waveform’s frequency is much higher than the scope’s sample rate. This sampling can be done by two methods: random and sequential.
Random Method of Sampling
Random method of sampling is the most common method of sampling. It uses an internal clock which adjusted in such a way that it runs with respect to input signals and the signal trigger samples are taken continuously, no matter where it was triggered. Samples those are collected are regular with respect to time but random with respect to trigger.
Sequential Method of Sampling
In this technique, samples are taken with respect to triggered and it is independent of time setting. Whenever the trigger is detected, the sample is recorded with a small delay. Make sure that the delay should be very short but well defined. When next trigger occurred, it gets registered with a little incremental time delay with respect to previous one. The delayed sweep can have the range from few microsecond too few seconds. Let us suppose delay for the first time is ‘t’ then the delay for the second time will be little more than ‘t’ and in this manner samples are being taken many times with added delay until the time window is filled.
