What is a Sampling Oscilloscope & Its Working

Before going to discuss a sampling oscilloscope, one should know the main working principle of a normal oscilloscope. An oscilloscope is one kind of device which uses one or several electrical signals and generates the signal on the display simultaneously. The best example of an oscilloscope is a CRO which is known as a cathode ray oscilloscope. Generally, a CRO (cathode ray oscilloscope) doesn’t provide a broad bandwidth of operation. Therefore, the direct tracing for extremely high-frequency signals may not be achievable because the brightness of the output image will decrease on the display at a high frequency of operation. To overcome this, we need a technique to trace high-frequency signals properly namely the sampling technique. The sampling oscilloscope is a device that applies the sampling technique to trace the waveform. So, an advanced version of the digital oscilloscope namely sampling oscilloscope is used with some additional features.

What is a Sampling Oscilloscope?

The device which is used to generate a signal by gathering several samples of an electrical signal is known as a sampling oscilloscope. This is an advancement for digital oscilloscopes including some extra features for special purposes. This device uses the principle of the Stroboscopic light method for analyzing speedy electrical signals.

In this oscilloscope, different samples are taken from different parts of the signals over uninterrupted cycles, so that the overall image will be displayed on display like a continuous signal. Here, a signal can be formed with thousand points and it is to be noted that the resultant signal can be amplified using a low bandwidth amplifier before getting illustrated on the display.

The main purpose of this sampling oscilloscope is to notice the high-frequency waveform that lies in the 50 GHz range. A signal with high-frequency can be achieved at the oscilloscope’s output when evaluated with the sample rate of the slope.

Sampling Method for Signal Tracing

In the sampling method, the number of dots can make a complete signal by combining them. In this, every dot comes from one consecutive cycle of the signal individually, and then the next consecutive dot comes from the next little part of the consecutive cycle of the signal.

Sampling Oscilloscope Block Diagram and Explanation

The block diagram of a sampling oscilloscope is shown below.

From the above figure, we can notice that the input signal is applied to the sampling gate. Once the sampling signal is given to this sampling gate, then it will open to sample the input signal. It is important that sampling can be done in synchronization through the frequency of the given input signal.
When the vertical amplifier is used in the circuit, then it holdups the input signal & once the amplification of the signal is done then the signal can be given to the vertical plates.

Once the sampling cycle starts, the oscillator will be switched ON through the trigger signals, so that linear ramp output voltage can be generated. The signal which is generated from the ramp generator can be given to the voltage comparator unit.

Here, the ramp signal is evaluated through the staircase signal, which is produced from the staircase generator. While evaluating, once the two signal’s amplitude is equal, then it improves the staircase through a single step then a sampling pulse can be generated. Again, this will open the sampling gate & the cycle can be repeated similarly.

The steps dimension which is generated through the staircase generator decides the image resolution at the output. Once the steps dimension is lesser, then the number of samples will be larger. Thus, the image resolution will be higher.

The following image shows the waveforms for different blocks of the sampling oscilloscope. In the sampling oscilloscope, the frequency samples can be less as one-hundredth of the i/p signal frequency. Therefore, the input signal frequency of 1 GHz simply 10 MHz amplifier BW is required.

Sampling Method

The trigger pulse triggers an oscillator before every sampling cycle then generates liner voltage. Once the two voltages amplitude is equivalent, then the staircase will move single step & a sampling signal can be generated to open the sampling gate intended for a sample of the input voltage. The signal resolution mainly depends upon the size of the steps for the staircase generator. There are different methods in sample taking however two methods are normally used like real-time sample & equivalent sample technique.

Real-Time Sample Method

In this technique, a digitizer performs at high-speed to list the highest points within a single sweep. The main function of this is to capture transient events accurately which have high-frequency. The transient pulse is so unique so the level of current otherwise voltage at any instant of time cannot be connected through its adjacent ones.

These actions do not do again themselves, thus they should be listed within an equal time frame while they take place. The sampling frequency can be extremely high like 500 MHz & the sample rate can be 100 samples/sec. So a high-speed memory is necessary to store a high-frequency signal.

Equivalent Sample Method

This technique performs on the prophecy as well as estimation principle which is achievable simply through the cyclic signal. In this technique, a digitizer gets samples from several repetitions of waves. So, it may use single otherwise additional samples from every repetition. The resultant signal’s frequency is higher as compared to the scope sample rate. So, this kind of sampling can be done through two techniques like Random & Sequential.

Random Method

This technique is the most common one for sampling. This technique mainly uses a clock inside and it can be adjusted to work with input signals & the trigger samples from the signal are taken constantly to trigger. The collected samples are standard to time however random to activate. In this method, samples are recorded at the usual time interval samples however that is independent of the triggering rate.

Sequential Method

In this method, samples are used to trigger. Once the trigger is noticed, the sample can be recorded through a tiny delay. Check the delay because that should be extremely short however well defined. Once the subsequent trigger happens, then it gets listed through a small incremental time delay as compared to the previous one.

The range of delayed sweep is from microseconds to seconds. For instance, if the delay is ‘t’ for the first time, after that the delay for the next time will be high than ‘t’. In this way, samples are used several times with additional delay until the filling of the time window is done.

The delayed sweep can be defined as the technique that is used to add a time among the points of starting & the triggering points of the scope sweep. It enhances the flexibility of the oscilloscope device. The signal which is not delayed can be amplified through the delayed sweep oscilloscope. In other applications, it is most frequently used to measure the signal’s rise time otherwise to measure the modulation of pulse time.

Advantages

The advantages of sampling oscilloscope include the following.

• It is used to measure the extremely high-speed signals using a device that has less bandwidth
• The sampling technique is very useful in changing the input signal immediately into a signal within a less-frequency field.
• It can respond as well as store the data in rapid bit form.

Disadvantages

The disadvantages of sampling oscilloscopes include the following.

• The main drawback of this oscilloscope is that it can simply measure the continuous wave.
• The frequency range of the sampling oscilloscope device mainly depends on its design.

Applications

The applications of sampling oscilloscopes include the following.

• The sampling oscilloscope is used to observe a high-speed signal on the display by using different electrical signals.
• Used to record the signals through a recorder
• The signals can be measured accurately
• This oscilloscope is not only used for high-frequency signals (hundred MHz) but also used for low frequency (few kHz) signals. It is feasible to calculate each element of a signal to 200 µs after a set-off point and the signal repetition rate can be provided like 10 Hz or above.

Thus, this is all about an overview of sampling oscilloscope and it is a modified oscilloscope because it avoids the difficulties of BW requirements. These instruments are mainly to display signals which have above 300 MHz frequency. So sampling technique is mainly used to attain good signal display. This display is made up of 1000 dots. For each dot, the vertical deflection can be acquired from the next points increasingly in every consecutive cycle of input. The signals horizontal deflection can be attained by supplying the staircase signal toward the horizontal deflecting plates. Here is a question for you, what are the different kinds of oscilloscopes?