Fundamentals of Using a Tektronix Oscilloscope

Take the Tektronix TDS2000 series oscilloscope as an example

#1 Digital Storage Oscilloscope

Glossary

1. Digital is relative to analog, converting continuous analog signals into discrete digital signals

2. Storage refers to the ability to store data collected by AD (there is a concept called storage depth)

3. The fundamental purpose of an oscilloscope is to display waveforms

#2 Rationale

The waveform first passes through the probe, is amplified by the front-end amplifier, then converted by the analog-to-digital conversion unit, stored in the acquisition memory, and then displayed on the monitor.

The parameter settings mentioned below are the parameters that need to be adjusted in each setting link to achieve the purpose of correctly matching the measured waveform.

The speed of sampling conversion and high-speed storage and processing of collected data are the core technologies of the oscilloscope.

#3 Basic Usage

The purpose of using an oscilloscope is to observe waveforms. Before using it, you need to understand the performance parameters of the oscilloscope, and in the process of observing waveforms, you need to set the parameters of the oscilloscope. So how to set the parameters? What are the performance parameters of an oscilloscope?

##Oscilloscope performance parameters

The following parameters are related to the performance of the oscilloscope. The higher the parameters, the better the performance, and of course the more expensive the price.

1. Bandwidth

Bandwidth refers to the cutoff frequency when the signal amplitude is reduced to 0.707 times of the original value (power drops by half), which means the gain is reduced by -3dB.

Calculation: -3dB=10*log(1/2).

The larger the bandwidth, the higher the frequency of the signal that can be measured, otherwise the high-frequency signal will be filtered out.

2. Sampling rate

Sampling rate refers to the number of times AD samples in 1 second. According to the sampling theorem, as long as the sampling rate is higher than 2 times the signal frequency, the sampled signal can be restored. Of course, in order to be more accurate, the general sampling rate = (5~10) * signal frequency.

3. Storage Depth

The record length is also called the storage depth. After AD acquisition, all digital oscilloscopes will store the triggered signals in a storage space. The size of this storage space is the storage depth, and **storage depth = current time base * 10 grids * sampling rate.

4. Number of channels

That is, the number of signal paths that can be measured at the same time, and different channels can be compared and analyzed.

TDS2000 parameters:

Bandwidth 200MHz

Sampling rate 2Gs/s

Storage depth 2.4K

Number of channels 2*

Capture serial port waveform

Taking capturing the serial port waveform as an example, briefly describe the parameters that need to be set

1. Serial port parameters

Baud rate 115200, 8 data bits, 1 stop bit

2. Channel 1

Use the oscilloscope's channel 1 probe + to connect the serial port's TX, and probe - to connect GND

3. Time base selection

The time base is selected to be about 10 times the period of the waveform, which is convenient for observing the waveform. The baud rate is 115200, and the period is about 17.36us. Here, the time base position of 250us is selected.

4. Channel parameters

The input coupling mode in the channel parameters needs attention, which is divided into AC/DC/GND coupling

The DC mode is marked as DC, the DC part of the signal will be processed and displayed, and the corresponding display waveform is the full state of the signal;

The AC mode is marked as AC, the DC part of the signal will not be displayed, and the corresponding displayed waveform is the AC part;

The grounding mark is GND, which actually disconnects the input and grounds it. The purpose is to eliminate interference and make it easier to find the zero point.

5. Trigger parameters

The selection of trigger parameters is critical:

Trigger source: Select channel 1, CH1

Trigger mode: There are rising edge/falling edge. Here, we choose to output high level in idle state, so we choose falling edge trigger

Trigger mode: AUTO/NORMAL/SINGLE, that is, automatic, normal, single sequence

Since we need to capture a single waveform for analysis, we can choose SINGLE or NORMAL mode. The difference is that after SINGLE is triggered once, it needs to be manually set before the next trigger. Here we choose SINGLE mode.

6. Serial port sends data

The serial port sends data, and you can see the TX waveform perfectly captured on the oscilloscope:

Adjust the time base to 10us and adjust the Position horizontal knob at the same time.

The measurable period of the waveform is 17.39us, which is almost consistent with the theoretical value of 17.36us.

#4 Problems and Prospects

1.Why is the sampled waveform distorted when the time base is large?

According to the storage depth = current time base * 10 grids * sampling rate, enter the parameters for calculation:

Select 100ms as the time base:

2.4K=10*100ms*sampling rate. The sampling rate is: 2.4Ks/s

Select 100us as the time base:

2.4K=10*100us*sampling rate. The sampling rate is: 2.4Ms/s

Obviously, according to the sampling theorem, for the 67.5K frequency signal of the serial port 115200bps, the sampling rate of 2.4Ks/s cannot restore the waveform, resulting in waveform distortion.

2. The sampling speed of ADC is as high as 2Gs/s (high-end oscilloscopes can reach 100Gs/s). How can data be sampled, stored and processed at high speed?

This is considered the core technology of high-end oscilloscopes. First, it involves the production of high-speed ADC chips, and then the storage and processing of high-speed data (serial or parallel? How to increase the speed of serial? How to calibrate/align data in parallel?). This is still an area where China has not been able to make breakthroughs. The high-end ADC chips are under an embargo, and the production of high-precision instruments is hindered. The road to independent research and development is arduous and long!