Explore how important an oscilloscope is in your work?

With the continuous development of science and technology, I just learned about oscilloscopes, and now there is a measurement artifact, the mixed domain oscilloscope. It seems that I really can't stop my love of learning! Then join the audiophiles and immerse yourself in the world of mixed domain oscilloscopes! Let's learn more together...
 
What is MDO?
 
Before describing its internal technology, it is best to understand what an MDO is made of.
 
In the past, three different instruments were required to make analog, digital, and RF measurements:
● Oscilloscope, used to make time-correlated measurements on analog signals in the time domain
● Logic analyzer, used to make time-correlated measurements on digital signals in the time domain. A mixed signal oscilloscope (MSO) is an oscilloscope with added digital channels
● Spectrum analyzer, used to measure RF signals in the frequency domain
A mixed domain oscilloscope (MDO) is the first tool to integrate a mixed signal oscilloscope (including logic and protocol analysis functions) with a modern spectrum analyzer.
 
They are optimized for measuring analog, digital, and RF signals with a full set of input channels:
● 2 or 4 analog time domain channels with 100MHz, 200MHz, 350MHz, 500MHz, or 1GHz bandwidth, with serial bus decoding and triggering
● 16 digital time domain channels with timing resolution down to 60.6ps, with serial bus decoding and triggering
● 1 spectrum analyzer channel with up to 6GHz input frequency range
 
How do MDOs work?
 
To provide the above capabilities, especially RF measurement performance, MDOs use a unique architecture that may not be familiar to users of traditional spectrum analyzers or oscilloscopes.
 
The figure below is a simplified block diagram of a traditional swept spectrum analyzer. The traditional architecture is the swept superheterodyne spectrum analyzer (SA), which is what enabled engineers to make frequency domain measurements decades ago. The
 
current generation of spectrum analyzers includes many digital elements, such as ADCs, DSPs, and microprocessors. But the basic swept frequency method remains largely the same.
 
The SA performs power-versus-frequency measurements by down-converting the signal of interest and then sweeping the transmission band through the resolution bandwidth (RBW) filter. The RBW filter is followed by a detector that calculates the amplitude at each frequency point in the selected bandwidth.
 
The advantage of this method is that it provides a high dynamic range, but the disadvantage is that only data for one frequency point can be calculated at a time. As a result, the measurement data is only valid for relatively stable, non-changing narrowband input signals.
 
The figure shows the structure of a vector signal analyzer (VSA). The VSA represents a more modern spectrum analyzer structure in which the local oscillator is stepped rather than swept.
 

 
The resulting signal is filtered and then converted to digital. This results in a band-limited time-domain signal that can be converted from the time domain to the frequency domain using a DFT (discrete Fourier transform). The resulting frequency-domain information is then used to plot a small portion of the spectrum on the display around the LO frequency.
 
The LO is then stepped to the next higher frequency, and the process is repeated until the entire spectrum is plotted. Stepped analyzers are superior to swept analyzers when dealing with time-varying RF, but only if the bandwidth of interest is within the step width, which is usually quite narrow (10MHz to 25MHz).
 

 
The following is a simplified block diagram of an MDO. The highlighted blocks are only available in the MDO4000 series, while the others are How does an MDO work?
 
The RBW filter is followed by a detector that calculates the amplitude at each frequency point in the selected bandwidth. The advantage of this approach is that it provides a high dynamic range, but the disadvantage is that data can only be calculated for one frequency point at a time.
 
As a result, the measurement data is only valid for relatively stable, non-changing narrowband input signals. The following is the architecture of a vector signal analyzer (VSA). The VSA represents a more modern spectrum analyzer architecture in which the local oscillator is stepped rather than swept. The
 
resulting signal is filtered and then converted to digital. This results in a band-limited time domain signal that can be converted from the time domain to the frequency domain using a DFT (discrete Fourier transform). The resulting frequency domain information is then used to plot a small portion of the spectrum on the display around the local oscillator frequency.
 
The local oscillator is then stepped to the next higher frequency, and the process is repeated until the entire spectrum is plotted. When dealing with time-varying RF, stepped analyzers are superior to swept analyzers, but only if the frequency span of interest is within the step width, which is usually quite narrow (10MHz to 25MHz).
 

 
The following is a simplified block diagram of an MDO. The highlighted blocks are only found in the MDO4000 series, while the others are common to both the MDO4000 and MDO3000 series. Modern vector signal analyzers use essentially the same architecture. The
 
main differences between an MDO and a regular VSA are that an MDO has:
● A much higher ADC sampling rate, thus enabling an ultra-wide capture bandwidth
● A smaller number of fixed downconversion ranges
 
At the heart of an MDO is the same Tektronix-developed analog-to-digital converter used in most Tektronix oscilloscopes. This 8-bit analog-to-digital converter samples at 10 GS/s and has an input bandwidth of over 5 GHz.
 
In all MDOs, jitter is added to the signal to improve SFDR. After the data is acquired into memory, digital downconversion (DDC) is performed using a combination of hardware and software techniques to greatly enhance signal fidelity.
 
This process accomplishes three things:
● The data record is converted to an I (in-phase) and Q (quadrature) composite data format
● The center frequency is shifted to DC, which reduces the IQ sample rate to half the rate
● The data is filtered and compressed to a sample rate sufficient to cover the bandwidth
 
The digital signal processor in the MDO performs an FFT to convert the RF time domain data to frequency domain data in the form of a spectrum. The entire spectrum is multiplied by calibration factors to adjust for flatness and phase. A
 
user-selectable detection method is then used to determine how to compress the 1000-2,000,000 point FFT output to a 1,000 pixel wide display. Positive peak, negative peak, average, and sample detectors are provided.
 
Finally, the resulting spectrum is logarithmically scaled and displayed.
 
Summary
 
Both the MDO3000 and MDO4000 Series offer the convenience of implementing the functionality of multiple instruments in one integrated platform.
 
The MDO4000 Series is ideal for EMI debugging and integrating wireless transceivers by displaying both the time and frequency domains in one synchronized view.
 
While using oscilloscope technology to build a high-fidelity spectrum analyzer is a significant departure from tradition, there are many techniques that can be used to effectively achieve this goal, including:
 
● Improving fidelity using dedicated spectrum analyzer inputs
● Using digital downconverters and DFT techniques to exploit process gain to improve sensitivity
● Using jitter to improve SFDR
● Improving shielding
 
By using these techniques, the MDO achieves the fidelity required to make spectrum measurements while retaining the advantages provided by an integrated oscilloscope, including:
 
● Wide capture bandwidth of at least 1 GHz at any center frequency, and up to 3.75 GHz in some cases
● Cost and space advantages, sharing a single chassis, display, interfaces, power supply, etc.
● Convenience of an integrated instrument
● On the MDO4000 Series
    - Time correlation of analog and digital channels
    - Ability to view RF time domain data outside of the typical zero span provided by a spectrum analyzer