Zoom is a display tool that expands the view of the selected waveform. The source trace can be expanded horizontally and vertically for detailed visual analysis or further processing. Each zoom trace can have its own horizontal and vertical scale setting, enabling views of the source trace using multiple horizontal and vertical scales. All digital oscilloscopes offer zoom functionality.
Zoom is important because oscilloscopes can acquire gigasamples of data per acquisition, with a vertical resolution of 12 or more bits. This data must be displayed on a screen with a resolution of approximately 1920 x 1080 pixels. If a full acquisition is displayed, the data has to be compacted to fit on the screen. Expanding the data with a zoom trace so that it fits within the screen resolution allows a view of all the acquired data.
Zoom demo
Zoom can be invoked from this oscilloscope’s front panel using the Zoom button. It can also be evoked interactively by touching the touchscreen and dragging the resulting box over the area to be expanded. Zoom traces can also be controlled from the Zoom Trace dialog boxes (Figure 1).
Figure 1 An example of several zoom instances used to analyze a remote keyless entry system waveform. The Zoom dialog box is used to control each zoom trace. Source: Art Pini
The source waveform from a remote keyless entry (RKE) system appears as trace M1 in the top grid. The waveform comprises an amplitude-modulated RF carrier. The modulation encodes the commands to lock a car door. Zoom is used to expand the fifth pulse in the acquired waveform horizontally. Note that the zoomed area is highlighted by increased intensity on the source trace. The expanded version appears in trace Z1 (second down from the top). The Z1 trace is controlled using the Z1 zoom dialog box at the bottom of the display.
The trace’s horizontal and vertical scale and offset can be adjusted interactively while observing the effects on the screen. The trace annotation box for the Z1 trace shows the vertical and horizontal scaling for the zoom trace. Trace Z1 has a horizontal scale of 150 microseconds per division, compared to the 5 milliseconds per division scale of the M1 source trace, representing an expansion of thirty-three times.
The zoom trace reveals variations in the RF carrier amplitude at the start and end of the burst. These keying transitions affect the generation of spurious signals that can interfere with other RF services. The zoom trace Z2 expands the view of the trailing edge of the first zoom trace and displays it in detail in the third grid from the top. Here, we have an example of Zoom on Zoom.
The analysis continues by demodulating the signal in Z2 by low-pass filtering the absolute value of the waveform. The demodulated signal can be measured to obtain the signal amplitude’s slew rate and the decaying amplitude’s time constant. This is an example of a math operation on Zoom. The math trace F1 performs demodulation; the result is displayed in the bottom grid. This example used two zooms, each with a different horizontal scale.
Horizontal and vertical scale factors
Zoom, in the oscilloscope used for this article, can be applied to any waveform, acquired signals, math, memory, or even other zoom traces. Zoom traces are waveforms like any other. They can be expanded further using another zoom trace, allowing the same signal to be viewed with multiple horizontal or vertical scale factors.
Math operators can be applied, allowing arithmetic, filtering, or FFTs to be performed on them. The number of available zoom traces generally matches the number of acquisition traces; however, all non-acquisition traces, like math or memory traces, have zoom functionality in this family of oscilloscopes.
Figure 2 provides an example of zoom being used to expand a signal vertically.
Figure 2 The echo in an ultrasonic range finder signal is expanded vertically to see the details of a double signal return. Source: Art Pini
A double echo in an ultrasonic range finder is zoomed vertically to see the detail of the waveform that is not easily discerned on the acquired waveform. The vertical resolution of this waveform is twelve bits or 4096 levels. At least a four-to-one vertical zoom is required to render the full resolution on a display with 1080-pixel vertical resolution. A ten-to-one vertical expansion shows the echo at 5 mV per division, providing a detailed view of the waveform structure.
Multi-Zoom
Some applications use multiple zoom traces with the same expansion factor for comparison purposes. Consider the measurement of an I2C data signal and clock signals shown in Figure 3.
Figure 3 Using time-locked multi-zoom to verify the timing between an I2C data and its associated clock signal. Source: Art Pini
The signal in the top grid is an I2C data signal. The grid immediately below that is the associated I2C clock. These waveforms are expanded synchronously using a feature called multi-zoom. Multi-zoom locks the selected zoom traces together. This feature allows common horizontal control of all zoom traces. They can be expanded or contracted synchronously, locked in time, or offset by a user-defined time offset.
In the example, the zoom traces Z1 and Z2 are the expansions of the data and clock signal, respectively. They are locked in time with no offset. The expanded view makes it easier to see the relative timing of the signals. So, the start condition, where the data signal is forced to a low state, followed by the clock signal being forced low, is easy to discern. The zoom traces incorporate the address field of the I2C packet. The expanded view afforded by the zoom displays is useful in evaluating physical layer issues like signal levels, period, with, transition times, and timing.
The multi-zoom feature also includes an auto-scroll mode to automatically scan through the entire waveform at a user-set rate (Figure 4).
Figure 4 The zoom auto-scroll controls allow automatic scrolling of the zoom horizontal location of the zoom trace to scan through long records. Source: Art Pini
Automatic scrolling is very helpful when moving narrow zoom windows through very long acquisitions that might require an extreme number of turns of a knob. It offers two scan rates and the ability to jump to the extreme values.
Comparing waveform segments
Zoom displays can help compare waveforms. For instance, an acquired I2C data signal contains multiple data packets; Zoom can be used to display these packets on the same expanded timescale for comparison (Figure 5).
Figure 5 Using zoom traces to separate and compare I2C data packets on the same expanded time scale. Source: Art Pini
Packets 1, 2, and 4 from the acquired I2C data bus acquisition are separated and compared using three zoom traces with the same scale factors but with different offsets. It is easy to see the difference in the length of packet 2; the data content of the three packets differs in the last half millisecond of the waveforms.
Using Zoom to window signals
Zoom can select, or window, specific regions of an acquired signal for further processing. This allows the examination of selected parts of a signal separately. Consider analyzing an RKE system that uses frequency shift keying (FSK) to encode commands (Figure 6).
Figure 6 Using zoom traces to isolate the one and zero state frequencies in an RKE system using FSK modulation. Source: Art Pini
The trace in the upper left grid represents 10 ms of a 260-ms-long RKE command. The RKE fob uses FSK to encode the digital one and zero states. The trace below the acquired trace shows that the demodulated FSK data is an NRZ serial signal. The upper-right grid shows the FFT of the acquired RKE signal. The signal has a frequency-modulated 434-MHz carrier. The FFT shows two peaks characteristic of frequency hopping, one corresponding to the frequency of the one state and the other to the frequency corresponding to the zero state.
Zoom can be used to separate the parts of the acquired signal corresponding to the signal’s 0 and 1 states. Zoom trace Z1 (third grid down on the left) shows the part of the RKE signal matching the zero state shown in the demodulated signal. The duration of the zoom trace is adjusted to fit within the duration of the digital state.
Similarly, the zoom trace Z2 (bottom left) has been used to select the part of the signal in the one-state. The intensified segments on the acquired waveform correspond to the selected regions. FFTs of the zoom traces show that each digital state contributes a specific frequency to the signal.
Measurement parameters identify the zero frequency as 433.888 MHz and the one state as 433.964 MHz. The magnitude of the frequency shift between the two digital states is determined by taking the difference between the two measured frequencies, which is 76 kHz. Zoom has separated the frequencies associated with each digital state.
Note that the FFT’s frequency resolution is proportional to its input’s record length and that the zoom traces are shorter than the acquired waveform and thus will have poorer resolution. This does not matter in this example, where the goal is to determine the frequencies of the two digital states.
Expanding waveforms with zoom
Zoom is a useful tool for studying and analyzing acquired waveforms by providing an expanded view of the signal vertically or horizontally. These traces provide enhanced visual acuity, allowing the instrument’s full amplitude and time resolution to be displayed on the screen. They also select specific parts of a signal, allowing for the analysis of only those portions of the signal that are of interest.
Arthur Pini is a technical support specialist and electrical engineer with over 50 years of experience in electronics test and measurement.
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