Digital Oscilloscope
Understanding the Digital Oscilloscope
The digital oscilloscope replaced the analog CRT oscilloscope by converting the continuous input voltage into a stream of digital samples. This seemingly simple change unlocked capabilities that are impossible with analog technology: storing waveforms indefinitely, performing mathematical operations on captured data, triggering on complex patterns, and transferring acquisition data to a computer for post-processing.
The signal path in a modern DSO begins with a wideband analog front end that conditions the input (attenuating, amplifying, and impedance-matching). The signal then enters the ADC, which samples at rates up to 256 GSa/s in current flagship instruments. The digital samples flow into acquisition memory (up to 2 Gpoints on high-end models), where the trigger system examines them in real time. Once a trigger condition is met, the oscilloscope freezes the acquisition and displays the captured waveform.
Real-Time vs. Equivalent-Time Sampling
There are two fundamentally different sampling architectures. A real-time oscilloscope digitizes the entire waveform in a single trigger event. Its ADC runs fast enough to satisfy Nyquist for the full analog bandwidth. This is essential for capturing non-repetitive events like glitches, protocol violations, and radar pulse anomalies. An equivalent-time (sampling) oscilloscope takes one sample per trigger, shifting the sample point slightly on each subsequent trigger. Over thousands of triggers, it reconstructs the waveform with extremely fine time resolution, achieving bandwidths over 90 GHz with relatively modest ADC speeds. The trade-off is that the signal must be repetitive and an external trigger is required.
BW = 0.35 / trise (for Gaussian response)
A 10 ps rise time requires BW ≥ 35 GHz
Required Sample Rate (Nyquist):
fs ≥ 2 × BW (minimum), 2.5 × BW (practical)
For 70 GHz BW: fs ≥ 175 GSa/s
Effective Number of Bits (ENOB):
ENOB = (SINAD − 1.76) / 6.02
At 50 GHz, a typical real-time oscilloscope achieves 5 to 6 ENOB, compared to 8+ ENOB at DC. The ENOB roll-off limits dynamic range at high frequencies.
Oscilloscope Types for RF Applications
| Type | Bandwidth | Sample Rate | RF Use Case | Example Instrument |
|---|---|---|---|---|
| General Purpose DSO | 200 MHz to 6 GHz | 5 to 20 GSa/s | Power supply noise, clock jitter, debug | Keysight InfiniiVision 6000X |
| High-Performance RT | 8 to 36 GHz | 80 to 128 GSa/s | Radar pulse analysis, 5G NR demod | R&S RTO6, Tektronix MSO6B |
| Ultra-High BW RT | 50 to 110 GHz | 128 to 256 GSa/s | PAM-4 eye diagrams, 112G SerDes | Keysight UXR, Tektronix DPO70000SX |
| Equivalent-Time Sampling | 70 to 90+ GHz | N/A (sequential) | Compliance eye masks, TDR/TDT | Keysight N1000A, Tektronix DSA8300 |
Frequently Asked Questions
What is the difference between a real-time oscilloscope and an equivalent-time sampling oscilloscope?
A real-time oscilloscope digitizes the entire waveform in a single trigger event. Its ADC runs fast enough to capture every cycle. An equivalent-time oscilloscope takes one sample per trigger, shifting the sample point slightly on each subsequent trigger, reconstructing the waveform over thousands of triggers. Real-time captures non-repetitive events; equivalent-time achieves higher bandwidth for repetitive signals and is the standard for SerDes compliance eye diagrams.
Why does an RF engineer need an oscilloscope instead of a spectrum analyzer?
A spectrum analyzer shows frequency-domain power. An oscilloscope shows the actual voltage waveform in time. For pulsed radar, it captures rise time, droop, and overshoot. For high-speed SerDes, it builds eye diagrams that reveal jitter, intersymbol interference, and noise margin. For envelope tracking PA design, it shows the timing relationship between the RF envelope and supply modulator output. These time-domain details are invisible to a spectrum analyzer.
How much bandwidth do I need in a digital oscilloscope?
For analog signals, use 5 times the fundamental frequency. For digital signals, use the fifth harmonic of the data rate. A 28 GBaud PAM-4 link needs at least 70 GHz. For rise time measurements, BW must be at least 0.35 divided by the rise time. A 10 ps rise time requires 35 GHz minimum to avoid the scope's own rolloff distorting the measurement.