Test and Measurement Equipment Instrument Selection Informational

How do I select an oscilloscope for characterizing high speed RF waveforms?

Q: How much memory do I need?

Memory depth = sample rate × capture time. For a 100 GSa/s scope capturing 10 μs: 1 Mpoints. For capturing a full 5G NR slot (0.5 ms at 100 GSa/s): 50 Mpoints. Deep memory (100 Mpoints to 2 Gpoints) is needed for: long captures with high time resolution, segmented memory (capturing many short events over a long period), and serial protocol decoding (I2C, SPI, MIPI). For most RF waveform characterization: 10-100 Mpoints is sufficient.

How do I select an oscilloscope for characterizing high speed RF waveforms? An oscilloscope for RF waveform characterization must have sufficient analog bandwidth, sample rate, and dynamic range to accurately capture the signal in the time domain: (1) Analog bandwidth: the oscilloscope bandwidth must exceed the highest frequency component of the signal. Rule of thumb: bandwidth ≥ 5× the fundamental frequency for clean waveform capture. For a 5 GHz RF signal: bandwidth ≥ 25 GHz (to capture harmonics and rise times). For pulsed radar at 10 GHz: bandwidth ≥ 20-30 GHz. For 5G NR FR1 modulated signals (3.5 GHz carrier): bandwidth ≥ 8-12 GHz. For digital signals with fast edges (SerDes, clock): bandwidth = 0.35 / rise_time. A 10 ps rise time requires ≥ 35 GHz bandwidth. (2) Sample rate: must satisfy Nyquist (≥ 2× bandwidth), but practical requirements are higher. For oscilloscopes: sample rate ≥ 2.5× the analog bandwidth. A 25 GHz scope should have ≥ 62.5 GSa/s (gigasamples per second). For single-shot capture (non-repetitive events): the full sample rate is needed on all channels. For equivalent-time sampling (repetitive signals): the effective sample rate can be much higher (100+ GSa/s) using sequential sampling across many trigger events. (3) Dynamic range and vertical resolution: traditional oscilloscopes: 8-bit ADC → 48 dB dynamic range. This is sufficient for digital waveforms but marginal for RF measurements (where 60-80 dB dynamic range is desirable). High-resolution oscilloscopes: 10-12 bit ADC → 60-72 dB dynamic range. Better for characterizing low-level artifacts (noise floor, spurs, distortion). Example: Keysight UXR (10-bit), Tektronix 6 Series B MSO (12-bit via high-res mode). (4) RF-specific features: FFT and spectral analysis: view the signal in the frequency domain (like a spectrum analyzer). Useful for identifying spurs, harmonics, and modulation sidebands. Demodulation and EVM: some oscilloscopes include vector signal analysis software (e.g., Keysight 89600 VSA on the oscilloscope). This turns the scope into a wideband VSA for EVM, constellation, and modulation analysis. Time-gated FFT: analyze the spectrum of a signal during a specific time window (useful for pulsed and burst signals). (5) Major instruments: Keysight UXR (Infiniium): 13-110 GHz, 10-bit, 256 GSa/s. The benchmark for high-bandwidth RF and SI measurements. Price: $100,000-800,000. Tektronix DPO70000SX: 70 GHz, 200 GSa/s. Excellent for SerDes and radar. Price: $100,000-500,000. Tektronix 6 Series B: 10 GHz, 12-bit (high-res mode), 50 GSa/s. Great value for sub-10 GHz RF. Price: $15,000-60,000. R&S RTP: 16 GHz, 10-bit, 40 GSa/s. High dynamic range for mixed-signal work. Price: $30,000-100,000.

Category: Test and Measurement Equipment

Updated: April 2026

Product Tie-In: VNAs, Spectrum Analyzers, Signal Generators

Oscilloscope for RF Waveforms

Oscilloscopes are essential for time-domain RF measurements that spectrum analyzers cannot perform: pulse shape, rise time, transient behavior, and real-time signal capture.

When to Use an Oscilloscope vs a Spectrum Analyzer

(1) Use an oscilloscope for: time-domain waveform shape (pulse rise/fall times, overshoot, ringing). Simultaneous multi-channel timing (trigger synchronization). Wideband signal capture (entire signal bandwidth in one acquisition). Eye diagrams for SerDes and high-speed digital links. Pulsed radar envelope and pulse-to-pulse characterization. (2) Use a spectrum analyzer for: narrowband spectral measurements (phase noise, spurs at specific offsets). EMC pre-compliance (narrowband RBW, quasi-peak detector). Power measurements at specific frequencies. Higher dynamic range (120+ dB vs 48-72 dB for scopes). (3) Use both: the oscilloscope captures the time-domain waveform; the spectrum analyzer confirms the spectral purity. Many labs use both instruments simultaneously for PA characterization.

Oscilloscope Selection Criteria

BW ≥ 5× fundamental for waveform capture
Sample rate ≥ 2.5× analog BW
8-bit ADC: 48 dB dynamic range
10-12 bit ADC: 60-72 dB dynamic range
Rise time BW: 0.35 / t_rise

Common Questions

Frequently Asked Questions

Do I need a real-time or sampling oscilloscope?

Real-time (single-shot): captures the waveform in one trigger event. Required for non-repetitive signals (transients, radar pulses, digital data). Most modern oscilloscopes are real-time. Sampling (equivalent-time): builds up the waveform over many trigger events. Only works for repetitive signals. Achieves very high effective bandwidth (70-100+ GHz) with lower-cost hardware. Used for: SerDes eye diagrams (repetitive data patterns), clock jitter measurements, and TDR/TDT characterization.

What probes do I need for RF measurements?

For direct connection (50 ohm): use a coaxial cable from the DUT to the scope input (50 ohm input impedance). SMA to BNC or SMA to scope-specific connector. No probe needed if the DUT has a 50 ohm output. For on-board probing: active probes (e.g., Keysight InfiniiMax, Tektronix P7600 series) with probe tips that contact PCB pads. Active probe bandwidth: 13-33 GHz. Loading: < 0.1 pF (minimal impact on the circuit).

How much memory do I need?

Memory depth = sample rate × capture time. For a 100 GSa/s scope capturing 10 μs: 1 Mpoints. For capturing a full 5G NR slot (0.5 ms at 100 GSa/s): 50 Mpoints. Deep memory (100 Mpoints to 2 Gpoints) is needed for: long captures with high time resolution, segmented memory (capturing many short events over a long period), and serial protocol decoding (I2C, SPI, MIPI). For most RF waveform characterization: 10-100 Mpoints is sufficient.

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