Can an oscilloscope replace a spectrum analyzer?

Partially. Oscilloscope FFT gives basic frequency information but with limited dynamic range (~48 dB vs. 80-120 dB for a spectrum analyzer). For quick frequency checks it's adequate; for RF measurements, EMC testing, or finding low-level spurious signals, a dedicated spectrum analyzer is necessary.

Do I need both instruments?

For RF and mixed-signal work, yes — each reveals information the other cannot. For general electronics debugging, an oscilloscope with FFT may suffice. For serious RF or EMC work, a spectrum analyzer is essential. Mixed-domain instruments combine both if budget allows.

What bandwidth oscilloscope do I need?

Rule of thumb: oscilloscope bandwidth should be 5× the highest fundamental frequency of interest. For 100 MHz digital signals, use a 500 MHz oscilloscope. For rise time measurement, bandwidth ≥ 0.35/rise_time.

Spectrum Analyzer vs Oscilloscope

Quick Answer

An oscilloscope displays voltage versus time (time domain), while a spectrum analyzer displays amplitude versus frequency (frequency domain). Use an oscilloscope to see waveform shape, timing, rise times, and pulse characteristics. Use a spectrum analyzer to see frequency content, harmonics, spurious signals, and modulation. Modern digital oscilloscopes include FFT math functions that provide basic spectrum analysis, and some advanced instruments combine both capabilities. The choice depends on whether your question is about 'when' (oscilloscope) or 'what frequencies' (spectrum analyzer).

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Oscilloscope: Seeing Signals in Time

An oscilloscope captures and displays voltage as a function of time. It answers questions like: What is the peak voltage? How fast does the signal rise? What is the period? Are there glitches or ringing? Is the signal clipped? Digital storage oscilloscopes (DSOs) sample the input at high rates (1-100 GS/s), store the data, and display it. Bandwidth ratings (50 MHz to 30+ GHz) determine the fastest signals the scope can accurately capture. Multiple channels (2-8 typical) allow simultaneous viewing of related signals — clock and data, input and output, voltage and current.

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Spectrum Analyzer: Seeing Signals in Frequency

A spectrum analyzer displays signal power versus frequency. It answers: What frequencies are present? How strong is each harmonic? Are there unwanted spurious signals? What is the signal's occupied bandwidth? What is the noise floor? Spectrum analyzers are essential for RF work — checking transmitter output for harmonics and spurious emissions, measuring receiver sensitivity, verifying filter performance, and identifying interference sources. Dynamic range (ability to see small signals near large ones) is typically 80-120 dB, far exceeding what an oscilloscope's FFT can achieve.

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When to Use Which Instrument

Use an oscilloscope when you need to see timing relationships between signals, measure pulse widths and rise times, debug digital logic signals, capture transient events, or view eye diagrams for serial data. Use a spectrum analyzer when you need to measure frequency content, check for harmonics and spurs, verify occupied bandwidth, measure phase noise, test EMC compliance, or characterize filters and amplifiers in the frequency domain. For power supply design, you might use an oscilloscope to view switching waveforms and a spectrum analyzer to check for electromagnetic interference.

Modern Convergence: Oscilloscopes with FFT

Most digital oscilloscopes include an FFT math function that computes a basic spectrum display from the time-domain data. This is convenient for quick frequency checks but has significant limitations compared to a dedicated spectrum analyzer. Oscilloscope FFT dynamic range is limited by the ADC resolution (typically 8 bits = ~48 dB) versus 80-120 dB for spectrum analyzers. Frequency resolution is limited by the capture time window. Amplitude accuracy depends on windowing function selection. For serious RF measurements, EMC testing, or finding signals more than 50 dB below the carrier, a dedicated spectrum analyzer is necessary.

Mixed-Domain Oscilloscopes and Signal Analyzers

Advanced instruments blur the boundary between oscilloscope and spectrum analyzer. Mixed-domain oscilloscopes (like the Tektronix MDO series) include a dedicated spectrum analyzer input alongside traditional oscilloscope channels, allowing simultaneous time and frequency domain viewing with proper dynamic range in each. Signal analyzers (like the Keysight UXA) combine spectrum analysis with advanced demodulation, allowing both frequency-domain and time-domain analysis of complex modulated signals like Wi-Fi, LTE, and 5G. For embedded systems work, combining logic analyzer, oscilloscope, and spectrum analyzer capabilities in one instrument maximizes productivity.

Frequently Asked Questions

Oscilloscope shows voltage vs. time (time domain). Spectrum analyzer shows amplitude vs. frequency (frequency domain). They reveal different aspects of the same signal — choose based on whether you're asking about timing or frequency content.

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