A signal generator is an electronic instrument that produces signals with specific, settable electrical waveforms. Signal generators are used to design, test, and manufacture systems and components: you feed a known signal into a device and measure what comes out. Depending on the application, the output can be repetitive or single-shot, and modern instruments can set frequency, amplitude, and modulation with calibrated precision. For a deeper primer, see what a signal generator is and how to use a signal generator.

The most basic units produce sine, square, and triangle waves. More capable instruments generate pulses, ramps, noise, and arbitrary waveforms, and add amplitude, frequency, and phase modulation. At the top of the range, vector signal generators reproduce complete digitally modulated standards.

The confusion is common, but the distinction is simple. A signal generator is a source: it creates the signals. An oscilloscope is a measuring instrument: it displays a voltage as it changes over time. On a working bench, the generator and the oscilloscope work as a pair: the generator drives the circuit, the scope shows the response. If your question is "which instrument do I buy first," the answer follows from the job: to create test signals, buy a generator; to observe circuit behavior, buy a scope.

Several key specs separate signal generators that look alike on a category page. Work through these 6, in order, against your own test plan:

1. Waveforms and modulation
2. Frequency range
3. Frequency resolution
4. Sampling rate (for arbitrary waveform generators)
5. Phase noise and spectral purity
6. External frequency reference and output impedance

Signal generators are classified first by the waveforms they produce: sine, triangle, square, pulse, arbitrary, and so on. Match the list to your test plan, not to the longest datasheet.

Modulation is the second axis. Modulation is the process of varying a signal to convey information, and the common analog types are amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). Digital work adds another requirement: a vector signal generator adds I/Q modulation, the in-phase and quadrature architecture behind modern digital standards. Keysight's white paper The Essential Signal Generator Guide draws the line clearly: an analog signal generator produces continuous-wave signals with basic modulation, while a vector signal generator produces complex, digitally modulated signals. That distinction, analog or vector, is one of the most important choices on the order form.

The frequency range is the span of frequencies the generator can produce, specified as minimum and maximum. Choose it from the highest frequency your test plan needs, plus headroom, because range is a major price axis. Three tiers cover most buying decisions:

• Baseband and general benchtop work to 120 MHz: Trueform waveform generators (33522B at 30 MHz, 33622A at 120 MHz).
• RF work to 6 GHz: X-Series generators (EXG N5171B analog, N5172B vector, MXG N5182B vector), all 9 kHz to 6 GHz.
• Microwave work to 44 or 67 GHz: PSG generators (E8267D vector to 44 GHz, E8257D analog to 67 GHz) and the VXG M9384B (1 MHz to 44 GHz). Keysight’s current lineup also includes the VXG M9484C to 54 GHz, and frequency extenders take Keysight sources past 100 GHz.

A wider range usually costs more. A range that ends below your next project can cost a second instrument. State your highest test frequency clearly, then buy one tier of headroom if the budget allows.

Frequency resolution is the smallest step the instrument can make when changing frequency. For many applications, fine resolution matters: when testing narrowband filters, crystal devices, or receiver channels, you need to place the carrier precisely, not approximately.

If a generator has a frequency resolution of 1 Hz, it can generate signals that differ by only 1 Hz. Modern synthesized instruments can resolve far finer steps than that. Check the resolution spec against the narrowest channel spacing in your test plan before purchasing.

Sampling rate applies to arbitrary waveform generators, and it is often misunderstood. It is the rate at which the instrument's DAC updates its output, in samples per second: an AWG plays waveforms out of memory; it does not sample an input signal.

Two numbers follow from the sample rate, and Keysight's guide to arbitrary waveform generator specifications explains both. The Nyquist limit requires the DAC to produce at least 2 points per period, so a 1 GSa/s generator can theoretically produce 500 MHz. In practice, the reconstruction filter reduces usable bandwidth: Keysight's guidance puts usable output at up to roughly 40% of the sample rate, so at most about 400 MHz from that same 1 GSa/s instrument. The specified bandwidth also depends on the analog output path. Memory depth matters too: memory divided by sample rate is your maximum play time, and effective number of bits (ENOB), not the DAC's headline bit count, reflects the real-world resolution.

The Trueform 33622A illustrates the practical arithmetic: 1 GSa/s and 14 bits deliver a specified 120 MHz of bandwidth, not 500 MHz.

Two generators with identical frequency ranges can behave very differently in a receiver test, and phase noise is often why. NIST defines phase noise as the rapid, short-term, random fluctuations in the phase of a wave, specified in dBc/Hz: decibels below the carrier, per hertz of bandwidth, at a stated offset from the carrier. The definitions behind those datasheet numbers are standardized in IEEE Std 1139-2022, so the spec is based on metrology, not marketing language.

Spectral purity is the umbrella term: phase noise plus harmonics and spurious outputs. Keysight's application note How Signal Generator Spectral Purity Affects Measurements explains why it matters: noise on your stimulus appears in your measurement, so a noisy source can mask exactly the behavior you are trying to see.

Read the spec as a curve, not a single number, and compare instruments at the same carrier and offset. Class examples from the Keysight MXG and EXG data sheets, both at a 1 GHz carrier and 20 kHz offset: the MXG N5182B with low-phase-noise Option UNX offers -146 dBc/Hz typical (-141 dBc/Hz specified), against -122 dBc/Hz typical for the EXG, a 24 dB gap. A standard MXG without that option is specified at -131 dBc/Hz (-134 dBc/Hz typical), still about 12 dB quieter than the EXG in typical values. That gap can be the difference between an adequate source and one that limits your measurement. Microwave sources are rated at their own carrier and offset: with Option UNY, the PSG E8257D offers -126 dBc/Hz typical (-120 dBc/Hz specified) at 10 kHz offset at a 10 GHz carrier, so set it beside figures quoted at that same point, not beside the X-Series pair.
 

Two practical specifications complete the checklist:

• External frequency reference (phase lock). Modern generators can lock to an external reference, typically 10 MHz, through reference input and output connectors. Locking every instrument on the bench to one house standard, the practical meaning of “phase lock mode” on older spec sheets, keeps frequencies consistent across the setup. A fixed, repeatable phase relationship between generators additionally requires a shared local oscillator, for example through Option 012 (LO in/out) on the X-Series or a phase-coherent multi-channel instrument.
• Output impedance. RF and waveform generator outputs are designed as 50-ohm systems: the instrument expects a 50-ohm load, and amplitude calibration assumes it. Driving a mismatched load can cause reflections and level errors rather than instrument damage in most bench cases, but the displayed amplitude may not match the actual delivered level. Check the output connector type (BNC on bench generators; precision coaxial on RF sources), use 50-ohm cable throughout, and check whether the instrument can re-scale its displayed amplitude for high-impedance loads.
   

Types overlap with the features above, but buyers often shop by type and use case, so here is the map. For the full taxonomy, see the types of signal generators guide. Audio-frequency and video signal generators are left out: any modern function or waveform generator covers the audio band, and video pattern generators are a specialist broadcast category outside this store's focus.

An arbitrary waveform generator (AWG) generates electrical signals with waveforms you define, replayed from memory through its DAC. That can include simulating real-world signals: an AWG can replay a captured cardiac waveform to test a medical device that detects heartbeats, or reproduce a sensor’s output, including its noise.

The trade-off is cost per point: deep memory, high sample rates, and good ENOB cost money. If your bench needs complex or captured waveforms, an AWG is the right tool. If you only need standard waveforms, a function generator one tier lower can do the job for less. The arbitrary waveform generator versus function generator guide walks through that decision.

A vector signal generator is an RF signal generator with a built-in I/Q modulator, so it can produce digitally modulated signals, including 5G NR, Wi-Fi, satellite links, and radar waveforms. The transmitter-quality requirements those signals must meet, including channel bandwidths and error vector magnitude, are defined in standards such as 3GPP TS 38.101-1 for 5G NR. That is why receiver test benches specify vector sources.

The spec that often separates vector generators is modulation bandwidth: how wide a signal the I/Q system can reproduce. The X-Series (N5172B, N5182B) provides up to 160 MHz of RF modulation bandwidth, depending on installed options, which covers most sub-6 GHz standards. The VXG M9384B extends to 2 GHz of modulation bandwidth for wideband 5G, satellite, and radar work. Vector instruments need periodic calibration to maintain I/Q accuracy, which is one reason to choose Keysight-calibrated used units over unknown-provenance units.

A pulse generator produces electrical pulses with settable shape, width, period, and edge times. Pulse and pattern generators support high-speed digital timing tests, serial-data stress tests, and component characterization: the 81134A pulse pattern generator, for example, produces patterns and pulses to 3.35 GHz on 2 channels.

Timing accuracy is the key spec: check jitter and transition-time specs against your fastest signal, and keep the instrument calibrated, since a drifted pulse generator may report edges at the wrong time.

An RF signal generator creates continuous-wave and modulated signals at radio frequencies for testing and troubleshooting receivers, amplifiers, and RF modules in cellular, satellite, and general wireless work.