Function and Signal Generators
A signal generator supplies a known, controllable test signal, so that you can see how a circuit responds to it. With a generator and an oscilloscope you can measure an amplifier's gain and frequency response, find a filter's cutoff, check how a circuit handles a square wave, simulate a sensor, or inject a signal into a piece of equipment to find where it stops working. Signal generators range from simple audio oscillators to precision microwave synthesizers; for most bench work the function or arbitrary waveform generator is the workhorse.
Signal sources compared: bench function and arbitrary waveform generators for everyday work, RF signal generators with calibrated output levels, and low-cost DDS and clock-generator modules.
View LargerSignal Generator Types
- Function generator: sine, square, triangle, ramp and pulse waveforms from below 1 Hz to a few megahertz (older analog types) or tens of megahertz (modern digital types). The everyday bench generator.
- Arbitrary waveform generator (AWG): a function generator that can also play any waveform stored in memory — an ECG trace, a sensor's output, a noisy or distorted signal — at a programmable sample rate. Almost all modern bench function generators are AWGs built on direct digital synthesis.
- RF signal generator: a precise sine-wave source from kilohertz to gigahertz with a calibrated output level down to very small signals (−120 dBm or less) and analog and digital modulation, for testing receivers, filters and RF circuits.
- Low-cost modules: DDS boards (AD9833, AD9850), clock generator chips (Si5351) and PLL synthesizer boards (ADF4351) produce useful test signals for a few dollars, but without calibrated output levels or good spectral purity.
- Pulse and pattern generators: specialist instruments for fast, precise digital pulses and data patterns.
Function and Arbitrary Waveform Generators
A modern two-channel function generator offers far more than its name suggests:
- Amplitude and DC offset: a sine wave can be centered on any DC level — for example 0–3.3 V to drive a logic input, or a small signal on a 2.5 V bias to drive a single-supply amplifier.
- Duty cycle and pulse width: square waves and pulses with adjustable duty cycle and edge times, ideal as PWM signals or clocks.
- Two channels: with an adjustable phase between them, for testing phase detectors, driving differential inputs, or providing a clock and a data signal together.
- Arbitrary waveforms: drawn on screen, built from equations, imported from a file, or captured on an oscilloscope and played back — useful for reproducing a fault signal or simulating a sensor.
- Noise: a white-noise output for testing filters and for adding noise to a signal to check a circuit's tolerance.
Key Specifications
- Maximum sine frequency: often quoted in the model name (a “30 MHz” generator). Square-wave and pulse limits are usually much lower.
- Sample rate and vertical resolution: 125 MS/s to 1 GS/s and 14–16 bits are typical; they set how fine an arbitrary waveform can be.
- Amplitude range and flatness: how small and how large a signal it can produce, and how much the amplitude changes with frequency (for example ±0.3 dB up to 10 MHz).
- Harmonic distortion: how pure the sine wave is — typically −50 to −65 dBc. For testing low-distortion audio amplifiers you need much better, which is why audio test sets and sound cards with low-distortion oscillators are used for that job.
- Rise and fall time and jitter: for square waves and pulses, typically a few nanoseconds.
- Frequency accuracy: set by the internal crystal (often ±1–20 ppm), and improved by a 10 MHz reference input.
Output Levels and the 50-Ohm Trap
Almost every generator has a 50 Ω output impedance. The amplitude it displays assumes a particular load, and getting this wrong is the most common surprise on the bench:
- With the generator set to a 50 Ω load, it displays the voltage you will get across a 50 Ω termination. Connect it to a high-impedance input (a 1 MΩ scope input, an op-amp) and you get twice the displayed voltage, because the internal 50 Ω and the load no longer divide the voltage in half.
- With the generator set to High-Z, the display is correct for a high-impedance load but half the displayed value appears across a real 50 Ω load.
The generator is left in its default 50 Ω setting and set to 1 Vpp. The scope (1 MΩ input) shows 2 Vpp. Nothing is wrong: the generator's internal source is 2 Vpp, and with 50 Ω in series it would deliver 1 Vpp only into a 50 Ω load. Either set the generator to High-Z, or terminate the cable at the scope with 50 Ω.
RF generators specify level in dBm into 50 Ω. 0 dBm = 1 mW = 0.224 V RMS = 0.632 Vpp across 50 Ω; −107 dBm = 1 µV; every 20 dB is a factor of ten in voltage.
Modulation, Sweep and Burst
- Frequency sweep: the generator sweeps from one frequency to another, linearly or logarithmically, and provides a trigger at the start of each sweep. With the sweep on one scope channel and an amplifier or filter output on the other, the scope draws the circuit's frequency response directly.
- Burst: a set number of cycles, then silence — for testing ultrasonic transducers, tone decoders and how circuits respond to the start of a signal.
- AM, FM, PM and PWM: modulating the output with an internal or external signal, for testing demodulators, PWM controllers and FM receivers.
- Gated and triggered output: the output starts on an external trigger, so that a test signal can be synchronized with other equipment.
RF Signal Generators
An RF signal generator is optimized for spectral purity and level accuracy rather than waveform variety. Its calibrated attenuator delivers precise levels down to −120 dBm or below, its phase noise and harmonics are low, and it offers AM, FM and often digital modulation. It is the instrument for measuring receiver sensitivity, testing filters and amplifiers at radio frequencies, aligning radios, and checking wireless modules. Low-cost synthesizer boards can stand in for frequency, but not for level: their output varies by several decibels with frequency and they produce strong harmonics, so a filter and a calibrated step attenuator are needed before they can be used for sensitivity measurements.
Affordable Options
- Entry-level DDS function/AWG generators (FeelTech/FeelElec FY6900, JDS6600 and similar): 15–60 MHz, two channels, around $80–$150. Excellent value for audio, logic and general use; check the output level accuracy before relying on it.
- Bench AWGs from Siglent, Rigol and Owon (for example SDG1032X, DG800/DG900 series): 25–100 MHz, better accuracy, lower distortion and more features, around $250–$600.
- Module-based sources (AD9833, AD9850, Si5351, ADF4351 boards with a microcontroller): a few dollars to tens of dollars; ideal as built-in test sources in projects.
- Used laboratory RF generators (HP 8640, 8656, 8657 and similar): excellent calibrated RF sources available second-hand.
- A computer sound card with free generator software makes a very low-distortion audio oscillator up to about 20 kHz.
Output Level Converter
Generator Output Level Converter
Enter a level in dBm (into 50 Ω) to see the equivalent RMS and peak-to-peak voltage across 50 Ω, the open-circuit voltage at the generator, and the voltage a high-impedance input would see.
Frequently Asked Questions
Why does my scope show twice the amplitude I set on the generator?
The generator is set for a 50 Ω load but connected to a high-impedance scope input. Its display assumes the 50 Ω load divides its internal voltage in half; without that load, you see the full open-circuit voltage, which is twice the displayed value. Set the generator's load setting to High-Z, or terminate the cable at the scope with 50 Ω.
Can a function generator drive a loudspeaker or a motor?
Not usefully. A generator can typically deliver about 10–20 Vpp into a high impedance but only a few hundred milliwatts at most into a low impedance, and its output is protected by current limiting. To drive a speaker, transducer or motor, use the generator as the input to a power amplifier. Never connect a generator's output to a point in a circuit that already has a voltage on it without a coupling capacitor or checking the output's voltage limits.
Why does the square wave from my generator ring on the scope?
Usually because the cable is not terminated. The generator's fast edges travel down the coaxial cable and reflect from the high-impedance scope input, as described in the Cables, Connectors and Signal Integrity module. Terminate the cable with 50 Ω at the scope end (and set the generator to its 50 Ω load setting) and the edges become clean. Ringing that remains is in the circuit you are testing, or in the scope probe's ground lead.
Can I use a wireless development board as a signal generator?
Transmit-capable development boards can generate RF signals and complex modulation, which makes them useful for testing receivers and demodulators. Their output level is not calibrated and they produce spurious signals and harmonics, so they need external filtering and attenuation for accurate work. Any signal radiated from an antenna must also comply with radio regulations, so use them connected by cable to the device under test, through an attenuator.
Test Your Knowledge
Answer the questions below to check your understanding of this lesson. Every answer can be found in the lesson above.