Field Guide · hardware

Also known as: sig gen, RF source, VSG

A signal generator is a bench instrument that synthesizes a known, controlled RF signal to serve as a reference stimulus1 — a pure carrier at a chosen frequency and power level, optionally carrying a defined modulation. Where a spectrum analyzer measures signals, a signal generator creates them, making it the essential source for testing receiver sensitivity, calibrating gain, and exercising a decoder with a repeatable input.

Synthesizer Modulator Level / attenuator RF out I/Q baseband
A signal generator: a synthesizer sets the carrier frequency, a modulator imprints information, and a calibrated attenuator sets the exact output level.

Overview

The value of a signal generator lies in its known, traceable outputs. Frequency is set by a precise PLL or DDS synthesizer referenced to a stable TCXO or OCXO, so frequency stability and low phase noise are headline specs. Output level is set by a calibrated step attenuator, so a “−100 dBm” setting really is −100 dBm at the connector — which is what lets you measure a receiver’s sensitivity meaningfully.

Variants

Analog (RF) signal generators produce a continuous-wave carrier and support basic built-in modulation — AM, FM, phase, and pulse. They are the simplest and are ideal as a clean local-oscillator-quality reference or for sensitivity testing with a tone.

Vector signal generators (VSGs) add an I/Q modulator fed by an internal arbitrary-waveform engine, so they can synthesize essentially any digital modulation — QPSK, C4FM, π/4-DQPSK, OFDM, and full standard waveforms such as LTE or Wi-Fi frames. A VSG can replay a captured IQ file, letting you generate a realistic P25 or DMR signal on the bench with controlled level, noise, and impairments.

Simpler sources fill the low end: DDS modules and the transmit side of an SDR like a HackRF or LimeSDR can generate test signals, though without the calibrated level accuracy and spectral purity of a bench instrument. A tracking generator paired with a spectrum analyzer or the tinySA forms a low-cost scalar network-measurement setup.

In practice

  • Sensitivity / SINAD testing — inject a known low-level modulated signal and find the level at which the decoder or SINAD meets a threshold.
  • Reference / LO substitution — supply a clean carrier to stand in for a local oscillator when characterizing a mixer or filter.
  • Impairment injection — a VSG can add controlled noise, frequency offset, or phase noise to stress a demodulator’s tracking loops.
  • Level discipline — always mind the calibrated output range and never exceed a device-under-test’s damage threshold; use external attenuators for very low levels below the generator’s floor.

Relevance to SDR

For SDR and trunking work, a signal generator — especially a vector one replaying a captured IQ waveform — is how you exercise a decoder repeatably: feed a known-good P25 or DMR signal at a controlled level to confirm the receive chain, then walk the level down to measure sensitivity, or add impairments to probe where lock breaks. GopherTrunk itself is a receiver and generates no RF; a bench signal generator (or the TX side of an SDR) is an external aid used to validate the receive path rather than something GopherTrunk drives.

Sources

  1. Signal generator — Wikipedia, on analog and vector RF signal generators and their use as reference sources. ↩

See also