Signal Generators: Introduction

A signal generator produces a controlled electrical signal at a known frequency and amplitude. It is the complement of the spectrum analyzer in the test bench: where the spectrum analyzer measures what signals are present, the signal generator puts a specific signal exactly where you need it. Together they enable a complete set of receiver and filter measurements: sensitivity (what is the weakest signal the receiver can detect?), selectivity (how well does it reject signals on adjacent channels?), and spurious responses (what out-of-band signals cause it to produce false responses?).

Signal generators range from simple audio-frequency oscillators to precision microwave synthesizers. For amateur radio testing work, the most useful types are the RF signal generator (covering HF through VHF/UHF) and the direct digital synthesis (DDS) generator, both of which are now available at amateur-friendly prices. An AD9850-based DDS module generating signals up to 40 MHz costs under $10 and can serve as a basic signal source for receiver sensitivity testing. Purpose-built units from companies like Siglent and Rigol with RF coverage to 3 GHz cost $300–$600 and offer amplitude accuracy and modulation capabilities that the inexpensive DDS modules cannot match.

Signal Generator Types

Function Generator

Produces sine, square, and triangle waves from a few Hz to a few MHz. Primarily useful at audio and sub-audio frequencies. The output is not well-calibrated in terms of harmonic content — a function generator's sine wave contains visible harmonics — and its frequency coverage is too low for RF work. Most useful for testing audio amplifiers and low-frequency circuits, oscilloscope triggering, and AFSK audio tone generation for testing packet radio modems.

RF Signal Generator

The workhorse of RF test work. An RF signal generator synthesizes frequencies from the audio range through HF, VHF, and UHF. Precision laboratory models cover 9 kHz to 3 GHz or beyond. Affordable bench units suitable for amateur radio work cover 100 kHz to 3 GHz. An RF signal generator has calibrated output level control — typically adjustable from −127 dBm (about 0.2 nanovolts into 50 Ω) to +10 dBm or higher — with level accuracy of ±1–2 dB. This calibrated output is what makes it useful for receiver sensitivity testing: you can set the generator to −117 dBm and know that you are applying a signal just at the receiver's theoretical noise floor.

DDS Signal Source

Direct digital synthesis uses a high-speed digital-to-analog converter driven by a lookup table in a digital chip to synthesize frequencies. DDS modules based on the AD9850, AD9851, or Si5351 chips are inexpensive and cover HF frequencies well. The Si5351 is particularly popular in amateur radio applications and can produce signals at any frequency from about 8 kHz to 160 MHz with excellent frequency resolution (sub-Hz) but with moderate harmonic content that must be filtered for use as a clean signal source. Output level is fixed (not calibrated) and must be padded down with external attenuators to reach the low signal levels needed for sensitivity testing.

VHF/UHF Synthesizer Module

ADF4351, ADF4356, and similar PLL synthesizer chips can produce signals from about 35 MHz to 4 GHz. These are often used as signal sources for 2m and 70cm equipment testing. Output level is moderate (around 0 dBm) and requires external attenuation for sensitivity testing. Phase noise is higher than in purpose-built RF signal generators.

Comparison of the four signal source types available to the amateur radio test bench. A calibrated RF signal generator is the only type with a known, accurate output level suitable for MDS and sensitivity measurements. DDS modules are inexpensive and frequency-agile but require a calibrated step attenuator to reach the sub-−90 dBm levels needed for receiver sensitivity testing. The output level scale shows the range relevant to HF receiver work.

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Key Specifications

Frequency Range and Resolution

The frequency range must cover the bands of interest. For HF receiver testing, 1–30 MHz minimum is needed. For VHF/UHF work, coverage to 300 or 450 MHz is essential. Frequency resolution is how precisely the frequency can be set — modern synthesized generators have sub-Hz resolution, more than adequate for any amateur application.

Output Level Range and Accuracy

The most critical specification for receiver testing. For minimum discernible signal (MDS) measurements, you need output levels below −120 dBm. Level accuracy of ±2 dB is adequate for most amateur measurements. Better generators achieve ±0.5 dB across their frequency range. Cheaper generators (especially DDS modules) have no absolute level calibration — you must add a calibrated step attenuator if you need known output levels.

Frequency Accuracy and Stability

The generator's frequency accuracy depends on its internal oscillator reference. Consumer-grade RF generators use a TCXO (temperature-compensated crystal oscillator) with ±1 ppm frequency accuracy. For receiver testing, ±10 ppm is adequate. For alignment of frequency-critical circuits (crystal filters, WSPR transmitters), ±1 ppm or better is needed. GPS-disciplined oscillator (GPSDO) references bring frequency accuracy to parts in 10¹¹.

Understanding Output Levels

Signal generator output levels are specified in dBm (decibels relative to 1 milliwatt into 50 Ω). Understanding the relationship between dBm and microvolts EMF (the unit commonly used in receiver specifications) is essential for sensitivity testing.

The relationship between dBm and voltage in a 50 Ω system:

• 0 dBm = 1 mW into 50 Ω = 223.6 mV RMS = 447 mV peak-to-peak
• −60 dBm = 1 µW = 224 µV RMS
• −107 dBm = 1 µV RMS into 50 Ω (the signal level at which a typical HF receiver first produces audio)
• −117 dBm = 0.316 µV RMS = the thermal noise power in 1 Hz of bandwidth at room temperature
• −127 dBm = 0.1 µV RMS — typical minimum output of a laboratory RF signal generator

Receiver specifications often quote sensitivity in µV EMF (open-circuit voltage from a 50 Ω source). The relationship between µV EMF and dBm: P(dBm) = 20·log₁₀(V_emf/1 µV) − 107 (for a 50 Ω system). A receiver specified at 0.5 µV for 10 dB SNR corresponds to about 20·log₁₀(0.5) − 107 = −6 − 107 = −113 dBm.

Modulation Capability

For receiver testing beyond simple sensitivity, a signal generator must be able to modulate its output. The most useful modulation modes for amateur radio testing are:

Spectral Purity

A signal generator must produce a clean signal at the desired frequency without significant energy at other frequencies. Impurities in the generator output — harmonics, sub-harmonics, or spurious outputs — can cause the receiver or filter under test to respond to these unwanted frequencies, giving a false measurement. Key spectral purity specifications:

Harmonics: The second, third, and higher harmonics of the fundamental frequency. A quality RF signal generator specifies harmonics below −30 dBc (30 dB down from the carrier). Lower-quality DDS sources may have harmonics only 15–20 dBc down, requiring a low-pass filter to clean them up before use in testing.

Spurious outputs: Non-harmonic spurious signals produced by mixing of the synthesizer's reference and VCO signals. Specified as dBc below the carrier. A good RF signal generator has all spurious outputs below −60 dBc.

Phase noise: The spread of energy around the carrier due to frequency instability. Specified in dBc/Hz at a given offset from the carrier. For receiver noise floor testing, the generator's phase noise must be lower than the receiver's thermal noise floor in the measurement bandwidth, otherwise the generator's phase noise skirt will dominate the measurement.

Ham Radio Measurement Applications

• Receiver minimum discernible signal (MDS): Set the generator to a known low level, apply to the receiver input, observe the audio output SNR
• Receiver selectivity (adjacent channel rejection): Set the generator to a frequency offset from the receiver's operating frequency; measure how much the output drops relative to the on-frequency level
• Image rejection: Set the generator to the image frequency and measure the output relative to the on-frequency response
• Filter alignment: Sweep the generator frequency through a filter while monitoring the output to map the filter's response curve
• AGC threshold: Vary the generator level and observe at what input level the receiver's AGC begins to act

Affordable Signal Generator Options