Quick Answer
Signal generator calibration at Prism verifies: output frequency accuracy (using calibrated frequency counter), amplitude accuracy (using calibrated True RMS voltmeter), waveform distortion (THD using audio analyser), DC offset accuracy, and frequency range coverage from the instrument's minimum to maximum rated frequency.
Key Takeaways
- Signal generator calibration covers: frequency accuracy, amplitude, DC offset, distortion (THD).
- Reference: Keysight 53230A frequency counter (OCXO-based, < 1 ppm accuracy).
- Calibration frequency range: 0.01 Hz to rated maximum (typically 1–50 MHz for function generators).
- Amplitude calibrated at 50 Ω and 1 MΩ output impedance settings.
- Prism calibrates Keysight 33xxx, Rigol DG series, Tektronix AFG, Fluke PM5138 generators.
Signal Generator Parameters Calibrated
A signal generator is a source instrument: everything downstream of it inherits its error. If the generator says 1 kHz but actually delivers 1.004 kHz, every filter response, every amplifier gain measurement and every instrument you calibrate against it carries that error forward. Calibration establishes what the output actually is, at defined points across the range, against a reference whose own accuracy is traceable.
Traceability, in one line
A traceable result is one connected to the SI unit through an unbroken chain of calibrations, each with a stated uncertainty. For frequency, that chain runs from our OCXO-based counter back to the national standard at NPL India — which is what makes the number on your certificate defensible in an audit.
- Frequency accuracy (using NABL-calibrated frequency counter)
- Sine wave amplitude accuracy at 50 Ω (from 0.1 Vpp to maximum rated)
- DC offset accuracy (±max offset voltage)
- Total harmonic distortion (THD) of sine wave output
- Square wave duty cycle and rise/fall time
- DC voltage output accuracy (for generators with DC output)
The Calibration Procedure, Step by Step
The sequence below is what a signal generator goes through at our Vatva GIDC laboratory. The order matters: as-found readings are taken before anything is adjusted, because that is the record showing what your measurements were doing since the last calibration.
Stabilisation and visual check
The generator is powered and allowed to reach thermal equilibrium — typically 30 minutes, longer for OCXO-referenced units. Connectors, attenuator switches and the display are checked for damage that would affect the result.
As-found frequency sweep
Output frequency is measured against the reference counter at decade points across the full rated range, from the lowest setting to the maximum. Deviation is recorded in ppm at each point, before any adjustment.
Amplitude accuracy at defined loads
Sine output is measured with a True RMS voltmeter at both 50 Ω and 1 MΩ termination, from 0.1 Vpp upward. Load setting matters: a generator set for 50 Ω but measured into high impedance reads double.
DC offset and waveform integrity
DC offset is verified across its range, square wave duty cycle and rise time are measured, and total harmonic distortion of the sine output is assessed on an audio analyser.
Adjustment where applicable
Where a parameter falls outside the manufacturer specification and the instrument supports adjustment, it is corrected and the affected points re-measured as the as-left record.
Certificate issue
Results are issued with the expanded uncertainty, coverage factor, reference standard identity and ambient conditions recorded, along with the recommended due date.
Typical Accuracy Specifications by Parameter
Manufacturer specifications vary by model and grade. The figures below are representative of bench function generators in common industrial use, and give a sense of what tolerance a calibration is judging the instrument against.
| Parameter | Typical specification | Reference used |
|---|---|---|
| Frequency accuracy | ±10 to ±50 ppm (TCXO); ±1 ppm (OCXO) | Frequency counter, OCXO timebase |
| Sine amplitude accuracy | ±1% to ±2% of setting at 1 kHz | True RMS voltmeter |
| DC offset accuracy | ±(1% of setting + 2 mV) | DC voltage standard |
| Sine THD | < 0.1% at 1 kHz for quality units | Audio / distortion analyser |
| Square wave rise time | < 10 ns typical on bench units | Calibrated oscilloscope |
| Duty cycle accuracy | ±1% of setting | Universal counter |
Where a generator is used as a reference inside a production test rig rather than as a general bench tool, the acceptance tolerance should be tighter than the manufacturer figure — commonly a quarter of the tolerance of whatever it is being used to verify.
How Often Signal Generators Need Calibration
No standard fixes a single interval. ISO 9001 Clause 7.1.5 requires that you define the interval and be able to justify it, which in practice means starting from the figures below and tightening them where your own drift history or the criticality of the measurement warrants it.
| How the generator is used | Recommended interval |
|---|---|
| General bench work, development and fault-finding | 12 months |
| Reference source inside a production test system | 6 months |
| Used to calibrate or verify other instruments | 6 months |
| After any repair, overload or physical damage | Immediately, before return to service |
| Instrument with documented history of stable drift | May extend beyond 12 months with recorded justification |
Extending an interval is legitimate under ISO 9001, but only with evidence. Three consecutive calibrations showing drift well inside tolerance is the usual basis. Extending it because the instrument is inconvenient to release is not, and auditors ask for the drift records.
What We Commonly Find
Across the function and arbitrary waveform generators that pass through our electrical calibration laboratory, a handful of findings recur often enough to be worth anticipating.
- Amplitude error at low settings — accuracy is usually specified as a percentage of setting, so a generator that is fine at 5 Vpp can be well out at 100 mVpp
- Output impedance mismatch — the generator is set for 50 Ω but connected to a high-impedance input, giving readings exactly twice the expected value
- Timebase ageing — crystal references drift with age, and a unit that met ±20 ppm when new can exceed it after five or six years
- DC offset creep on older units, typically from capacitor ageing in the output stage
- Attenuator wear on instruments that see frequent range switching, showing as a step error at one particular amplitude range
None of these are visible from the front panel. They only appear when the output is measured against a traceable reference, which is the point of the exercise.
What Your Certificate Should Show
Check this before you file it
A certificate that reports only 'pass' is not evidence. An auditor needs the measured values, the tolerance they were judged against, and the uncertainty of the measurement itself.
- As-found readings at each frequency and amplitude point, before any adjustment
- As-left readings where an adjustment was made
- Expanded measurement uncertainty with the coverage factor stated (normally k = 2)
- The reference standard used, with its own certificate number and due date
- Ambient temperature and humidity during the calibration
- The NABL accreditation number of the issuing laboratory, and confirmation the parameter falls inside that scope
Prism calibrates signal generators, function generators and arbitrary waveform generators under NABL accreditation CC-2480, alongside the rest of our electrical calibration scope. Send us the make, model and rated frequency range and we will confirm what falls inside scope before you ship anything.
Frequently Asked Questions
How accurate is signal generator frequency output?
Most function generators use a TCXO or OCXO crystal reference with accuracy of ±10–50 ppm. Calibration verifies the actual frequency accuracy against a NABL-calibrated frequency counter (< 1 ppm reference). For applications requiring < 1 ppm frequency accuracy, GPS-disciplined signal generators are recommended.
Does Prism calibrate RF signal generators?
Prism calibrates RF signal generators up to 3 GHz for amplitude accuracy and frequency accuracy. For RF generators requiring power level calibration in dBm, a calibrated power meter and power sensor are used.
How often should signal generators be calibrated?
Annual calibration is standard. Signal generators used as references in production test systems or calibration setups should be calibrated every 6 months.
What is THD for a signal generator and why does it matter?
THD (Total Harmonic Distortion) measures the purity of the sine wave output. For a 1 kHz sine wave, THD < 0.1% is typical for quality generators. High THD causes errors when testing circuits or calibrating amplifiers — the reference 'sine wave' contains unwanted harmonics.
Can Prism calibrate arbitrary waveform generators (AWGs)?
Yes. AWG calibration covers the same parameters as function generator calibration. Waveform fidelity and DAC linearity can also be assessed.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
Hours: Mon–Sat, 9:00 AM – 7:00 PM