Feature: T&M
Figure 4. EVM is the difference between the actual measured signal and the ideal reference signal
Frequency accuracy is vital to RF applications, especially
where bands have data that are close together. For example, in wireless communication, it is important to understand how well a receiver can process a signal whilst rejecting the signal from an adjacent channel. Poor frequency accuracy will cause the signals to appear too close or too far, giving a misleading indication of the performance of the device under test (DUT).
Ageing rate and frequency Te ageing rate indicates how fast the reference driſts from its specified value aſter calibration, in parts-per-million (ppm). An oscillator is a primary component where ageing comes into
play. A quartz crystal’s physical properties within an oscillator exhibit a gradual change over time, resulting in frequency driſt. Te ageing of a crystal is ongoing and requires that a signal generator keep to a regular calibration maintenance schedule.
Spectral purity An ideal signal generator produces a sinusoidal wave at a single frequency without the presence of noise. In the real world, every signal generator has unwanted amplitude and phase impurities added to the generated signals, since the components in the instrument are non-ideal themselves; see Figure 3. Te characterisation of spectral purity evaluates harmonics, sub- harmonics, spurs and phase noise. Regardless of signal stability, it is important to choose a
signal generator with better spectral accuracy than the DUT. In datasheets, it is common to see the spectral purity represented as the single-sideband (SSB) phase noise. To understand the characteristics of spectral purity, it is helpful
to know that: • Phase noise is a frequency-domain view of the noise spectrum around the oscillator signal, describing the frequency stability of an oscillator.
• Harmonics and sub-harmonics are integer multiples of the sinusoidal fundamental frequency output. Non-linear characteristics of components in the signal generator cause harmonics.
• Spurs are non-random or deterministic signals, created by mixing and dividing signals to get the carrier frequency. Tese signals can be harmonically or non-harmonically related to the carrier.
• Broadband noise floor is the added unwanted noise produced by the non-ideal components of a signal generator. Noise floor
26 June 2024
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magnitude can vary across frequencies.
• Continuous wave (CW) is the desired frequency set to a specific amplitude. Users select and modify the CW, as needed.
EVM measurement Figure 4 shows the error vector magnitude (EVM) difference between the ideal reference signal and the measured signal at a given time. Tis result defines the performance of a digital radio transmitter or receiver. Signal generators with smaller EVM percentages enable users
to design and test systems to reach higher data capacity and better power efficiency whilst increasing wideband signal power.
The impact of distortion on performance Distortion is an alteration of the original waveform. Tere are two major types of non-linear distortion in signal generators: harmonic and inter-modulation distortion. Harmonic distortion happens when a smooth voltage change of a pure sinusoidal wave incurs an interruption caused by an abrupt voltage change. Inter-modulation distortion is a spurious output that occurs when mixing two or more signals with different frequencies. Spectral re-growth is the distortion generated by amplitude and
phase shiſts in digital modulations. It spreads outside the main channel; adjacent channel power ratio (ACPR) measurements examine this characteristic. Similar to spectral purity, it is important to choose a signal
generator with a superior distortion test margin so that it does not mask the performance of the DUT. Seeing this type of digital distortion is becoming increasingly important as wireless standards require more data throughput.
Understanding the fundamental sources Understanding the fundamental sources of signal generator errors simplifies the process of determining what instrument is right for a test project. When evaluating this, consider the ambient temperature a source will be in during testing. Tis will add relevance to the accuracy specifications in datasheets. In addition, have a plan for how oſten the single source will be recalibrated. Calibrating frequently and consistently is the best way to reduce the ageing effect. Lastly, evaluate the amount of EVM error and distortion in the power the signal source naturally generates, to avoid measuring the instrument’s performance. Collectively considering sources of errors leads to selecting a signal generator with the greatest signal purity for a task.
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