**1. Overview of Current Vector Signal Analyzer Calibration Methods**
Currently, the most common method for calibrating a vector signal analyzer (VSA) involves using a standard vector signal source. This approach is simple and user-friendly, but it has limitations in terms of accuracy, stability, and repeatability, as the quality of the "standard source" can vary significantly.
In contrast, international metrology institutions such as Germany’s PTB, the UK’s NPL, and the US’s NIST employ high-speed sampling oscilloscopes combined with multi-carrier signal sources to synchronize time and phase. They use synchronization triggers to perform system calibration, and then calculate the oscilloscope’s sample values through software. These calculated values serve as a reference for amplitude and phase parameters, enabling precise metering calibration of the VSA.
The advantage of using a multi-carrier signal source, oscilloscope, and synchronizer is that it allows vector parameters to be traced back to power level, time, and frequency. However, this method also has drawbacks. The oscilloscope’s frequency range is limited, and its uncertainty is relatively high. Additionally, synchronization delays can introduce significant phase errors, especially in microwave measurements. The overall system complexity increases, leading to more uncertainties and potential inaccuracies.
**2. The Calibration Technique Proposed in This Paper**
This paper introduces a novel calibration technique based on the continuous wave frequency offset (CWO) method, which is used to measure the residual error of the signal analyzer. It enhances the measurement accuracy of analog modulation signal analyzers.
a. **Continuous Wave Frequency Offset Method**: This method measures carrier frequency error, power error, and residual error in vector signal analysis, which characterizes the noise floor of the signal analyzer and helps evaluate various demodulation indicators.
b. **Continuous Wave Frequency Offset with Additional Analog Modulation**: This approach is used to calibrate and verify the error vector magnitude (EVM), amplitude error, and phase error of the vector signal analyzer.
c. **Multi-Carrier Method**: This technique is applied to calibrate and verify the magnitude accuracy of I/Q offset (carrier leakage) in the vector signal analyzer.
**3. Quantitative Definitions**
**3.1. I/Q Signal**
A vector is a graphical representation that uses a rotating arrow to describe a signal in a Cartesian coordinate system. The length of the arrow represents the peak amplitude, while the angle between the arrow and the positive horizontal axis represents the phase. As the arrow rotates counterclockwise, it indicates the frequency of the signal.
An I/Q signal is decomposed into two components: one in-phase (I) and one quadrature (Q). These components have the same peak amplitude and frequency but are 90 degrees out of phase. Typically, a cosine signal represents the I component, and a sine signal represents the Q component.
**3.2. Error Vector Magnitude (EVM)**
The error vector (EV) is the difference between the actual measured signal (m) and the ideal reference signal (R). The error vector magnitude is usually expressed as a percentage of the reference signal's amplitude. This metric is crucial for evaluating the performance of modulated signals.
**3.3. Amplitude Error**
The amplitude error is the difference between the actual measured signal (m) and the ideal reference signal (R). It is typically expressed as a percentage of the reference signal's amplitude.
**3.4. Phase Error**
The phase error is the difference in phase between the actual measured signal (m) and the ideal reference signal (R).
**3.5. Origin Offset**
The origin offset refers to the magnitude of the vector difference between the actual measured signal’s origin and the ideal reference signal’s origin. It is usually expressed as a ratio (in dB) relative to the reference signal’s amplitude.
**3.6. I/Q Imbalance**
I/Q imbalance includes both amplitude (gain) imbalance and phase (orthogonal) imbalance. These imbalances affect the accuracy of I/Q signal decomposition and can lead to distortion in the signal constellation.
**3.7. Carrier Frequency Error**
The carrier frequency error is the difference between the actual measured signal’s frequency and the ideal reference signal’s frequency.
**3.8. Average Power**
The average power is the mean power of the actual measured signal over a given period.
**4. Continuous Wave Frequency Offset Method (CWO)**
**4.1. Scope of Application**
This method is used to measure and evaluate the residual intrinsic error (VSA noise) of the demodulation analysis parameters of the vector signal analyzer. It is applicable to digital modulation methods such as MSK, PSK, and QAM.
It supports the calibration of the following parameters in spectrum and vector signal analyzers: frequency error, power level error, residual EVM, residual amplitude error, residual phase error, I/Q origin offset (carrier leakage), residual I/Q imbalance, residual gain imbalance, and residual phase imbalance.
**4.2. Metrology and Calibration Equipment**
The equipment required for this method includes an RF microwave signal generator with a suitable frequency range. The traceability standards for this method are frequency and power level.
**4.3. Principle of Continuous Wave Frequency Offset (CWO)**
The goal of the CWO method is to generate a calibration signal that corresponds to the vector signal analyzer’s response for digital demodulation standard constellation points or a portion of them. Based on the I/Q vector demodulation principle, accurate I/Q vector diagrams and constellation points can be obtained by adjusting the I/Q phase difference corresponding to the frequency offset between the calibration signal and the VSA’s center frequency.
For digital modulation schemes like MSK, PSK, and QAM, the vector constellation diagram contains N symmetric constellation points with the same origin and equal amplitude. These are referred to as target constellation points. The calibration signal generated from a signal generator, which is traceable in frequency and power, is a continuous sine wave with a frequency offset Δf relative to the VSA’s center frequency.
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