With RF devices becoming ever more ubiquitous, knowing how to make fast, flexible, RF measurements is vital. As the saying goes, a chain is only as strong as its weakest link, and in the same way an RF measurement is only as reliable as the most uncertain component or practice.
Exceptional accuracy and flexibility have made the vector network analyzer (VNA) the instrument of choice for many RF measurements. A VNA may be one of a number of instruments used in a test system. In this case, RF networks are considered to be a series of interconnected electrical components, not to be confused with wireless or computer networks. To fully realize these benefits, you must understand the weaknesses of your measurement, correcting or minimizing them where possible.
Like all RF instruments, network analyzers require a certified calibration laboratory to perform a yearly factory calibration. However, the key to the exceptional measurement accuracy achieved by network analyzers is performing a user calibration more frequently.
Common systematic errors;
1) Transmission and reflection tracking – Frequency response errors, or transmitted and reflected signal loss, result across all frequencies and must be characterized and corrected.
2) Source and load mismatch – Many RF systems have a characteristic impedance of 50Ω. To properly measure the impedance of the Device Under Test (DUT), any impedance errors in the system must be accounted for.
3) Isolation – Errors can occur from crosstalk between various components in the test setup, VNA ports and measurement reference planes.
4) Coupler directivity – VNAs rely on directional couplers to separate the transmitted signals from the reflected signals. Ideally, a directional coupler measures the forward or reverse travelling signals and produces no output for signals travelling in the opposite direction. Because this is rare, the coupler leakage must be accounted for.
To correct for these systematic errors, you can perform a user calibration and measure a set of known standards. Next compare the measured value to the value of the known standard to calculate the error for each data point, and finally apply the appropriate error correction for each frequency point in the measurement. Using broadband Short, Open, Load and Through (SOLT) standards is a common, accurate calibration method for RF frequencies. Many factors determine how often a user calibration is performed including the required measurement accuracy, environmental conditions and the repeatability of the DUT connection. A fresh user calibration may be necessary hourly or weekly.
Go to electronicsweekly.com to know more about RF measurement.