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Using Calibration to Optimize Performance in Crucial Measurements
The increasing demand for high frequency components with various applications and the need to test such components require effective and efficient calibration of the measurement systems that are used. In order to have a wide field of application calibration, devices need to be both economic and technical. In particular, the relationship between price and performance is an important consideration. The ideal is for the calibration to be quick and easy and carried out with as little disruption as possible. In many technical fields such as telecommunications, aerospace and military applications, a universal method of calibration is useful and sufficient, including calibration of portable VNAs for field applications.
To meet these requirements, Rosenberger developed a versatile OSL(M) Calibration Kit, which consists of components required to meet Open – Short – Load and Mismatch (optional) standards. The mismatch option makes it possible to verify the calibration directly with the same connection (transmission line), which yields an accurate measurement.
Figure 1 Sectional view of the male revolving calibration adapter.
Rosenberger has developed the new generation RPC-N Revolving Calibration Adapter, which is simple to use, has state-of-the-art operation and performance and has been designed for field applications. By rotating the hand wheel, which is mounted on the centre axis, the adapter can be adjusted into the appropriate position for the specific standard required (as shown in Figure 1). Thus, the correct component is engaged and aligned with the coaxial transmission line. Springs ensure secure contact between coaxial components.
The RPC-N Revolving Calibration Adapter is available in two frequency ranges – DC to 8 GHz and DC to 18 GHz. It is simple to handle, with only one connector interface for all components. Also, it is versatile, has good electrical characteristics and is easy and quick to use, thus saving time.
As has been mentioned previously, a mismatch will be used for direct verification. The magnitude of the reflection coefficient at the load is the result of the forward and backward voltage ratio. The reflection coefficient changes the phase, dependent on the distance to the load. The complex plane (see Figure 2) shows the qualitative case of real mismatch reflection coefficient, as well as the verification mismatch that would be achieved after a successful calibration.
A key advantage is having the same transmission line for calibration and verification. The substrates for the load and the mismatch have the same resistive structure and only the load and mismatch at the end of line (at the load) are compared. Figure 3 shows the results of measurements using a VNA, which shows a curve without ripple. The plot shows the difference between independent mismatch and verification mismatch after calibration using the RPC-N Revolving Calibration Adapter.
Figure 2 The qualitative case of real mismatch reflection coefficient.
Figure 3 The results of measurements using a VNA.
Particularly for verification with mismatching, a reasonable VSWR should be applied. A VSWR of 1.2 is a good compromise between the difference to absolute magnitude of the calibration load (important for the upper frequency range) and possible error vector magnitude of the calibration error.
Additional error vector magnitude will be added to the reflection coefficient in the case of calibration error. The value of the resulting reflection coefficient depends on the kind of calibration error.
Following are practical applications of the RPC-N Revolving Calibration Adapter. All applications were executed with a one port calibration on the VNA and the connecting interface was PC-N. Calibration standards for the Revolving Calibration Adapter were Open, Short and Load. Typical parameters were entered into the VNA, which was a basic industrial version that only uses electrical length of open and short.
Figure 4 Results of measurements for the DC to 18 GHz male RPC-N revolving calibration adapter.
The next step, directly after calibration, is the mismatch measurement, which is a simple and useful verification of the calibration. This complete cycle of calibration and verification can be realized with only one connection.
Figure 4 shows the results of measurements for the DC to 18 GHz male RPC-N Revolving Calibration Adapter. All measurements are displayed in VSWR and clearly show the difference between successful and failed calibration. The plots show:
Figure 5 Measurement with LRL-calibration.
Figure 6 Measurement with OSL-revolving adapter calibration.
Figures 5 and 6 offer a comparison of the measurement of an independent DUT using LRL-calibration (Figure 5) and the RPC-N Revolving Calibration Adapter (Figure 6). The DUT is a long microwave cable with a 50 Ω termination and thus a typical application. The results show that OSL-revolving calibration is sufficient for normal requirements (not for precision standards).
Ruggedness is particularly important to users who want to use the revolving calibration adapter when working in the field, so during evaluation the adapter was drop tested. Different heights of up to 3.5 m, with different points of impact of the adapter on the ground, were tested. After testing, the mechanical and electrical properties remained unaltered.
The ingenuity of the RPC-N Revolving Calibration Adapter is its combination of good mechanical and electrical properties, excellent performance in industrial applications, and ease of use. A particular advantage is the facility for verification directly after calibration. Calibration using the revolving calibration adapter is considerably faster, than calibration with single standards, making it suitable for engineers who want an uncomplicated method for calibrating their network analyzers.
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