Bilateral comparison on transmission coefficient measurements between TÜBİTAK UME and SASO NMCC

This paper describes a bilateral comparison between two national metrology institutes, National Metrology Institute of Turkey (TÜBİTAK UME) and National Measurement and Calibration Center at Saudi Standards, Metrology and Quality Organization of the Kingdom of Saudi Arabia (SASO NMCC). The aim of the comparison is to check the newly installed SASO NMCC radio frequency and microwave laboratory infrastructure and assess compatibility between two institutes. Measured quantity of the comparison is transmission coefficient of fixed attenuators. The comparison was carried out in three months with 3 dB, 6 dB, 10 dB, 20 dB and 30 dB fixed attenuators. Measurements were performed using commercial Vector Network Analyzer in controlled environmental conditions. Two laboratories were used same measurement technique called as short, open, load and through. The analysis of the results provides good agreement between TÜBİTAK UME and SASO NMCC as a conclusion.


Introduction
National Measurement and Calibration Center of the Kingdom of Saudi Arabia (SASO NMCC) newly installed radio frequency (RF) and microwave laboratory.The laboratory frequency range is between 100 kHz to 50 GHz with type N, 3.5 mm and 2.4 mm connector types for scattering (S) parameters measurement.S-parameters are transmission coefficient and reflection coefficient with measurement range from 0 dB to -60 dB and from -1 to +1 respectively.In order to check S-parameters measurement infrastructure and establish international coherence of SASO NMCC, the National Metrology Institute of Turkey (TÜBİTAK UME) and SASO NMCC have organized and carried out a bilateral comparison on transmission coefficient measurement in 2016.Additionally, it is asked to measure reflection coefficient of the travelling standards.TÜBİTAK UME was acting as a Corresponding author: handan.sakarya@tubitak.gov.trpilot laboratory for the bilateral comparison and provided a technical protocol to define the measurement and organisational details according to BIPM guideline [1].TÜBİTAK UME provided five travelling standards have type N connectors and traceable to the International System of Units (SI).The comparison was carried out in three months with 3 dB, 6 dB, 10 dB, 20 dB and 30 dB fixed attenuators.In this paper, results of this comparison exercise using attenuators are described.

Travelling standards and time schedule
Travelling standards belonging to the TÜBİTAK UME and identification of each fixed attenuator described in Table 1.These standards were chosen for their high accuracy and stability in time.

S-Parameter measurement method and results
S-parameter measurements were made using commercial vector network analyzers (VNA) belonging to the TÜBİTAK UME (Rohde&Schwarz ZVA50) and SASO NMCC (Keysight N5225A) in controlled environmental conditions.VNA calibration was carried out using Agilent 85054B type N calibration kit with short-open-load-through (SOLT) calibration method at both of the NMIs.Male and female end of the travelling standards were connected to port 1 and port 2 of the VNA respectively for all measurements reported here.
Results were delivered to the pilot laboratory in the format of real, imaginary and logarithmic (dB) value.The uncertainties of measurement were calculated according to JCGM 100 "Guide to the expression of uncertainty measurement" and "EA-4/02 expression of the uncertainty of measurement calibration" documents for the coverage probability of approximately 95% [2,3].Since VNA was used in the measurements, the uncertainty was calculated in accordance with the EURAMET guidelines for VNA [4].The uncertainty in S-parameter measurements was evaluated by using excel macro software developed by TÜBİTAK UME which is including uncertainty components, i.e. repeatability, reproducibility, VNA non-linearity, mismatch, cable stability etc.
Approved measurement frequencies and attenuation values for the comparison are 50 MHz, 100 MHz, 300 MHz, 500 MHz, 1 GHz to 18 GHz with 1 GHz step and 3 dB, 6 dB, 10 dB, 20 dB, 30 dB, 40 dB, 50 dB, 60 dB respectively.Although, attenuation values at aforementioned frequencies were measured and reported to the pilot laboratory, the uncertainty budget declared only for 50 MHz, 1 GHz, 10 GHz and 18 GHz frequency point measurements.Also reflection coefficient measurement results of the 40 dB, 50 dB and 60 dB attenuators were not declared because the connection order of the attenuators was not defined in the protocol.
Comparison reference values "x ref " (Eq. 1) and associated uncertainties "U ref " (Eq.2) were calculated using the measurement results of TÜBİTAK UME.
Measurement results of the laboratories and comparison reference value with expanded uncertainties were given in Table 2. Degrees of equivalence, DoE, is calculated by subtracting the reference value from the each measurements (Eq. 3) and its uncertainty is calculated according to the Equation 4. Degrees of equivalence with respect to the reference value of each measurement can be found in Table 3. DoE for 1 GHz and 10 GHz of 10 dB and 60 dB fixed attenuators respectively are given in Figure 1.

Conclusion
The results from a bilateral measurement comparison of fixed attenuators' transmission coefficient, performed between TÜBİTAK UME and SASO NMCC, are reported. .The comparison involved two port devices have type N connector as travelling standards and completed within three months.All the results are presented at Table 2 and Table 3 with corresponding uncertainties are compatible within the stated uncertainties.As stated in Table 3, while calculated maximum DoE value is lower than ±0.075 dB up to 50 dB fixed attenuator, this value is increasing ±0.34 dB for 60 dB fixed attenuator.Finally, the comparison is regarded as successfully and the newly installed SASO NMCC radio frequency and microwave laboratory S-parameters measurement infrastructure is sufficient and measurement results of two institutes are compatible.
Participant laboratory measurement result U lab : Participant laboratory measurement uncertainty

Table 2 .
Measurement results of the laboratories and comparison reference value with expanded uncertainties.

Table 3 .
DoE with respect to the reference value.