NOISE SOURCE CALIBRATION

Electronic noise sources are calibrated using a variety of instruments including spectrum analyzers, noise figure meters, signal analyzers and network analyzers. Any suitable instrument must have a sensitive receiver that can measure noise levels over an adequate bandwidth with sufficient linearity and dynamic range. For automated measurements, the instrument should also be capable of controlling a two-level noise source while simultaneously measuring the received power for each noise level.

Table 2

Many older-model noise figure analyzers, such as the Agilent N8970 series, are essentially spectrum analyzers with additional built-in functions for measuring noise power levels and performing noise figure measurements. Newer signal analyzers, which combine the traditional functions of a spectrum analyzer with a wide range of other measurement capabilities, are rapidly becoming the tools of choice for performing noise measurements. Some of these newer signal analyzers are shown in Table 2.

The Y-factor method is a well-established measurement technique for calibrating noise receivers and noise sources. It uses a pair of noise sources or a single source that provides two different noise levels. When the Y-factor is expressed linearly, it is equal to the hot noise power divided by the cold noise power, as measured by the receiver.

When a receiver is calibrated, its noise figure is obtained by performing a Y-factor measurement with a calibrated noise source connected to its input. The measurement is usually repeated each time the receiver is used and certainly whenever the measurement bandwidth changes or when the attenuation or gain settings are changed. The amplitude response of the receiver is separately calibrated using a reference signal generator, a reference power detector and/or a set of reference attenuators. Amplitude calibrations are typically performed infrequently according to a maintenance schedule or when the operating environment changes significantly.

After the noise receiver is calibrated, it can be used to perform additional Y-factor measurements and determine the ENR of an uncalibrated noise source. The calibration accuracy of the reference standard is partially transferred to the noise receiver and then partially transferred to the uncalibrated noise source. Analyzing the cumulative effects of measurement uncertainty is one of the more difficult tasks associated with noise source calibration.

Advantages of the Y-factor calibration method include its relative simplicity and its acceptable accuracy, in most cases. Power ratio measurements can be obtained quickly, minimizing the effects of temperature drift in the receiver response. However, basic Y-factor measurements typically do not compensate for mismatch effects and various other sources of measurement uncertainty.

Many studies have focused on analyzing and improving the accuracy of noise source calibrations.20-22 Suggested enhancements can improve calibration results by applying considerably more effort in both measurements and calculations. By compensating for the measured effects of mismatches, the receiver noise parameters and temperature drift, one study achieved a two-sigma calibration accuracy of 0.046 dB for a noise source with an ENR of 5 dB.

The cold source measurement technique was developed as an alternative to the Y-factor method.23 Measurements are performed using a single reference noise level provided by a matched termination at room temperature. The cold source method relies on knowing the scattering parameters of the DUT.24,25 The accuracy of cold source measurements is generally superior to that of Y-factor measurements when the latter includes mismatch compensation.26 Additionally, the minimum noise figure and the noise parameters of a DUT can be obtained by performing multiple cold source noise figure measurements with different mismatches connected to the DUT input.

Cold source noise figure measurements require absolute noise power measurements, necessitating more accurate receiver calibrations. The measurement bandwidth must be known as well. Whereas the Y-factor method produces both the gain and noise figure of the DUT in one set of measurements, cold source noise figure measurements require independent gain measurements.

Table 3

Because cold source noise figure measurements involve far more measurements and data manipulation than traditional Y-factor methods, they are usually performed using a fully integrated and automated test system. Some newer vector network analyzer (VNA) models include the ability to perform cold source noise figure measurements. Examples of these VNAs are shown in Table 3. At frequencies beyond the operating range of VNAs, Y-factor measurements remain the most common method of performing receiver calibrations, noise source calibrations and noise figure measurements.

EXTENDED FREQUENCY COVERAGE

Most receivers that measure RF noise do so by converting input signals to an intermediate frequency (IF) where they are filtered, amplified and fed to a square-law detector or an analog-to-digital converter. To measure noise at frequencies beyond the limit of a given signal analyzer, an external down-converter can be used. Many general-purpose down-converters are suitable for this task.

If the down-converter uses a fixed local oscillator (LO) frequency, the noise receiver/analyzer is operated essentially the same as when measuring lower frequency signals. To preserve measurement accuracy, the noise figure of the down-converter should be comparable to that of the noise receiver. The down-converter should also provide adequate image rejection and good suppression of spurious signals.

Many down-converters built specifically for extending the frequency range of a noise analyzer are designed to accept a swept-frequency LO signal supplied by the analyzer. A frequency multiplier within the down-converter produces a higher-frequency LO signal. The result is a fixed IF for the down-converted signal fed back to the analyzer. This measurement strategy reduces the number of frequency conversions between the noise source and the IF amplitude detector, resulting in less measurement uncertainty.

Figure 5

Figure 5 Frequency extenders for noise figure analyzers.

Down-converters designed to extend the frequency range of noise figure analyzers are typically offered with a matching calibrated noise source. Available models provide full-band coverage up to 270 GHz. Full waveguide band coverage is generally provided with available models covering frequencies up to 170 GHz. An example of this setup is shown in Figure 5.

CONCLUSION

Fundamental aspects of noise generation and measurement remain firmly grounded while noise sources and instrumentation advance toward THz capabilities. At frequencies up to about 110 GHz, noise sources and the instrumentation required to measure noise are available from multiple manufacturers. At higher frequencies, industry support is somewhat harder to find. Fortunately, newer sources produce noise signals beyond 200 GHz and newer technologies may soon provide high-quality noise sources at THz frequencies. Meanwhile, high performance down-converters are available to extend the frequency coverage of existing noise measurement tools.

References

  1. ELVA-1, Web: https://elva-1.com/products.
  2. Eravant, Web: https://www.eravant.com/products/noise-sources.
  3. "Noise Figure Analyzers and Noise Sources," Keysight Technologies, Web: https://www.keysight.com/us/en/products/noise-figure-analyzers-noise-sources.
  4. "Calibrated Sources," Noisecom, Web: https://noisecom.com/products/calibrated-sources.
  5. NoiseWave, Web: https://noisewave.com.
  6. E. Maxwell and B. J. Leon, “Absolute Measurement of Receiver Noise Figures at UHF,” IRE Transactions on Microwave Theory and Techniques, Vol. 4, No. 2, April 1956.
  7. "Precision Calibration Noise Calibration System," Maury Microwave, Web: https://maurymw.com/product-category/precision-calibration/noise-calibration.
  8. C. T. Stelzreid, “Microwave Thermal Noise Standards,” IEEE Transactions on Microwave Theory and Techniques, Vol MTT-16, No. 9, September 1968.
  9. "Calibrated Thermal Targets," TK Instruments, https://www.terahertz.co.uk/tk-instruments/products/calibratedthermaltargets
  10. W. W. Mumford, “Broad Band Microwave Noise Source,” U.S. Patent No. 2,706,782, April 1955.
  11. Y. Chen, M. Yamaguchi, M. Wang and X.-C. Zhang, “Terahertz Pulse Generation From Noble Gases,” Applied Physics Letters, Vol. 91, No. 25, December 2007.
  12. B. Vidal, “Broadband Photonic Microwave Noise Sources,” IEEE Photonics Technology Letters, Vol. 32, No. 10, May 2020.
  13. Y. Zhang, W. Liu, Y. Guo, J. Liu, Z. Jia, Y. Sun, A. Wang and Y. Wang, “Generation of Flat Terahertz Noise by Mixing Incoherent Light Fields,” Photonics, Vol. 2023, No. 10, p. 778.
  14. N. J. Keen, “Avalanche Diode Noise Sources at Short Centimeter and Millimeter Wavelengths,” IEEE Transactions on Microwave Theory and Techniques, Vol. 24, No. 3, March 1976.
  15. M. M. Radmanesh and J. M. Cadawallader, “Solid State Noise Sources at mm-Waves: Theory and Experiment,” Microwave Journal, October 1991.
  16. S. J. Mukhopadhyay, S. Kanungo, V. Maheshwari and M. Mitra, “Terahertz IMPATT Sources Based on Silicon Carbide,” Lecture Notes in Electrical Engineering, Vol. 727, February 2021, Web: https://link.springer.com/chapter/10.1007/978-981-33-4489-1_5.
  17. R. S. Roeder, M. C. Smith, L. P. Dunleavy and S. . Lardizabal, “Variable Microwave Cold/Warm Noise Source,” U.S. Patent No. 6,439,763, August 2002.
  18. “Fundamentals of RF and Microwave Noise Figure Measurements,” Keysight Technologies, Application Note, 2019.
  19. “Noise Figure Measurement Accuracy: The Y-Factor Method,” Keysight Technologies, Application Note, 2021.
  20. J.-M. Collantes, R. D. Pollard and M. Sayed, “Effects of DUT Mismatch on the Noise Figure Characterization: A Comparative Analysis of Two Y-Factor Techniques,” IEEE Transactions on Instrumentation and Measurement, Vol. 51, No. 6, 2002.
  21. L. Belostotski, “A Calibration Method for RF and Microwave Noise Sources,” IEEE Transactions on Microwave Theory and Techniques, Vol. 59, No. 1, January 2011.
  22. K. Wong, J. Gorin and G. Lu, “Quantifying the Error Contribution of Noise Parameters on Y-Factor Noise Figure Measurements,” ARFTG Microwave Measurement Conference, 2017.
  23. N. Otegi, J. M. Collantes, and M. Sayed, “Cold-Source Measurements for Noise Figure Calculation in Spectrum Analyzers,” ARFTG Microwave Measurement Conference, 2006.
  24. “The Cold Source Technique for Noise Figure Measurements,” Rohde & Schwarz, Application Note, 2021.
  25. “Performing Differential Noise Figure Measurements,” Anritsu, Application Note, 2018.
  26. J. Dunsmore, “Noise Figure Verification of Y-Factor and Cold Source Methods,” International Conference on Noise and Fluctuations, 2017.