February 15, 2019
The complexity of today’s high-speed designs has created many a challenge for signal integrity engineers developing communications devices and systems. When time-to-market and tight cost parameters are added to the equation, finding the right test solution to meet emerging applications can be the difference between a product performing according to specifications and one that doesn’t make it into mass production.
When it comes to vector network analyzers (VNAs), engineers need an instrument that can effectively conduct noise figure measurements. This has become particularly more daunting due to the proliferation of multi-port devices in emerging designs.
In recent years, numerous improvements have been made in noise figure measurements through better algorithmic understanding of the measurements, more sensitive receivers, and less error-prone methods of processing noise power measurements. One advance is the utilization of the cold source noise figure measurement method, which has benefits compared to the more traditional Y-factor technique.
Cold Source Method
The cold source noise figure measurement method was developed to eliminate the requirement for a multi-state noise source. A simpler, better controlled noise source – nominally a termination at room temperature – can be used in the cold source approach. In this case, the noise figure is found from a more easily populated equation, shown below.
- Because an absolute noise power is required (numerator N) with the cold source method, a means of a receiver power calibration is necessary. Anritsu has developed highly accurate power calibrations and very linear broadband receivers to accommodate this requirement.
- An effective measurement bandwidth (B) is needed. Since measurement bandwidth is largely determined by the digital IF system of the VNA, B can be pre-determined.
- To isolate the noise figure of the device under test (DUT), the receiver’s noise contributions must be taken into account. Similar to the conventional Y-factor method, a measurement of receiver noise is required, however, this approach only requires the cold source to be attached to the receiver input. Taking the receiver noise into account, the equation above can be re-expressed in the form below:
One thing is immediately obvious when looking at these equations. Errors in gain or the noise power measurement will propagate to noise figure on roughly a dB-for-dB basis, if the composite receiver gain is sufficiently high.
Advantages Over Y-factor Method
The cold source approach, which is used in the noise figure measurement capability of the Anritsu VectorStar® VNAs (figure 1), addresses some of the limitations associated with Y-factor. Y-factor uses a noise source that can produce a low noise output power (cold = Nc) and an elevated one (hot = Nh). This noise source becomes the input noise signal to the DUT.
The ratio of measured noise powers in these two states was termed the Y-factor (Y=Nh/Nc) and could lead to a quick calculation of noise figure. By making a noise figure measurement of the receiver itself and the system (DUT + receiver), it is possible to deconvolve the DUT noise figure.
One advantage the Y-factor noise figure measurement method is that no absolute power calibrations are needed; they are all based on ratios. The measurement also is not dependent on DUT gain.
On the other hand, calibration and mismatch errors are a concern when using the Y-factor method. Also, the noise source used in the Y-factor method needs to be periodically calibrated to confirm measurement accuracy. Mismatch errors become a consideration due to the match changes of the source when switching between the hot and cold states. This can result in measurement errors, particularly as the DUT input match worsens and effort is not made to correct.
Cold Source Method
The cold source method simplifies the measurement by only requiring a load at the input of the DUT. A power calibration is used to determine the noise power of the receiver and thus the measurement becomes an absolute noise power difference. Anritsu uses the cold source method as part of its Single Ended and Differential Noise Figure option for the VectorStar that allows the VNA to measure 2-, 3- and 4-port devices in single-ended, differential, and common mode operation with a variety of processing options. The new differential noise figure option adds the ability to perform levels of vector correction in both 2-port and multi-port devices for even greater accuracy, particularly when mismatch is significant.
To learn more, download a new application note from Anritsu entitled Performing Differential Noise Figure Measurements. The educational document explains the two methods, as well as outlines the steps to conducting the measurements using the VectorStar VNA.