Figure 1. Noise Figure Measurement Representative Diagram.
is the equivalent hot temperature of the noise source and the cold temperature is assumed equal to a room temperature of 290 K = T0
(per IEEE definition). By making a noise figure measurement of the receiver itself (Frcvr) and of the system (Fsys
), it is possible to deconvolve the DUT noise figure with the familiar Friis' equation (Eq. 2) below: 
Here the DUT gain, G, could be measured separately (via S-parameters) or it could be determined from the change in measured noise powers during calibration and measurement. One advantage the hot-cold noise figure measurement method had was that no absolute power calibrations were needed (all based on ratios). This was particularly important in the past, when wide dynamic range power measurements over large bandwidths were more difficult.
However, because of the signal levels and bandwidths involved, accurate noise source calibration (for Th) is challenging and normally left to only a few metrology laboratories. In addition to issues in calibrating the noise source, a larger challenge occurred when the match of the noise source changed between the hot and cold states . This could lead to large errors, particularly as the DUT input match worsened. If one wanted to perform a number of additional measurements, these errors could be partially corrected using a source correction method (e.g., ).
Cold-Source Noise Figure Measurement Method
cold-source noise figure measurement method was developed to eliminate the requirement for a multi-state noise source, which would allow the use of a simpler, better controlled noise source (nominally a termination at room temperature). In this case, the noise figure is found from a more easily populated equation (Eq. 3), but it has some subtleties:
• k = Boltzmann’s constant
• N = noise power added
• G = gain
• B = bandwidth
calculate the noise figure, there a few key steps to accomplish. First, an absolute noise power is now required (numerator N), so a means of a receiver power calibration is needed. Anritsu’s highly accurate power calibrations and very linear broadband receiver facilitate this. Other methods are possible to determine added noise power, including the use of a calibrated hot noise source (only during the calibration step). In both cases, an absolute power reference is being created. Second, 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. This bandwidth value may also be determined in the absolute power calibration step, if a noise source is used. Third, to isolate the noise figure of the DUT, the noise contributions of the receiver must be taken into account. As with the hot-cold method, a measurement of receiver noise is required. This time only a cold source is attached to the receiver input. Taking the receiver noise into account, Eq. 3 above can be re-expressed in the form below (Eq. 4):
A few things are immediately obvious. 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). These relationships will be discussed throughout this application note, but it is worth remembering the approximate dependence.
Another potential source of errors in the noise figure measurement is the interaction of the receiver with the DUT output. If the receiver noise power is strongly affected by the source impedance, then significant errors could be encountered by not accounting for this interaction. This is accomplished by error correcting with the receiver noise parameters. To understand the relationship (e.g., ), recall that the noise response of a device (receiver in this case) may be characterized by four parameters represented by Fmin
, and Γopt
(the latter is complex and counts as 2 parameters). Fmin
is the minimum noise factor of the device when presented with an optimum source match (given by Γopt
) and Rn
is a sensitivity parameter describing the reaction of noise factor to changes in source match. The net noise factor of the device is then given by (Eq. 5):
is small relative to the system impedance Z0
, then the net noise factor is relatively insensitive to changes in the source reflection coefficient, Γs
. Commonly, the effective receiver Rn
is small and the dependency is reduced since the receiver is normally a matched/feedback amplifier with some loss in front of it from cables, switching, etc. The noise circles of an example, typical receiver structure are shown below at 50 GHz in Figure 2. Even if the DUT output is very poorly matched (|S22| >– 1 dB), the effective receiver noise figure only changes by ≈ 0.65 dB from the minimum. Assuming the DUT has ≈ 10 dB of gain and ≈ 2 dB noise figure, this would only add 0.1 dB of uncertainty. If the DUT had ≈ 20 dB of gain and ≈ 2 dB noise figure, this would only add 0.01 dB of uncertainty. With many common receivers, even less sensitivity is displayed although the effect is, of course, a function of which pre-amplifiers are used in the composite receiver.
If further reduction in uncertainty is desirable, it is relatively straightforward to incorporate additional correction by including three additional measurements during the receiver noise calibration step (e.g., ). They are measuring the noise power with three reflect standards in addition to the cold noise source. Here one is just drawing on the long history of noise parameter measurement to help with an increase in receiver correction.
One way of measuring noise power is a dedicated noise receiver with a large, variable gain system and appropriate filtering. This can be expensive and cumbersome. Typically, these configurations have wide bandwidths, which can lead to problems with narrowband devices; errors can result if the noise measurement bandwidth is close to the DUT bandwidth and images.
NF Measurement Procedure
Figure 2. Variable Impedance Effects on Example Receiver Noise Figure.
The noise figure measurement procedure consists of a few relatively straightforward steps:
Step 1. Measurement of DUT gain or S-parameters (over an appropriate frequency range and at an appropriate power level)
Step 2. Perform an optional user power calibration over the frequencies of interest to optimize receiver calibration accuracy
Step 3. Assembly of composite receiver for the noise figure measurement
Step 4. Receiver calibration (transferring the traceable power accuracy to the receiver)
Step 5. Basic setup (frequency range, number of points, …)
Step 6. Recall DUT gain and receiver calibration data
Step 7. Noise calibration (measurement of the receiver noise power so it can be removed from the calculations)
Step 8. DUT measurement
- Test Preparation / Setup (Steps 1-4)
The first pre-requisite for performing a noise figure measurement is obtaining DUT gain or S-parameter data. The process of measuring DUT gain or S-parameters and saving to a *.s2p file is a standard VectorStar operation and will not be covered in this application note. For information on these topics, please see the VectorStar MS4640A Series VNA Measurement & Calibration Guide. It is important to ensure the data obtained includes the frequencies of interest for the noise figure measurement and that the device is not in a compressed state when the measurements are made.
Figure 3. Basic configuration for b2 composite receiver calibration.
The second pre-requisite is to perform a b2 receiver calibration; see Figure 3 for basic configuration. The receiver calibration must be made on the composite receiver. The required composite receiver gain is somewhat dependent on the DUT, but 40 dB of gain is a reasonable starting point. The noise figure of the composite receiver itself is generally not as critical, but less than 5 dB or so is generally desirable. If using cascaded amplifiers, the amplifier closest to the DUT dominates the composite receiver noise figure. Include a filter to isolate the LO harmonic of interest. The VectorStar MS4640C series uses harmonic downconversion. The fundamental frequency range is 2.5 to 5 GHz and 5 to 10 GHz.
It is recommended, but not required, to perform a power calibration on Port 1, prior performing the receiver calibration. The power calibration requires a power meter and sensor. Both the power and receiver calibrations are standard VectorStar operations and will not be covered in this application note. For information on these topics, please see the VectorStar MS4640A Series VNA Measurement & Calibration Guide
. It is important to ensure the power calibration and receiver calibration are performed at the power level expected at the output of the DUT and the resulting power level through the user supplied amplifers in the composite receiver does not put the VectorStar receiver into compression; the VectorStar b2 receiver 0.1 dB compression point (P0.1dB) is ~–15 to –5 dBm, depending on frequency. In addition, the calibrations need to include the frequencies of interest for the noise figure measurement.
Instrument Setup (Step 5)
Select the desired frequency range.
# of Points is the number of frequency steps within the sweep range.
# RMS Points is the number of measurements per sweep point used in the noise power computation. Trade-offs can be made on measurement speed versus data jitter by adjusting the measurement number of RMS points.
IFBW should be the same as that used in the receiver calibration.
Temperature is nominally the temperature of the cold termination used during the measurements and defaults to 290 K, specified by the IEEE definition.
Measurement Procedure (Steps 6 - 8)
Return to the NF Setup menu. Here three tasks needed completed.
✓ Get DUT S-Param Data – This will be a *.s2p file. See Figure 4.
✓ Recall Recevier Calibration – This will be a *.rcvr file. A reminder will be shown to ensure the calibration was performed on the composite receiver. See Figure 5.
✓ Perform Noise Calibration – A diagram will be shown as a reminder of the calibration setup. See Figure 6.
After completing each task, the menu button will display a checkmark ☑.
Figure 4. Retrieving *.s2p DUT S-parameter file.
Figure 5. Recalling the composite receiver calibration, a *.rcvr file.
Figure 6. Performing a noise calibration.
Depending on how many frequency points are used, noise calibration may be the most time-consuming of the setup/calibration steps (allow on the order of 0.2 seconds to 2 seconds per frequency point depending on IFBW, averaging, and number of RMS points). If the frequency range is changed after calibration, interpolation and extrapolation of both the noise calibration data and DUT S-parameter data will be employed. Uncertainties may degrade, particularly if extrapolation is invoked by going outside the original frequency range of either step.
Insert the DUT and begin measuring noise figure.
NF response traces include: Noise Figure, Noise Temperature, and Noise Power.
Figure 7. Basic configuration for noise figure measurement.
Figure 8. Example noise figure trace.
To provide a measurement example, the NF of a Mini-Circuits ZJL-6G+ low power amplifier was measured between 3.5 - 4 GHz.