Noise Figure Measurement     

The Noise Figure Meas submenu controls the measurement of the noise figure of a DUT. This measurement requires option R&S ZVAB-K30.

Noise Figure Measurements

• Noise Figure Setup Guide opens a dialog which helps you perform all the necessary steps for a noise figure measurement in the right order. All configuration and calibration dialogs for the noise figure measurement can be accessed from the setup guide.  

• Define Noise Figure Meas... provides basic parameters for the noise figure measurement. Open this dialog to modify the definitions of the setup guide.  

• Noise Figure Cal... opens a dialog to assess and compensate the internal noise at the NWA receiver and source. Open this dialog to modify the calibration procedure of the setup guide.   

• Correction Off (de)activates the noise figure calibration for the active channel.  

Performing a noise figure measurement

Basic Test Setup and Noise Figure Calculation

The schematic test setup for a noise figure measurement is shown below.

The NWA source provides a superposition of a CW signal and a noise signal. The latter generally consists of the thermal noise NT, 1 and a possible additional contribution ΔN:

PNoise, 1 = NT, 1 + ΔN

The definition of the noise figure FD is based on a pure thermal noise signal (ΔN = 0):

FD = (PSignal, 1 / NT, 1) / (PSignal, 2 / PNoise, 2) = PNoise, 2 / (NT, 1 * GD) where GD = PSignal, 2 / PSignal, 1

ΔN is supposed to be amplified like the CW signal. Hence, the noise signal at the output of the DUT equals to:

PNoise, 2 = GD * (FD * NT + ΔN)

According to the equations above, it is possible to calculate both the gain GD and the noise figure FD of the DUT, if the four powers PSignal, 2, PSignal, 1, PNoise, 1, PNoise, 2 can be measured with sufficient accuracy. The measurement requires two steps:

• Source noise calibration of the driving NWA port --> PSignal, 1, PNoise, 1. This step is required only once for a given channel configuration.

• Transmission measurement of the DUT --> PSignal, 2, PNoise, 2. The analyzer uses two different detector settings to separate the CW signal and the noise signal contributions. The Average detector yields the CW signal power; the RMS detector yields the sum of the CW signal and noise power.

Conditions for the measurement

• Option R&S ZVAB-K30, Noise Figure Measurement, provides the measurement including the necessary (wide-band) IF filters. Internal source attenuators are recommended but may be replaced by an external attenuator.

• The measurement algorithm is based on the assumption that the DUT operates in its linear range so that all input signal contributions are amplified by the same gain factor.

• A power calibration (source power calibration using an external power meter and subsequent receiver power calibration) prior to the noise figure calibration and to the actual measurement is essential for an accurate measurement of the noise power at the NWA receiver.

• The Noise Figure Calibration is important to assess the internal noise figure of the NWA receiver and generator.

• The power calibration and noise figure calibration correspond to a transmission normalization of the test setup. An additional system error correction (S-Parameter Calibration) using an AVG detector calibrates the gain GD and improves the accuracy of the noise figure measurement.

The Noise Figure Setup Guide helps you perform all the necessary steps consistently and in the right order.

Extensions

• The network analyzer provides two alternative measurement procedures for maximum flexibility concerning the signal levels or maximum speed. See Sequential vs. Simultaneous measurement.

• To prevent excess input levels (especially for analyzers without source attenuators), it is possible to loop an external attenuator in-between the NWA source port and the input of the DUT. An external attenuator can also improve the match at the input of the DUT. Proper noise figure calibration ensures that the gain and noise figure of the external attenuator is compensated.

• Conversely, it is possible to use an external low-noise preamplifier in order to reduce the noise figure of the NWA receiver and thus improve the measurement accuracy. The gain and noise figure of the pre-amplifier is also compensated during the noise figure calibration. Refer to the background information in section Noise Figure Calibration to learn more about the relationship between receiver noise figure, preamplifier gain and measurement accuracy.
  A suitable low-noise preamplifier (option R&S ZVAX-B203) is available in combination with the Extension Unit, R&S ZVAX24.  

• Options R&S ZVA-B16, Direct Generator and Receiver Access, improve (i.e. reduce) the noise figure of the NWA source and receiver. On analyzers equipped with options R&S ZVA-B16, it is also possible to measure the noise figure using a single port. Connect the NWA port connector to the DUT and route back the output signal to the MEAS IN connector of option R&S ZVA-B16. On a four-port analyzer, this allows you to measure four DUTs in parallel.

• On four-port analyzers, it is possible to perform two noise figure measurements in parallel using two different port combinations.

• Network analyzers with upper frequency limits in the microwave range have an enhanced coupling attenuation below 1 GHz. This can impair the accuracy of the noise figure measurement.  

Performing a noise figure measurement

Noise Figure Setup Guide

The Noise Figure Setup Guide gives access to all necessary steps of a noise figure measurement. The dialog is similar to a wizard, however, everything is arranged in a single dialog. The panels numbered 1. to 7. suggest a meaningful order for the different steps. In some cases, it is possible to change the order or skip a step entirely (e.g. a calibration may be skipped if a valid calibration for the test setup is already available).

The buttons in the different panes open dialogs or wizards; all of them are also accessible from the menus of the network analyzer. In general, it is advantageous though to open the dialogs from the setup guide, because this will adjust some of the settings to the requirements of the noise figure measurement.  

The setup guide provides the following panels/steps:

1. DUT Port Connection defines the basic test setup, i.e. the input and output ports of the DUT/NWA. With option R&S ZVA-B16, Direct Generator and Receiver Access, it is possible to measure the noise figure at a single port (<DUT Input> = <DUT Output>); see section Basic Test Setup and Noise Figure Calculation.

2. Source Power Calibration gives access to the Source Power Cal dialog for the previously selected DUT Input port and base power. The base power setting overwrites the Channel – Power Bandwidth Average – Power setting as soon as the calibration dialog is opened.   

3. Receiver Power Calibration gives access to the Receiver Power Cal dialog for the previously selected DUT Output port. If a preamplifier is connected between the DUT and the receive port of the analyzer, or if the output signal of the DUT is fed to the MEAS IN connector of option R&S ZVA-B16, Direct Generator and Receiver Access, in order to bypass the coupler and improve the receiver noise figure, the check box should be selected. This activates the input fields for base power and source step attenuator settings (if available), which you may have to adjust to the test setup in the receiver path in order to avoid excess receiver input powers (receiver compression; see data sheet). An active check box has no additional effects.
The base power setting overwrites the Channel – Power Bandwidth Average – Power setting as soon as the calibration dialog is opened. The step attenuator setting overwrites the source attenuator settings for the DUT Input port in the Channel – Power Bandwidth Average – Step Attenuators dialog.

4. Define Noise Figure opens the Define Noise Figure Measurement dialog, providing basic settings for the noise figure measurement.  

5. Noise Figure Calibration opens the Noise Figure Calibration dialog to assess and compensate the internal noise at the NWA receiver and source.  

6. S-Parameter Calibration opens the Calibration wizard for the previously selected input and output ports and the Cal Method. If this step is skipped, the previous power and noise figure calibrations effectively ensure a transmission normalization; the noise figure trace line contains no calibration label. With an additional One Path Two Port or Full Two Port calibration, the system error correction of the gain GD (and thus the noise figure measurement in total) is improved; the trace lines for the calibrated ports indicate Cal. A Full Two Port calibration requires more calibration measurements but provides best accuracy; see section Calibration Types.
Note: An S-parameter calibration is required for S-parameter measurements in parallel to the noise figure measurement. The large IF bandwidth required for noise figure measurements limits the accuracy of the calibration and the subsequent S-parameter measurements; the AVG detector can improve the results.

7. Configure Traces provides a selection of traces which are commonly of interest for the noise figure measurement. The selected traces are created and displayed when the Replace or Add buttons are clicked. The noise figure trace (NF21) is smoothed automatically (label SMO in the trace line). The large IF bandwidth limits the accuracy of the S-parameter measurement; the AVG detector can improve the results. Use the Trace – Measure menu if you wish to modify the trace settings.
To avoid misleading results, S-parameter selection is enabled only after an S-Parameter Calibration has been performed.   

Define Noise Figure Meas...

Provides basic parameters for the noise figure measurement.  

• Detector Meas Time sets the time that the analyzer uses to acquire data with each of the detector settings (Avg, RMS; see Detector). The two detectors are used to separate the CW signal and the noise signal at the output of the DUT. A smaller measurement time enhances the total measurement speed, possibly at the expense of accuracy. With the default measurement time, the great majority of noise powers should be measured correctly. If you reduce the detector measurement time, ensure that the variation of the measurement results does not increase significantly.

• Measurement of Gain and Noise selects a measurement procedure for maximum flexibility concerning the signal levels or maximum speed; see background information below.

• OK activates the noise figure measurement using the current settings. Note that the filter settings are automatically adjusted to the requirements of the noise figure measurement: An IF bandwidth of 5 MHz and a filter with high selectivity is activated. The filters are available if option R&S ZVAB-K30, Noise Figure Measurement, is installed.

Sequential versus Simultaneous measurement

Noise Figure Cal...

Opens a dialog to assess and compensate the internal noise at the NWA receiver and source. The noise figure calibration should be performed after regular power calibration using an external power sensor following the instructions in section Power Calibration. It is required for various reasons (see also Basic Test Setup and Noise Figure Calculation):

• The Receiver Noise Calibration determines the noise figure of the internal NWA receiver and ensures an accurate measurement of the output signals PSignal, 2, PNoise, 2.

• The Source Noise Calibration determines the noise figure and gain of the internal NWA source and provides the accurate source signal contributions PSignal, 1, PNoise, 1.

• If an additional external attenuator is used (see Sequential vs. Simultaneous measurement), the noise contribution of this attenuator must be compensated in addition.

The necessary calibration steps are automatically performed with the channel settings of the active noise figure measurement.

Noise Figure Calibration procedure

• DUT Input and DUT Output denote the input and output ports of the DUT/NWA.

• Gen Atten for Src NCal reduces the source level during the source noise calibration (calibration step no. 2) by the selected value. Increase the attenuation factor to protect a pre-amplifier with sensitive input from excess input levels. In general, a small attenuation factor improves the accuracy of the source noise figure measurement.

• Gen Atten for DUT Meas reduces the source level during the measurement (i.e., after the noise figure calibration). Select an appropriate factor to ensure that the DUT is operated in its linear range. If Gen Atten for DUT Meas differs from Gen Atten for Src NCal, then the source noise calibration is extended by a second calibration sweep which compensates the effect of the additional attenuator (Attenuator Cal).

• IF Gain a configures the receiver path for the reference channels (a-waves). The default setting Low Distortion is appropriate for relatively high RF levels. Changing the default setting can be appropriate e.g. if an external pre-amplifier and a large Gen Atten for Scr NCal value is used.  

• IF Gain b configures the receiver path for the measurement channels (b-waves). The default setting Low Noise is appropriate for relatively low RF levels and ensures maximum dynamic range.  

Correction Off

Activates or deactivates the Noise Figure Calibration in the active channel. Correction Off is available only after a calibration has been performed and applied to the active channel.

With deactivated correction, the analyzer calculates an approximate noise figure, neglecting the influence of internal noise, attenuators etc. This can introduce large deviations between the uncorrected and the corrected traces.