Noise Figure FAQ for Network Analyzers

Last revision: 3/16/2009
Got ideas for other noise questions? Send them to david_ballo@agilent.com

Measurements

Q. Why is the PNA-X’s measurement technique more accurate than the Y-factor method?
A. The PNA-X method eliminates two large sources of error: mismatch and noise-parameter effects. Mismatch errors are removed with standard vector error correction, and noise parameter effects are removed by using an impedance tuner during the noise figure measurements to correct for imperfect system source match.

Q. Can the PNA-X measure high-gain LNAs?
A. Yes, the PNA-X can measure amplifiers with gain (dB) plus noise figure (dB) of around 60 dB. For higher values, it is recommended to simply add an attenuator to the output of the amplifier. This should have a negligible effect on noise figure accuracy.

Q. Can the PNA-X measure lossy devices? What about low gain plus noise figure devices?
A. The cold-source technique works well in measuring lossy devices. We have accurately measured a 40 dB attenuator, which cannot be done with the Y-factor technique. The technique also works well with low gain, low noise figure devices. In these cases, it may be necessary to increase the number of noise averages to minimize trace noise and improve measurement accuracy.

Q. Can the PNA-X accurately measure noise figure of devices with poor input or output match?
A. Yes. The combination of vector error correction for measuring S-parameters and source-match-correction for measuring noise power means that the match of the DUT has very little contribution to the overall measurement error.

Q. What bandwidths are available in the noise receivers?
A. 800 kHz, 2 MHz, 4 MHz, 8 MHz, and 24 MHz. The default bandwidth is 4 MHz.

Q. Does it matter which noise bandwidth I use?
A. In general, the wider the bandwidth, the lower the jitter of the noise measurement (lower jitter is better). However, the bandwidth of the instrument should always be narrower than the bandwidth of the DUT. For narrowband devices, it may be necessary to use one of the narrower noise bandwidth settings of the PNA-X.

Q. Does the PNA-X indicate when my amplifier has too much gain?
A. Yes, there are two indications. One is when the IF ADC over-ranges, and the other is when the front-end detector in the noise receiver detects excessive RF power. These indicators will show on the screen momentarily during overload conditions. Temporary overload sometimes occurs when measuring unshielded amplifiers, for example, when a wireless LAN or mobile phone signal is picked up and amplified. If the overload condition shows while the noise-receiver gain is at the lowest setting (0 dB), then attenuation on the output of the amplifier is needed.

Q. If I get an overload indication, is the entire measurement bad?
A. If the overload indicator is on throughout the measurement, then the NF data is suspect and should not be trusted without further investigation. Since the filters in the noise receivers are well past the front-end amplifiers, an interfering signal can cause the front-end amplifiers to compress, which can distort the noise measurements. If the overload indicator only comes on briefly, then the data away from the interfering signal may be OK, but again, caution is urged. Often there will be interfering signals that cause spikes in the measured results, but do not trip the overload detectors. For example, signals from mobile phones and WLAN routers often sneak into unshielded power cables or onto PC boards. In these cases, it is likely that the data around or in-between the spikes is accurate.

Q. Can I measure noise figure and S-parameters at the same time without throwing a mechanical switch?
A. Yes. Noise-figure measurement channels support S-parameters as well as noise figure, effective input temperature, and noise-power parameters (DUT and system relative noise power [output power above kTB in dB] and DUT and system noise-power density [output power in dBm/Hz]).

Q. Can I measure noise figure, S-parameters, and IMD in one setup?
A. Yes, this can be done by setting up two measurement channels: one for S-parameters and noise figure, and a second one for IMD. The IMD channel configuration is likely to have different switch or attenuator settings, so single sweep mode must be used to prevent excessive wear on the switches or attenuators. Additional measurement channels can be set up for other measurements like harmonics or phase-versus-drive. All of the measurement channels and their associated calibrations can be saved in one instrument state.

Q. Are the noise receivers used for S-parameter measurements?
A. No, the noise receivers are only used for measuring noise power, which is then used to calculate noise figure, Teff, etc. When measuring S-parameters in a noise-figure channel, the standard measurement receivers are used, and the noise receivers are placed in a “protection” mode to prevent potential damage due to large CW signals.

Q. Can I measure the ENR of a noise source?
A. Yes. In addition to measuring noise figure, the PNA-X can also measure calibrated noise power, either as an absolute power value or relative to kTB (normalized to a 1 Hz bandwidth). The results can be expressed in log format or as a noise temperature. From the noise power measurements of a noise source in its on (and optionally off) states, one can easily calculate ENR. Using the trace equation feature, one can define a trace with the right math to directly display ENR either using a single sweep for T-hot and calculating T-cold based on the ambient temperature [for example, ENR (dB) = 10*Log[(T-hot/290) - (297/290)], or by using the active trace for T-hot and a memory trace for T-cold [ENR (dB) = 10*Log[(T-hot/290) - (T-cold/290)].

Q. Does the PNA-X support noise figure measurements on mixers or converters?
A. Not at this time. We plan to add this capability in a later firmware release.

Q. Can I de-embed a network while measuring noise figure?
A. Not at this time. We plan to add this capability for noise-figure channels in a later firmware release. Note that de-embedding is fully supported in standard S-parameter measurement channels.

Q. Can I control the number of impedance states used when making a noise figure measurement?
A. Yes. The minimum number of impedance states is four, but the user can select up to seven states for increased accuracy.

Q. Can I manually control the noise measurement switches?
A. Yes, they can be controlled from the Path Configuration dialog box. For instruments with the noise-figure option, there is an additional tab for setting the noise hardware. This is very useful when both a standard S-parameter and a noise figure channel are present and continuous triggering is desired for both channels. In the S-parameter channel, the port 1 bypass switch should be set to “external”, and the port 2 switch should be set to “noise receiver”. Note that the ECal configured as an impedance tuner will be in series on the port 1 side, giving an additional 4.5 dB of loss to any S-parameter measurements.

Q. What formats are available for saving my measurement data from a noise figure channel?
A. Noise-figure-related and S-parameter data can be saved as a comma-separated-variable file (filename.prn) or as a Citifile (real and imaginary). In addition, S-parameter data can also be saved in an .s2p file.

Q. Does the PNA-X compensate the port power levels to correct for the internal loss of the noise figure hardware?
A. Yes and no. On the port 2 side, where all of the lossy elements are within the instrument, the delivered port power matches the value shown on the user interface. In other words, the PNA-X adjusts the power levels to compensate for the loss of the bridge, switches, and cables associated with using the noise receivers on port 2. On the port 1 side, where the loss of the impedance-tuner ECal may vary from module to module, the PNA-X does not compensate the power reading. Therefore, the delivered power at the test port is typically about 4.5 dB lower than what the user sets. A source calibration (using a power meter) can be done to eliminate this offset between the requested and actual port powers.

Q. How many sweeps does it take to perform a noise measurement?
A. It depends. For noise figure, the total number of sweeps is two (for the forward and reverse S-parameters) plus the number of sweeps corresponding to the number of different impedance states that the ECal module is set to. The default number of impedance states is four, so the default number of sweeps for a noise figure measurement is 2 + 4 = 6 sweeps. For noise power measurements, only two sweeps are required: one for taking a noise power reading with the noise receivers, and one for measuring the output match (S22) of the DUT using the standard receivers.

Q. Can I measure the noise parameters of my device?
A. Not directly. Agilent’s source-correction technique is based on mathematics using the noise correlation matrix. Noise parameters can be calculated from this matrix, but they tend to be noisy, and Agilent does not provide specified or typical noise parameter accuracy. The accuracy of the noise parameter data degrades as gamma-opt gets near the outer portion of the Smith chart. This is because the cluster of source impedances we use to measure noise figure is around the center part of the Smith chart (typically gamma of 0.5 or less). This allows us to calculate 50-ohm noise figure very accurately, but outside of gamma of 0.5 or so, the accuracy of Fmin might degrade to several dB. Similarly, the accuracy of gamma-opt and Rn degrades as gamma-opt moves away from the central portion of the Smith chart.

Q. Does the PNA-X provide noise-correlation matrix values?
A. Yes, they can be saved to an ASCII file (“filename.nco”) with a similar format to an .s2p file.

Q. Are there any commercial noise-parameter systems that use the PNA-X?
A. Yes, Maury Microwave has written a PNA-X driver that can be used with their mechanical- and electronic-tuner-based noise parameter systems. They also support the PNA-X for their load pull systems. For noise parameter systems, the PNA-X is used for both S-parameter and noise power measurements. A noise source is used for calibration, but there is no need for an ECal module used as an impedance tuner, as the Maury tuners provide the necessary changes in source impedance. With recent changes to their software, Maury has achieved over two orders of magnitude improvement in measurement speeds, while at the same time, improving the accuracy of the noise parameter measurements.

Calibration

Q. Is it necessary to use noise averaging during calibration?
A. It is not required, but it is a good idea to use noise averaging during the calibration to produce a clean cal. Noise that is present in the calibration cannot be removed in subsequent measurements. A noise-average value of 10 or more is recommended during calibration. For the measurement, this value can be lowered if faster measurements are desired, at the expense of more trace noise and less accuracy.

Q. Can the PNA-X measure devices in lossy fixtures?
A. Yes, the effect of a lossy fixture can be removed by calibration. This includes wafer probes.

Q. Can the ECal module that is used as an impedance tuner also be used to perform the 2-port calibration?
A. No. A separate ECal module or mechanical cal kit must be used for the S-parameter portion of the calibration process.

Q. Which ECal modules are supported with the noise figure option?
A. The following ECal models are supported: 85092, 85093, N4431, N4432, N4433, N4690, N4691, N4692, N4693, N4694, N4696

Q. Do I need the noise source during the measurement?
A. No, the noise source is only used during calibration. The cold-source technique does not use an excess noise source during measurements.

Q. Which noise source do you recommend?
A. We recommend the Agilent 346C noise source, which has a 16 dB nominal ENR and covers the full frequency range of the PNA-X. The 346B is also a good choice for coverage to 18 GHz.

Q. Can the 346A noise source be used?
A. Yes, the calibration can be performed with a 346A source, but the lower ENR value (6 dB) will give a less precise measurement of the PNA-X’s noise receiver compared to using a 346B or C model. Using more noise averages will help overcome this issue.

Q. Does the PNA-X noise figure option support N4000A series Smart Noise Sources (SNS)?
A. Not at this time. We are considering adding this capability in the future with a USB-to-SNS interface box. Please let us know how important this is for you.

Q. What temperature value should I use during the calibration?
A. This value should represent the average temperature of all of the components looking into test port 1. A good starting value is the ambient temperature, which is typically 298K. Although the ECal is heated internally to 304K, the loss between the ECal and the DUT generally negates its temperature rise above ambient. There might be a little heating from the instrument, so 299K or 300K might be a better estimate. Note that the difference between using 298 and 300K is only about 0.7%, which equates to a change in noise figure of only .03 dB.

Q. During calibration, is the match of the noise source measured in its hot and cold state?
A. Yes.

Q. After calibration, can I change the gain setting of the PNA-X’s noise receiver without having to re-calibrate?
A. Yes. During calibration, all three gain stages are measured (0, 15, 30 dB), so they can be changed after the calibration according to the gain of the DUT.

Q. Can I calibrate for non-insertable devices?
A. Yes.

Q. Can I view the error terms from a noise figure calibration in the Cal Set Viewer, including the gamma terms of the ECal module used as a tuner?
A. Yes.

Q. Can I calibrate my system if the noise source does not mate directly to test port 2?
A. Yes. The calibration can remove the effect of an adapter used to connect a noise source during calibration. An extra 1-port calibration is required at the point where the noise source is connected in order to align the noise cal plane and the 2-port cal plane.

Q. How do I calibrate for on-wafer measurements when I can’t connect a coaxial noise source at the on-wafer calibration plane?
A. The firmware allows the user to perform the characterization of the PNA-X’s noise receiver at any point in the coaxial portion of the test setup. The calibration routine adds an extra 1-port calibration so that the noise-characterization plane can be extended to the on-wafer calibration plane. The calibration routine handles the cases when an adapter is needed to connect the noise source and it is left in place for the on-wafer cal, or the adapter is removed after the 1-port cal, in order to connect the wafer probes to the test system.

Q. When measuring devices with two female connectors, I use an ECal module for the 2-port cal. However, when I try to do the 1-port cal on the adapter used to connect the noise source to the test system, my ECal does not show up as a cal kit choice. Why not?
A. Only ECals with one or more connectors that will mate to the adapter will show up. If you are using a female-female ECal module, you cannot use it to perform a 1-port cal on an adapter with a female connector. Instead, you need an ECal with at least one male connector. The most flexible ECal module is one with both male and female connectors, which will handle all different combinations of sexes. When using this type of ECal for female-female devices, the calibration routine is broken into three steps instead of one. Step one uses the male connector of the ECal for port 1, step two uses the male connector for port 2, and step three requires a thru connection.

Q. Are the noise parameter effects of the PNA-X’s noise receivers removed from measurements?
A. Yes. When two ECal modules are used during calibration, the one that is used for the 2-port calibration portion is also used to present a set of variable source impedances to the noise receivers to characterize their noise parameters. This is done because this ECal is closest to the noise receivers, and therefore provides a wider range of impedance values due to lower loss. Note that the ECal that will be used as an impedance tuner during measurements is not used as an impedance tuner during calibration if a second ECal module is available. When only the tuner ECal module is present during calibration, then it will be used in combination with the mechanical standards used during the 2-port calibration to provide the variable source impedances needed for a noise-parameter characterization.

Q. Is there a way to verify the absolute accuracy of a noise-figure measurement, using some type of verification kit?
A. Not at this time. However, we are investigating devices and techniques that could be used to verify the absolute accuracy of a noise figure measurement.

Q. Does Agilent have a noise-figure uncertainty calculator that includes the PNA-X’s source-corrected technique?
A. Yes. Contact your local Agilent application engineer to try it out.

Configuration

Q. Can I upgrade my existing PNA-X to add noise figure capability?
A. Yes. For a 2-port unit, order N5242AU-924, and for a 4-port unit, order N5242AU-929. The upgrade can be done at your local service center.

Q. Can I use an ECal module I already own?
A. Yes, you can use your existing ECal module for the S-parameter calibration or for the impedance tuner (but one ECal cannot serve both functions). If the ECal module used for the impedance tuner is female-female, order a 3.5 mm male-male adapter (85052-60014).

Channel Partners

Q. Does Cascade Microtech’s WinCal calibration software for on-wafer measurements include noise figure?
A. No, not at this time.

Q. Does Maury Microwave’s noise-parameter software and tuners work with PNA-X analyzers configured for noise figure?
A. Yes (see above in Measurements section).

Other Applications

Q. Does Agilent’s ICCAP software include the ability to measure noise figure?
A. No, not at this time.

Miscellaneous

Q. What is the noise figure of the PNA-X’s noise receivers?
A. While the internal noise figure is quite good, the loss of the test set components (cables, switches, couplers, etc) degrades the noise figure, especially at higher frequencies. Between 3 and 23 GHz, the noise figure at test port two is around 10 to 11 dB. At 26.5 GHz, it has degraded to about 13 dB