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   N9917A 18GHz Network/ Spectrum Analyser Review by NICHOLAS VINEN Keysight has a whole family of FieldFox instruments which can be optioned up for bandwidths from 4GHz up to 50GHz+. The model we are reviewing is a combination 18GHz Microwave Network/Spectrum Analyser. I f only because the Keysight FieldFox N9917A has such a huge range of features and functions, we imagine that even an experienced RF engineer would have a steep learning curve to become fully familiar with this instrument. But when they do, they’ll find it a very capable instrument indeed! Its main functions (depending on the installed options), are as follows: 1. Spectrum analyser 2. Real Time Spectrum Analyser (RTSA) 3. Vector Network Analyser (VNA) 4. Vector voltmeter 5. Time Domain Reflectometry (TDR) 6. Extended Range Transmission Analysis 7. Interference analysis 8. Cable and Antenna Analyser (CAT) A short description of each of those functions can be seen in a separate panel in this article. For such a potent instrument, the Keysight N9917A is 86  Silicon Chip not particularly large or impressive in appearance. It fact, it is quite unprepossessing. At first sight, it looks like a largish hand-held scope with many buttons, all with a charcoal finish. It can be used as a bench-top instrument, thanks to a stand which swings out from the back. It’s 183mm wide, 295mm tall and 70mm deep and it is fairly heavy at 3kg, no doubt mainly due to its battery. The FieldFox makes a fine spectrum analyser, however its real strengths appear to be in the area of cable, antenna and amplifier testing and in fault-finding. It comes in almost bewildering range of models with different capabilities and bandwidths but even once you have chosen your preferred combination, you will still need to specify from an exhaustive list of options, to get a unit that does exactly what you want. See http://literature.cdn.keysight.com/litweb/pdf/ 5990-9836EN.pdf If you purchase a FieldFox Spectrum Analyser then the Spectrum Analysis and signal generator functions are included. If you purchase the Network Analyser version then siliconchip.com.au Fig.1: return loss and distance-to-fault for a short (~5.5m) section of cable, open-circuit at the far end, from 30kHz up to 18GHz. Fig.2: VSWR vs distance for the same cable, indicating a spike in reflection power at the distance of the open-circuit fault. the VNA Transmission and Reflection functions, including DTF/RL/VSWR measurements are available. Combined SA/VNA units come with the Cable and Antenna Analyser (CAT) function as standard. Everything else is an option. The other available options for the VNA-capable versions include: time domain VNA, QuickCal calibration, 2-port VNA S-parameter analysis, 1-port mixed-mode Sparameters and TDR. Available options for the Spectrum Analyser version include: tracking generator, ERTA, pre-amplifier, interference analyser/spectrogram, channel scanner, RTSA and analog (AM/FM) demodulation. All versions of the FieldFox are also available with the following options: USB-based power measurement, USBbased power vs frequency, built-in power meter, pulse measurement with USB power sensor, remote control, GPS receiver and DC-bias variable voltage source. Our review unit came with all options enabled, giving it pretty much the full range of FieldFox capabilities. Lacking a manual, it only took us about 30 minutes to become familiar with the FieldFox’s user interface and figure out how to use most of the functions. Overall, there- fore, we would have to say that it is quite easy and intuitive to use, especially if you have prior experience with this sort of instrument. Below the 165mm diagonal, 640 x 452 pixel LCD screen are six soft buttons, five mode buttons, the power/standby switch, numeric keypad, jogwheel plus five navigation buttons. At the top of the unit are the two main input/output connectors (N-connectors on this unit, as with most of the FieldFox range) plus SMA connectors for the GPS antenna (if the option is fitted) and the reference/trigger input. Behind small waterproof rubber doors on the left side of the unit are the DC charging port, DC output (for when the bias supply option is fitted), headphone jack (for demodulated audio) plus a small speaker (ditto). At right, behind latching doors, are two USB host sockets, one mini USB device socket, an SD card slot, Ethernet port plus two SMB RF connectors for the reference/trigger output and IF output. The display is suitable for use indoors and outdoors, with adjustable brightness and various different colour schemes that you can choose from, which are set up to suit different situations. We tested it indoors and out and didn’t have any problems viewing the screen. Initial switch-on takes about 60 seconds, while shut Fig.3: time domain reflectrometry analysis for the same cable; this is another different way of finding the same fault. Fig.4: in VNA mode, displaying the real (amplitude) part of four S-parameters plotted against frequency at the same time, for the same cable. The forward and reverse loss plots are almost identical but reflection differs at each end due to different connectors being used. User interface and connectors siliconchip.com.au June 2017  87 Fig.5: differential and common mode reflection parameters for one end of the same cable as shown in Fig.4, plotted simultaneously and over the same frequency range. Fig.6: using either the internal power meter option or USB power meter option is easy; select the frequency, bandwidth and optionally radio standard and the received power level is displayed. down takes around 10 seconds. However, it does have a standby mode which can be initiated in just a couple of seconds and restoring the unit to operation from standby takes just a few seconds. So you would typically only need to boot the unit up once per day and you could leave it in standby between uses. The first step to setting the unit up after switch-on is to press the Mode button which reveals a choice of ten different modes (on our test unit): CAT / TDR, NA (Network Analyser), SA (Spectrum Analyser), RTSA, VVM, Power Meter (USB), Channel Scanner, Pulse Measurements, ERTA or Power Meter (built in). Selecting one of these loads the appropriate “application” which takes a few seconds. In each mode, you change the settings either by pressing one of the dedicated buttons below Mode, to change the frequency/distance range, display scale/amplitude, markers (up to six are supported) or access marker tools such as peak searching. Further settings can also be made by pressing one of the numeric keypad buttons, most of which are labelled with additional functions. These are: Measurements, Bandwidth selection, Sweeping, Measurement Set-up, Calibration, Trace set-up, System settings, Limit lines, Save/Recall, Presets and Run/Hold. The biggest hurdle to operating the FieldFox is understanding which options are available under each of these menus in each mode. Once you know that, it’s pretty easy to figure out how to change the parameters required to achieve your desired results. Fig.9: voltage standing wave ratio versus frequency plot for an antenna on the end of a cable. This provides an accurate means of tuning the antenna for a specific frequency. Fig.10: spectrogram of the 100MHz band centred around 2.4GHz, showing WiFi activity. Spectrogram plots are available in both spectrum analyser (SA) and RTSA modes. 88  Silicon Chip Operation and performance We started out by testing the FieldFox’s fault-finding capabilities. We don’t have any really long cables to test it with, especially not with built-in faults, but it was able to accurately identify the distance to open or short circuits on various cables we tested it with. Fig.1 shows the unit measuring the return loss and DTF of a short coaxial cable, using the screen colours designed for use in direct sunlight. Its reading of 5.5m was very close to the actual length. Note that we reduced the scale of the reading to make it more clear; the default DTF scale goes up to 100m and longer distances are possible, up to 5km. Figs.2 & 3 show the unit measuring the same cable in VSWR fault-finding and TDR mode respectively. Both show siliconchip.com.au Fig.7: single-ended cable loss analysis plot; we’re not convinced that this is an accurate way to measure cable loss but the facility is provided for when you have no other option. Fig.8: insertion loss for the same cable, measured with both ends connected. This tells a very different story and shows the cable and connectors are really only suitable for use up to a couple of Gigahertz. the same fault at around 5.5m, in a different manner. Fig.4 shows the flexibility of the unit when operating as a VNA (Vector Network Analyser). We have connected a series of cables and connectors between its terminals and it is displaying all four of the main S-parameters across the 18GHz frequency span. S12 and S21 show the cable loss in either direction while S11 and S22 show the amplitude of reflections at both ends across the whole frequency range. Analysing a cable isn’t a terribly interesting test case, but this does show the flexibility of the unit in setting up different displays. You can show one, two, three or four parameters on screen at one time and you can choose to display any parameters in any part of the screen, with different scales if necessary. The VNA mode could be used to test, measure and optimise individual sections of an RF circuit but to perform those tasks you would not only need the FieldFox unit but also suitable probes/cables and calibration hardware to allow the FieldFox to eliminate the characteristics of the probes and cables from its readings. The FieldFox supports several different means of calibration, which is extremely important to get accurate results, especially at higher frequencies. While the graphical representation of the S-parameters shown in Fig.4 can be used for fault-finding, in a lab setting where the FieldFox is being used to characterise RF circuitry or hardware, you would be more likely to off-load the parameters (via USB, Ethernet or SD card) onto a PC for further analysis. We found this very easy to do, using the Save/Recall menu. You can export the data in multiple formats, including CSV. Fig.5 shows the unit being used in “mixed mode”, this time showing two traces on the screen. At the top is the same S11 input reflection parameter visible in Fig.3, this time a bit clearer. Below it is shown the plot of Scc11, the cable’s common impedance profile. “C” here stands for “common mode” while “D” would refer to differential signalling. So for every normal S-parameter, there are four possible mixed-mode parameters: Sccxx, Scdxx, Sdcxx and Sddxx. The FieldFox is able to measure the differential impedance profile (Sdd11), the common impedance profile (Scc11), reflected common signal (Scd11) and reflected differential signal (Sdc11). Fig.6 shows how the internal power meter is used. It’s pretty simple; just choose a frequency, a bandwidth and optionally a radio standard and it shows the power level. Fig.7 shows how the unit can measure cable loss with a connection to just one end of the cable, while Fig.8 demonstrates the measurement of insertion loss when connecting to both ends. We’re not sure why they give such radically different results but we have to assume that Fig.8 is accurate and The right side of the unit with the three locking doors open. The LAN and USB device ports can used for remote control and offloading captured data. The SD card and USB device ports provide an alternative means for copying data from the analyser to a PC. siliconchip.com.au June 2017  89 Fig.7 shows that single-port cable loss measurements tend to underestimate losses. One thing we quickly learned in operating the FieldFox is that generic BNC/BNC type cables tend to have very high insertion loss, especially above a few GHz; even quite short ones. And of course, every adaptor and connector along the way degrades the signal. If you need to transmit a high frequency signal along a cable without significant loss, the FieldFox is an invaluable tool for evaluating whether the cable you’re using is up to the task. Fig.9 shows the measurement of the VSWR of a Diamond RH799 70-1000MHz stub antenna connected to the end of a BNC cable. As you can see from the marker information at upper right, despite being designed to operate below 1GHz, the minimum VSWR is 1.02 at 8.1GHz, indicating that this could be the most efficient frequency for the cable/antenna combination (ie, the lowest reflected power). Other troughs indicating high efficiency are at 4.58GHz (VSWR 1.5), 2.9GHz (1.4), 2.25GHz (1.375) and 365MHz (1.2). No doubt the cable plays a role in these figures. Fig.10 shows a spectrogram of the 100MHz band centred around 2.4GHz, received using that same antenna. The horizontal bars indicate sporadic activity at that frequency. You can see WiFi devices transmitting on around four dif- ferent bands between about 2.41 and 2.42GHz. At the time of the capture, no devices were transmitting, as indicated by the essentially flat black line. We used the regular spectrum analyser for this display since the RTSA has a much more limited frequency span (up to only 10MHz). Note that these signals show up quite clearly, despite the antenna being designed for sub-1GHz frequencies. It was able to pick up AM and FM radio just fine, too, and the FieldFox can even tune into and listen to them (in case you need to find out who’s interfering with your signal…). Conclusion It’s difficult to provide a full evaluation of the N9917A FieldFox analyser for a number of reasons. Firstly, there aren’t many other devices out there with such a wide range of capabilities. Secondly, we are not RF experts and so many of the capabilities of the device are new to us, and we have a limited familiarity with the potential applications of this technology. Also, given the large number of features, we don’t really have the space to fully do it justice. However, a few things have become clear from our time with the FieldFox. Firstly, if you load it up with options, it’s clearly a very powerful instrument and would be in- Here is a short summary of the various main functions that are available in Spectrum analyser A spectrum analyser analyses an AC signal and produces a plot, or a set of coefficients, representing the magnitude and phase of all the various different frequency sinewave components of that signal, at a particular moment in time and over a specified range of frequencies. For example, you can connect an antenna to a spectrum analyser to determine the carrier frequency and bandwidth usage of radio transmitters in the area. Each signal picked up will show up as spikes on the spectrum analyser plot, centred around the carrier frequency, with shape depending on the bandwidth. Analog transmissions normally have a bell-curve (Gaussian) shape while digital radios (eg, WiFI transmitters) tend to produce a more square shape. A spectrum analyser can also be used with a “tracking generator”, as a basic form of network analyser. The output of the tracking generator is fed into a network and the spectrum analyser analyses the output. The tracking generator’s frequency sweeps over the same frequency range as the spectrum analyser is capturing. The result is akin to a frequency response plot. Real Time Spectrum Analyser (RTSA) Spectrum analysers have various parameters that the user can adjust which control the trade-off between dynamic range, bandwidth and analysis time (eg, “resolution bandwidth” [RBW]). The greater the required dynamic range and the finer the bandwidth, the longer the spectrum analyser needs to capture and analyse the data. As a result, short signal bursts may be missed or “smeared”. This is the inevitable interaction between the frequency domain and time domain; ie, AC signals are only 90  Silicon Chip meaningful over a finite period of time. An RTSA is a spectrum analyser that provides a compromise more geared towards capturing fast-changing signals. It not only provides rapid analysis but also analyses time-overlapped data, such that any sporadic signal burst is guaranteed to be picked up. RTSAs can often display the results in a “waterfall” view or spectrogram (see Fig.10), to allow you to visualise all this data. Vector Network Analyser (VNA) A VNA is a device which produces a parameter matrix which describe the AC behaviour of an electrical network at a particular frequency. The FieldFox can operate as a two-port VNA which means it can analyse a network with one single-ended or differential input and one single-ended or differential output. That includes devices like filters, amplifiers, attenuators and transmission lines. The most common output from a VNA is a set of scattering parameters or S-parameters. In the case of a two-port network, this matrix comprises four complex numbers. They represent the gain and phase of the following aspects of that network: forward voltage gain (S21), reverse voltage gain (S12), input port voltage reflection coefficient (S11) and output port voltage reflection coefficient (S22). From these four parameters, you can also calculate the following (at least): complex gain, scalar gain, insertion loss, input return loss, output return loss, reverse isolation, reflection coefficient and voltage standing wave ratio (VSWR). To fully characterise a component or network, the analyser will generate a set of S-parameters over a stepped range of frequencies (see Fig.4). These measurements can be used for checking the persiliconchip.com.au valuable for field work which involves fault-finding, cable and antenna optimisation, measurement of interference and spectrum usage and so on. Secondly, it’s quite a practical and easy-to-use device and once you become familiar with its amazing capabili- At the top of the analyser, the input/output N-connectors and SMA sockets for GPS antenna and reference input all have waterproof caps. ties, you will find it very satisfying to use. Keysight have also apparently put quite a lot of effort into making it easy to calibrate for accurate results, which is absolutely critical for this type of device in a lab environment. Note though that SILICON CHIP does not have the equipment to make calibrated measurements and comment on their accuracy. Thirdly, the range of options is quite astounding and a single properly-configured FieldFox could easily replace a range of separate RF test instruments. We believe it would be an invaluable tool for an RF field engineer. If we have any criticism, it would probably be the display; while it’s quite large and bright, the resolution pales in comparison to today’s tablets and portable computers. Having said that, if you need to examine a signal in detail, you can always off-load the data and that’s what most users would need to do for proper analysis anyway. This is a serious tool and we believe potential customers given a demonstration of its capabilities would be able to quickly determine whether it’s the right tool for them. For more information Contact Keysight on 1800 629 485 or email tm_ap<at> keysight.com the Keysight FieldFox instruments: formance of antennas, seeing how cabling affects antenna performance, verifying RF amplifier stability, checking the correctness of RF PCB layouts, checking whether connectors are working properly and so on. They can also be used to characterise passive component networks such as filters. One important (and yet often overlooked) use for a VNA is to analyse the performance of cables and probes used in test equipment such as high bandwidth oscilloscopes, so that cable/probe loss can be compensated for by the scope, giving much more accurate measurements (See: www. microwavejournal.com/AgilentCableLoss). Vector voltmeter According to Wikipedia, a Vector Voltmeter is “a two-channel high-frequency sampling voltmeter that measures phase as well as voltage of two input signals of the same frequency”. This is one of the key components of a Vector Network Analyser but can have other uses, so FieldFox devices with VNA capability also provide you with the vector voltmeter function. Besides those measurements already available from a VNA, you can also use a vector voltmeter to measure the distortion of radio frequency waves and the complex impedance of mixers, to give two examples. Time Domain Reflectometry (TDR) This is a technique for detecting the location of shorts/ breaks/faults in a cable by making a connection at only one end. A signal is injected into that end of the cable and the device then “listens” for the reflection. By analysing the delay, phase and amplitude of the reflection, it is possible to get a rough idea of the location and type of fault. See our articles on TDR in the November and December 2014 issues for more siliconchip.com.au information and see Fig.3. Extended Range Transmission Analysis This is a system developed by Keysight to measure the gain or loss in a very long cable, where it would be impractical (or impossible) to connect both ends of the cable to the same measurement device. It involves using two FieldFox VNA/SA devices, one at each end of the cable. Essentially, it involves synchronising the two devices in such a way that they are able to operate as if they are one instrument. Interference analysis This involves using a spectrum analyser, with a persistence display, to look for transient interfering signals above a certain power threshold, within a given frequency range. Cable and Antenna Analyser (CAT) A device able to calculate parameters such as Insertion Loss, Return Loss and VSWR (see Figs.8-9) and also plot them against calculated distance in order to estimate the location (in cabling) of any problem spots (eg, kinks in cables) which result in poor performance of the system as a whole. This is related to but somewhat more complex than TDR and usually involves Frequency Domain Reflectometry (FDR). A major advantage of using FDR rather than TDR to calculate the Distance To Fault (DTF) in an RF system is that FDR can be performed at the system’s normal operating frequency, so the test signal can pass through filters and tuned circuits (as the normal signal would) and it also tests and analyses cables and antennas at their rated frequencies (see Fig.1 & 2). SC June 2017  91