most SA implementations require the use of an added LNA, at least over some of the band. Newer SAs have an IF structure almost as flexible as an NFA to optimize the performance of the system.
All of these systems of NFA utilize the “hot/cold” or “Y‐factor” method of measuring noise figure (more about this in Chapter 9) using as an input to the DUT a noise source that can be turned on and off. From careful measurement of the output noise, the gain and noise figure of the DUT can be discerned.
More recently, VNAs have been modified to operate as NFAs, utilizing an entirely different technique called the cold‐noise method. In this method, the output noise power is measured, along with the gain using the normal VNA measurement of gain, and the noise figure is computed from these values. No noise source is used in the measurement. This has an advantage of being faster (only one noise measurement is needed) but does have the disadvantage of being sensitive to drift in the gain of the VNA noise receiver. The Y‐factor method does not depend upon the gain of the NFA receiver, but this advantage is often offset by the fact that the gain measurements of the NFA are sensitive to match errors as are the noise measurements, and these are not compensated for.
The ultimate in noise figure analysis is a noise parameter test system. This system properly accounts for all mismatch effects. Some systems use both a VNA and an NFA to measure the gain and noise power, respectively. All noise parameter systems include an input impedance tuner to characterize the change in noise power versus impedance value. Recently, tuners have been combined with VNA‐based NFAs to produce compact, high‐speed noise parameter test systems. These newer systems provide the ultimate in speed and accuracy available today.
1.15.6 Network Analyzers
Network analyzers combine the attributes of a source, and a tracking spectrum analyzer, to produce a stimulus/response test system ideally suited to component tests. These systems have been commercialized for more than 40 years and provide some of the highest‐quality measurements available today. While there are many distinct manufacturers and architectures, network analyzers broadly fall into two categories: scalar network analyzers (SNAs) or VNAs.
1.15.6.1 Scalar Network Analyzers
These instruments were some of the earliest implementations of stimulus/response testing and often consisted only of a sweeping signal source (sometimes called a sweeper) and a diode detector, the output of which was passed through a “log‐amplifier” that produced an output proportional the power (in dBm) at the input. This was sent to the y‐axis of a display, with the sweep tune‐voltage of the sweeper sent to the x‐axis, thus producing frequency response trace. Later, the signal from the detector and the sweeper were digitized and displayed on more modern displays with marker readouts and numerical scaling.
Other SNA systems were developed by putting a tracking generator into a spectrum analyzer so that the source signal followed the tuned filter of the SA. This produced a frequency response trace on the SA screen.
SNAs had the attribute of being simple to use, with almost no setup or calibration required. The scalar detectors were designed to be quite flat in frequency response, and a system typically consisted of one at the input and one at the output of a DUT. However, for measurements of input and output match, or impedance, the SNA relied on a high‐quality coupler or directional bridge. If there was any cabling, switching, or other test system fixturing between the bridge and the DUT, the composite match of all were measured. There was no additional calibration possible to remove the effects of mismatch. As test systems became more complex and integrated, scalar analyzers started to fall from favor, and there are virtually none sold today by commercial instrument manufacturers.
1.15.6.2 Vector Network Analyzers
For microwave component test, the quintessential instrument is a VNA. These products have been around in a modern form since the mid‐1980s, and there are many units from that time still in use today. The modern VNA consists of several key components, all of which contribute to making it the most versatile, as well as most complicated, of test instruments; these are as follows:
RF or Microwave Source: This provides the stimulus signal to the DUT. RF sources in a VNA have several important attributes including frequency range, power range (absolute maximum and minimum powers), automatic‐level‐control (ALC) range (the range over which power can be changed without changing the internal step attenuators), harmonic and spurious content, and sweep speed. In the most modern analyzers, there may be more than one source, up to one source per port of the VNA. Older VNAs required that the source be connected to the reference channel in some way, as either the receiver was locked to the source (e.g. the HP 8510) or the source was locked to the receiver (e.g. the HP 8753). Modern VNAs, for the most part, have multiple synthesizers so that the source and receiver can be tuned completely independently.
RF test set: In older‐model VNAs, the test set was a separate instrument with a port switch (for switching the source from port 1 to port 2), a reference channel splitter, and directional‐couplers. The test set provided the signal switching and signal separation to find the incident and reflected waves at each port. Most modern VNAs have the test set integrated with the rest of the components in a single frame, but for some high‐power cases, it is still necessary to use external components for the test set.
Receivers: A key attribute of VNAs is the ability to measure the magnitude and phase of the incident and reflected waves at the same instant. This requires sets of phase synchronous receivers, which implies that all the receivers must have a common LO. In older, RF VNAs, the reference channel was common to ports 1 and 2, and the port switch occurred after the reference channel tap. Most modern analyzers have a receiver per port, which is required for some of the more sophisticated calibration algorithms. More about that appears in Chapter 3.
Digitizer: After the receiver converts the RF signals into an IF baseband signal, they pass to a multi‐channel phase‐synchronous digitizer that provides the detection method. Very old VNAs used analog amplitude and phase detectors, but since at least 1985, all VNAs utilize a fully digital IF. In modern VNAs, the digital IF allows complete flexibility to change IF detection bandwidths, modify gains based on signal conditions, and detect overload conditions. Deep memory on the IF allows complicated signal processing, and sophisticated triggering allows synchronization with pulsed RF and DC measurements.
CPU: The main processor of a VNA used to be custom‐built micro‐controllers, but most modern VNAs take advantage of Windows™‐based processors and provide rich programming environments. These newer instruments essentially contain a PC inside, with custom programming, known as firmware, which is designed to maximize the capability of the instruments' intrinsic hardware.
Front Panel: The front panel provides the digital display as well as the normal user interface to the measurement functions. Only the spectrum analyzer comes close to the sophistication of the VNA, and in more modern systems, the VNA essentially contains all the functions of each of the instruments mentioned so far. Thus, its user interface is understandably more complex. Significant research and design effort goes into streamlining the interface, but as the complexity of test functions increases, with more difficult and divergent requirements, it is natural that the user interface of these modern systems can be quite complex.
Rear Panel: Often overlooked, much of the triggering, synchronization, and programming interface is accomplished through rear‐panel interface functions. These can include built‐in voltage sources, voltmeters, general‐purpose input/output (GPIO) busses, pulse generators and pulse gating, as well as LAN interfaces,
