Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 87

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, USB interfaces, and video display outputs.

The detailed operation of a VNA is described in Chapter 2.

Extensions to traditional VNAs allow them to create multiple signals for two‐tone measurements and to have very low noise figures for noise figure measurements. But the main attraction of VNAs is calibration. A key attribute is that since they measure the magnitude and phase of waves applied to their ports, they can use mathematical correction to remove the effects of their own impedance mismatch and frequency response in a manner that makes their measurements nearly ideal. The details of VNA calibration are covered in depth in Chapter 3.

Thus, even though there is a wide variety of test equipment available for microwave component measurement, by far the most widely used is the VNA, and while many of the topics of component measurements in this book are extensible to any of the previous instruments, the specific implementation and examples will be illustrated primarily using the VNA, as that has become the predominant component test analyzer in use today.