Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 91

The basic block diagram for a component test system is a stimulus source, which is applied to the input of the device‐under‐test (DUT), and a response receiver at the output of the DUT. For S‐parameter measurements, the inputs consist of incident waves at all the ports, and the outputs consist of scattered waves at all the ports, so in general one would require a stimulus and two receivers at each port. In addition, there must be signal separation devices at each port to isolate the incident and scattered waves.

Early systems measured only transmission and/or reflection response, in only one direction, and thus consisted of at most a directional device (bridge or coupler) at the input and a receiver at the output. These systems were classified as transmission/reflection (TR) systems and were most commonly found as scalar network analyzers, although lower‐cost VNAs sometimes had TR test sets as well. The advantage of a vector TR analyzer is that the errors in the directional device could be removed with calibration and error correction.

Figure 2.1 shows the block diagram of a TR system. For simplicity sake the reference receiver will be normally at port 1, measuring the a1 wave, and the test receivers were limited to the two ports as well; normally the test receiver at port 1 is the reflection receiver (b1), and the test receiver at port 2 is the transmission receiver (b2). The source is typically split using a two‐resistor power splitter or a coupler to create a reference signal that is proportional to the incident signal on the DUT, followed by a directional‐coupler or directional bridge, whose coupled arm goes to the reflection test receiver, measuring b1. After the DUT, the transmission test receiver measures b2. Because of advances in VNAs and integration of dual reflectometers and receivers, this architecture is seldom seen in modern VNAs.

Figure 2.1 A TR network analyzer block diagram.

A full S‐parameter system extends the block diagram of the TR system by adding a reflectometer at each port and provides a source at each port. Older systems would switch the source between the ports using a test port switch. Two such block diagrams are shown in Figure 2.2. There are two distinct versions that use either one or two reference receivers.

Figure 2.2 S‐parameter block diagrams for a three‐receiver and four‐receiver VNA.

The three‐receiver version (upper diagram) was common in lower‐cost or RF network analyzers in the past but has largely been replaced with four receiver versions. Having individual receivers for all the reference and test port channels provides for more and better calibration choices, as will be discussed in Chapter 3.

Older analyzers such as the HP‐8510 used separate external sources and switched the source between the ports; others had internal sources, but the cost of the source was a major portion of the instrument cost, so a single switched‐source was used in these integrated analyzers. Often, the reference channel splitter was integrated into this switch as well. This provided a compact switch‐splitter assembly and allowed a lower‐cost alternative to individual splitters or directional‐couplers.

Modern network analyzers make use of a hybrid approach with two or more internal sources, such that more than one port at a time can have an output signal, as shown in Figure 2.3.

Figure 2.3 Multiple sources in a single VNA.

While there is no requirement for having more than one port active in traditional S‐parameter measurements, advanced measurements such as two‐tone intermodulation distortion (IMD), active‐load, or differential‐device test can make good use of these extra sources. In these systems it is common to use a directional‐coupler in both the reference and test arms for lower loss from the source to the test port, allowing higher maximum test port power. One synthesizer (that is, the frequency generation unit) may be shared between two ports (of a 4‐port VNA) with an individual output amplifier and leveling circuits available at each of the ports. This system can provide outputs at any of the ports at the same time or provide two different frequencies out of pairs of ports, which is useful for mixer test applications.