RF3500_unit3

I1 Z0 V1 Device V2
Vs
Z0
FIGURE 1. A SIMPLE TWO-PORT SYSTEM The low frequency Engineer would define Voltage Gain GV as V2/V1 This is fine for low frequency devices such as OPAmps, as their input impedances V1/I1 are very high. For these devices, the source and input voltages, VS and V1 respectively, are very nearly equal. High frequency devices generally feature low input impedances and therefore, the use of Voltage Gain as an RF parameter is somewhat meaningless. Clearly, two devices with identical Voltage Gains, but different input impedances, would behave differently in the system shown in Figure 1. The device with the lowest input impedance would have the lowest meaningful voltage gain as defined by V2/VS.
RF3500 Applications Course
Student Workbook UNIT 3
PRIMARY RF SYSTEM/CIRCUIT PARAMETERS. Unit objectives
Understand the following parameter defintions: Conventional Gain Insertion Voltage Gain Transducer Power Gain Reflection Coefficient Return Loss and VSWR Understand the application of these parameters in determining the performance of an RF circuit or system2
RF3500 Applications Course
A similar picture emerges when we consider Power Gain GP. The low frequency Engineer would define Power Gain as P2/P1 where P2 = V2I2 and P1 = V1I1. Clearly, two devices with identical Power Gains, but different input impedances, would behave differently in the system shown in Figure1. The device with the lowest input impedance would have the lowest meaningful power gain as defined by P2/PS, where PS = VSI1. 3.2 Insertion Voltage Gain. The RF Engineer circumvents the problems associated with the definition of Voltage Gain by introducing Insertion Voltage Gain GVI defined by V2/VM, where VM is the matched input voltage equal to VS/2. Clearly, this definition of gain gets around the problem associated with devices having the same Voltage Gain but different input impedances. This definition of gain has other, very significant, advantages. Another way to express Insertion Voltage Gain can be obtained by noting that V2 = VM when the device is removed and a connection made between input and output. Thus, by measuring V2 with the device replaced by this “Thru” connection and with the device inserted the Insertion Voltage Gain can be obtained. This interpretation and measurement of Insertion Voltage Gain illustrates two significant advantages of the parameter. Firstly, all frequency response errors associated with the source are automatically accounted for when making the measurement with the “Thru”. Secondly, all frequency response errors associated with the measurement instrument are simultaneously accounted for when making the measurement with the “Thru”. Thus, the calibration requirements for the measurement of Insertion Voltage Gain are not demanding. However, it should be noted that the definition and measurement of this parameter assumes a particular value for Z0 , known as the Reference Impedance. Thus, when considering Insertion Voltage gain, it is necessary to use and quote the associated Reference Impedance. Insertion Voltage Gain is a primary RF Engineering parameter. A knowledge of this parameter is essential to an understanding of RF topics such as S-Parameters.
999-0117-00 Rev AB
3- 3
RF3500 Applications Course
3.3 Transducer Power Gain. A similar picture emerges when we consider Transducer Power Gain GPT . This is also a form of insertion gain. Transducer Power gain is defined by P2/PAV , where P2 = V2I2 and PAV is the maximum available source power, that is, the matched power. As for Insertion Voltage Gain, the calibration requirements for Transducer Power Gain are not demanding. It is left to the student to show that GPT is equal to the square of GVI. Transducer Power Gain is a primary RF Engineering parameter. A knowledge of this parameter is essential to an understanding of RF Topics such as Noise Factor and Noise Figure. 3.4 Reflection Coefficient Γ. Reflection Coefficient Γ is, arguably, the most misunderstood concept in RF Engineering. A commonly given definition for Reflection Coefficient is the ratio of reflected power to incident power. The notion of power flowing down a wire and, what is more, being reflected is inconceivable. There is no physical or mathematical basis for this interpretation. There is, however, a basis for a voltage signal flowing down a wire and being reflected. This is a demonstrable fact. By applying a voltage pulse to a transmission line, we can observe it’s passage down the line by using a suitable measuring instrument. If at the far end, the line is incorrectly terminated, we can, also, observe a reflected signal. We will now throw caution to the wind and define Reflection Coefficient in terms of AC voltages. We will then attempt to place an interpretation in terms of “incident” and “reflected” on that definition.