Amplifier performance test

The test indicators of amplifiers can be divided into two categories: linear indicator test and nonlinear indicator test.

The test of linear indicators is based on the measurement of S parameters and is completed using conventional vector network analysis.

For the testing of nonlinear indicators, the traditional testing scheme adopts the spectrum analyzer plus signal source method, but this scheme has many disadvantages, such as the inability to achieve synchronous frequency sweep and power sweep testing, and the inability to perform phase measurements such as amplitude-to-phase conversion (AM/PM) measurement.

R&S ZVB adopts innovative hardware structure, with high output power and wide power scanning range, so there is no need to use a separate preamplifier, and the amplifier power compression characteristics can be determined in one scan. ZVB adopts a powerful automatic level control design and a highly selective and sensitive receiver, so it can perform harmonic testing of amplifiers in a wide dynamic range without using external filters.

　　Port matching characteristics measurement

　　The port matching characteristics mainly test the S11 and S22 parameters of the port. For example, the S11 parameter of port 1 is equal to the ratio of the reflected signal b1 to the incident signal a1:

　　The S11 parameter can also be called the input reflection factor G1. S11 is a complex number. In engineering, return loss (RL) and standing wave ratio (VSWR) are usually used to express the matching degree of the port. The relationship between S11 and these two parameters is as follows:

　　The conversion between the above two parameters and S11 is automatically completed by ZVB. Users only need to select [dB Mag]->Return Loss, [SWR]->Standing Wave Ratio in the [Format] menu to display the corresponding test curve. ZVB provides the trace statistics function [Trace Statistics], which can automatically display the maximum, minimum and peak-to-peak value of the trace, and can adjust the statistical frequency range by setting [Eval Range]. This function is very useful for in-band indicator testing of band-limited devices (such as filters).

　　In the process of circuit design, accurate input impedance information is more important for designers. For example, in the design of mobile phone boards, designers need to accurately test the input and output impedance of the front-end amplifier, and then design the corresponding matching network based on the input and output impedance information to achieve the maximum power transmission and the best overall sensitivity of the mobile phone. The relationship between input impedance and S11 is as follows:

　　The user can display the impedance test trace by selecting the [Smith] menu in the [Format] key. By setting the Marker, the input reactance and resistance corresponding to each frequency point can be easily measured. In addition, the virtual embedding function of ZVB standard can simulate the performance of the entire network after adding a virtual matching network to the input and output ports. This function greatly simplifies the workload of designers and can predict the performance of the adjusted DUT without actual circuit adjustment. The user activates this function by selecting [Virtual Transform] in the [Mode] menu.

　　Transmission parameter measurements

　　In addition to the measurement of port matching characteristics, the forward amplification and reverse isolation characteristics of the amplifier can also be obtained by testing S21 and S12 respectively. The forward transmission parameter S21 is equal to the ratio of the forward power b2 measured at port 2 to the excitation power a1 at port 1:

　　The reverse isolation of the amplifier is equal to the logarithm of the absolute amplitude of S12:

　　The user only needs to set the display format of S21 and S12 to dB ([format]->[dB Mag]), and the amplifier gain and isolation can be displayed on the ZVB at the same time.

　　The power compression characteristic test is mainly used to measure the linearity of the device under test (DUT). For amplifier testing, the output power 1dB compression point (P1dB) is usually used in engineering to characterize this characteristic. P1dB is defined as: As the input power increases, the amplifier gain drops to the output power value that is 1dB lower than the linear gain, as shown in Figure 2.

　　ZVB can not only measure the curve of parameter variation with frequency, but also the curve of parameter variation with input power. ZVB built-in signal source can provide a very large power sweep range, with a typical value of 60dB, and the 60dB power sweep range is completely realized by electronic attenuator instead of traditional mechanical step attenuator. Mechanical attenuator has poor amplitude repeatability and short service life, so ZVB is particularly suitable for testing the power compression characteristics of active devices.

　　The measurement of P1dB involves the curve of S21 changing with the absolute input power, and the vector network analyzer is usually used to measure the relative amount of S parameters. In order to improve its absolute measurement accuracy, it is recommended to use a power meter to calibrate the vector network. The NRP series power meter of R&S can be directly connected to the ZVB via the USB interface, thus eliminating the need for a power meter host and expensive GPIB cards. The ZVB power calibration process is divided into two processes: the internal source amplitude calibration of the vector network and the receiver amplitude calibration. In the first process, the power sensor is directly connected to the source port of the vector network, and the menu [Start Power Cal]-> [Souce Power Cal] under the [CAL] key is selected. The second step is to connect the calibrated source port and the receiving port to calibrate the receiver, and the menu [Start Power Cal]-> [Receiver Power Cal] under the [CAL] key is selected.

　　Harmonic Measurement

　　As the excitation power increases, the amplifier will enter the nonlinear working area, not only will the output power compression phenomenon occur, but also nonlinear frequency components will be generated. These new frequency output components are mostly integer multiples of the input frequency, which are called harmonic components. Designers are often more concerned about the amplitude difference between the input fundamental component and the harmonic component, because the larger the amplitude difference, the more power is converted into the required fundamental power rather than harmonic power under the same DC input power, which can also be regarded as improving the efficiency of the amplifier. ZVB breaks the limitation that the traditional vector network signal source and receiver must work at the same frequency, and can make the source and receiver work at different frequency points. Specifically for harmonic measurement, the vector network source can output the fundamental signal, while the receiver works at the harmonic frequency, and it is convenient to realize the sweep test of the fundamental input frequency or input power. Corresponding to the setting of ZVB: You can first add a new observation window and a new test channel through the method of [Chan Select]+[Add channel +trace+Diag Area]. Then select [Harmonics] under the [Mode] key to enter the harmonic test mode, and then measure the corresponding harmonics by selecting 2nd, 3rd or entering other harmonic orders.

　　For testing the change of absolute harmonic power to input fundamental power, it is also recommended to perform power calibration before testing. ZVB also provides a method for harmonic power calibration. Enter the power calibration dialog box through [Harmonic Power Cal]. The basic operation process is the same as when testing the amplifier power compression characteristics, except that the frequency during source power calibration is the entire test frequency and the frequency during receiver calibration is the harmonic frequency.

　　Amplitude-to-phase conversion measurement

　　In addition to power compression and harmonic frequency generation, the nonlinear characteristics of the amplifier also include phase nonlinearity, that is, the change in the amplifier insertion phase shift as the input power changes. AM/PM conversion is usually used to describe it in engineering, and its specific definition is: For every 1dB change in input power, the change in the insertion phase shift is expressed in degrees/dB.

　　As with the measurement of power compression characteristics, the ZVB sweep type [Sweep Type] should be set to power sweep [Power]. The test trace is S21, but the display format [Format] should be set to phase mode [Phase]. During the test, the ZVB can be used to provide convenient reading of the Delta Marker and Reference functions.