Network Analyzer VNA Test

Traditional vector network analyzers (VNAs) usually use the so-called "virtual" method when measuring balanced/differential devices: the network analyzer stimulates the device under test with a single-ended signal to measure its unbalanced parameters, and then the network analyzer converts the unbalanced parameters into balanced parameters through mathematical calculations. This method is sufficient for analyzing active/passive devices in a small signal (linear) state. However, when the device is in a large signal (non-linear) working state, the accuracy of the test results of this method is limited. Although people have come up with many ways to overcome this problem: for example, using "ideal" broadband power dividers or couplers, these methods cannot perform full system calibration. Fortunately, Rohde & Schwarz's multi-port network analyzers ZVA and ZVT can achieve accurate broadband differential device measurements and are easy to operate by adding new options.

The R&SZVx-K6 option is a new concept technology and has obtained multiple patents. The company has conducted experimental verification on a variety of active devices and found that the gain compression point results obtained by this method are indeed somewhat different from those of the "virtual" method. Figure 1 is a typical example. This experiment uses the R&SZVA40 network analyzer to test a 2GHz microwave monolithic integrated MMIC (monolithic-microwave-integrated-circuit) amplifier in two modes. It can be seen that in the case of small signals (linear), the measurement results of the two methods are the same, but when the amplifier is in compression (large signal), the measurement results of the two methods are significantly different. The gain measured using true differential excitation is compressed 4dB earlier than the result using the virtual method, and the measurement result of the maximum gain is also 0.5dB lower.

The improvements (advantages) of this new technology are as follows:

1. Currently, differential amplifiers are widely used in mobile phones, smart phones, data cards and other mobile devices. However, most of these devices are currently tested using virtual methods (because there was no true differential test technology before). This means that the device characteristics currently measured are not correct.

2. If the actual compressed power of the device is lower than that marked by the manufacturer (because manufacturers currently use virtual methods for testing), it means that many amplifiers are now operating in a compressed (overloaded) state, and their actual intermodulation products are much higher than the designed values.

3. Manufacturers who use virtual methods to design and produce mobile phones must currently use "power fallback" technology to achieve ideal linear power performance.

However, using the "power back-off" technology means that more (or higher output power) active devices are needed to achieve the specified output power, which may require redesigning the entire transmitter section.

Of course, if the characteristics of balanced components can be tested more accurately, component and system manufacturers can design ideal performance and operating conditions before the product leaves the factory (rather than after problems arise during use).

When using a traditional network analyzer to measure differential (balanced) devices, the instrument can only generate single-ended excitation, and convert the measured single-ended S parameters into differential S parameters through mathematical calculations. The instrument does not use a differential signal to stimulate the device under test, but measures it as a single-ended device. Then the measured single-ended S parameters are used to calculate the mixed-mode S parameters. Since no real differential signal is used to stimulate the device under test, the accuracy of this virtual method is difficult to guarantee. The accuracy of this method is acceptable in the small signal (linear) state, but it is difficult to guarantee in the large signal (non-linear) state.

When active devices are stimulated by large signals, their nonlinear characteristics gradually emerge (usually measured by 1dB or 3dB compression points). At this time, it is difficult to obtain ideal results by using traditional virtual methods to measure active devices. For example, if the 1dB compression point of an amplifier measured by the virtual method is higher than the actual value, if this parameter is used to guide the design, the designed amplifier may be in an overload state, thereby generating many nonlinear products. However, in the past, network analyzers could only provide virtual methods because the technology for controlling the amplitude and phase of the two signal sources output by the network analyzer was extremely complex.

This new technology developed by Rohde & Schwarz has for the first time enabled the network analyzer to output a true differential signal to stimulate RF microwave balanced devices with a maximum frequency of up to 40GHz. The method is based on patented control technology to control the amplitude and phase of two internal sources, as well as patented differential vector calibration technology. The two sources inside the R&S ZVA (2-, 3-, 4-port network analyzer) or the company's ZVT (multi-port network analyzer) can generate signals with the same amplitude and a phase difference of 0 or 180 degrees, with an uncertainty of less than 1 degree. Using this set of differential signals to stimulate the device under test, the differential or common mode response can be directly measured, and after vector correction, the mixed mode S parameters can be directly obtained.

The working principle of the traditional virtual method is as follows: at each frequency point, the 1st port of the network analyzer outputs a single-ended excitation, the transmission component is measured at the 2nd, 3rd, and 4th ports, the reflection component is measured at the 1st port, and then the single-ended excitation signal is output from the 2nd, 3rd, and 4th ports respectively, and the above test is repeated. 16 single-ended S parameters (S11 to S44) can be obtained, and then these 16 parameters are used to calculate the mixed mode S parameter Sxxyy. However, for nonlinear devices, the 1st and 2nd ports of the instrument cannot output excitation signals, so the performance of the device under test in the actual working state cannot be reproduced.

There are many challenges to overcome in generating a true differential signal: First, a 180-degree phase shift must be achieved between the two internal sources, and this phase difference must be precisely controlled to ensure the quality of the differential signal. In addition, this phase difference remains valid in the measurement and calibration reference plane. The loss, phase, and other characteristics of the test cable will vary, which brings many difficulties to accurate measurement.

The instrument is calibrated in accordance with the standard thru-open-short-match (SOLT) calibration method. This calibration is applicable even when the test cables are asymmetrical (e.g., different lengths) or when the test is on-wafer. The instrument can also generate signals with a phase difference of 0 degrees (in phase) for common mode testing. In previous instruments, phase drift over time and temperature was a serious problem. Here, the internal source uses a special algorithm and control circuit to continuously check and correct the phase difference of the internal source to ensure a strict amplitude-phase relationship of the differential signal.

The specific working steps of true differential technology to measure a 4-port balanced device are as follows:

The No. 1 logic port of the network analyzer (actually composed of two physical ports) sends out a differential mode signal with the same amplitude and a phase difference of 180 degrees, which is loaded onto the device under test. The differential and common mode responses of the transmission component are measured at port 2, while the differential and common mode responses of the reflection component are measured at port 1. Then the No. 1 logic port of the instrument generates a common mode signal with the same amplitude and a phase difference of 0 degrees, and the differential/common mode responses of the transmission and reflection signals are measured respectively.

The No. 2 logic port of the network analyzer sends a differential mode signal with the same amplitude and a phase difference of 180 degrees, which is loaded onto the DUT. The differential and common mode responses of the transmission component are measured at port 1, and the differential and common mode responses of the reflection component are measured at port 2. Then the No. 2 logic port of the instrument generates a common mode signal with the same amplitude and a phase difference of 0 degrees, and the differential mode/common mode responses of the transmission and reflection signals are measured respectively. The mixed mode S parameters of the DUT can be directly calculated from the above differential mode/common mode responses, and after system error correction, they are directly displayed on the instrument screen. The scanning time to complete all the above tests is only 300ms.

This technology can also realize amplitude and phase imbalance scanning (to simulate non-ideal conditions). For amplitude imbalance scanning, the amplitudes of the two signals are no longer equal, and one of them can be scanned in the range set by the user. Similarly, for phase imbalance scanning, the phase difference between the two signals is no longer maintained at 180 degrees, but varies within a set range. Both scanning methods are designed to simulate non-ideal working conditions and provide designers with more reference information.

Users can switch between virtual mode and true differential mode simply by clicking the mouse, and the test results of both methods can be displayed in real time in the same graph. Moreover, the calibration technology of both methods is the same, and users do not need to calibrate separately. The instrument also provides a simple and intuitive balanced device test wizard program. The true differential measurement technology does not require hardware updates and can be used on any 4-port ZVA series and any ZVT series network analysis with more than 3 ports.