Measurement Benefits of USB-Driven VNA
Historically, test and measurement instruments have been developed primarily with their own operating systems. In fact, many late model conventional test equipment devices contain embedded PCs running Windows or Embedded Windows. Unlike such conventional instruments, modular or USB-driven instruments contain no operating system. This architecture supports numerous advantages: it enhances instrument lifetime and stability by avoiding operating system and computer hardware obsolescence; it provides a single step to data purging by powering down the machine; it enables easy sharing of the instrument among users in a lab since all calibration and state files are saved on PC; and it greatly reduces the size, weight, power and cost of the instrument for the user.
In this paper, we describe the benefits of modular test equipment in the particular category of USB Vector Network Analyzers (VNAs), including increased measurement speed, ease of automation, and enabling of advanced measurement scenarios such as pulsed and high-power measurements.
The USB-driven VNAs separate the measurement module from the processing module, bringing the measurement results to any external PC using software . USB-driven VNAs are flexible, easily adaptable to multiple users, and are well-suited for lab, production, field, and secure test environment. The biggest advantage of a USB-driven VNA is that it doesn’t lock the user into a built-in computer that is already outdated. Usually, the development cycle for a new analyzer is around 24 months and by the time a new model goes into production, the PC is already two years old. If a customer purchases this analyzer three years into its life cycle, then they are buying a five-year old computer, which by today’s IT industry standard, is quite outdated. That on-board PC will get even more outdated quickly, and is extremely expensive to replace, since the replacement can only be done by the analyzer’s manufacturer or their authorized service center. Unlike the conventional VNA, USB-driven hardware lets the user have control over the processing module. The VNA contains the processing module and is connected to the PC using a USB connection. A typical setup with the USB-driven VNA is shown in Figure 1.
USB-driven VNAs are smaller, lighter, and cost- effective. With a USB- driven device, users can easily take advantage of the newer and faster processors, better display capabilities, and other new PC functionality by simply switching to a new computer at their discretion. Besides, a defining characteristic of a USB-driven VNA is external data storage. The analyzer can be easily and independently shared between multiple users and different locations. The USB-driven VNA is also well-suited to classified applications as well as ATE (Automated Test Equipment) applications.
Copper Mountain Technologies (CMT) provides a variety of USB-driven VNAs to accommodate a wide range of test and measurement needs . The CMT VNA family covers a wide span of frequencies starting from 20 kHz, and extending up to 20 GHz, with 1-port, 2-port or 4-port configurations. In this paper, we will discuss the various advantages of CMT VNAs in terms of measurement speed, automation, and advanced measurements, such as pulsed and high-power device measurements. The paper is organized as follows. In section 2, an introduction to CMT USB-driven VNA family will be given. The section 3 will discuss about the measurement speed of CMT VNAs. Different ways for CMT VNA automation will be the topic of the section 4. Advanced measurement techniques, such as pulsed and high-power device measurements by CMT VNAs will be discussed in section 5. Finally, section 6 will conclude the paper.
CMT VNA Families
The CMT VNA product line is divided into 4 types: R, TR, S2 and S4. The R instruments are 1- port reflectometer whereas the S4 contains 4-port VNAs. The TR and S2 groups both contain 2-port VNAs. TR instruments are capable of 2-port 1-path measurements (S11 and S21 only) while S2 instruments are capable of 2-port, 2-path measurements. The frequency range of operation and the characteristics of all the families are shown in Figure 2.
The measurement sweep time consists of 3 components: generator tuning and settling time, measurement time, and return time . Generator tuning and settling time is hardware dependent and this includes latencies associated with command and control internal to VNA. It also includes settling times of an associated phase-locked loop (PLL) or voltage–controlled oscillator (VCO) necessary to achieve a stable output signal frequency and power level for the next measurement point.
Measurement time is determined by the settling time of the IF filter. Narrower IFBW requires more time for the IF filter to settle. Moreover, there is a trade-off between measurement time, IFBW, and dynamic range. Reduced IFBW increases measurement time and dynamic range. Return time consists of fixed and variable delay components and a 100 us delay for returning the generator to start frequency. The breakdown of the total measurement time of a CMT VNA is shown in Figure 3.
Figure 3: Breakdown of total measurement time
Typical measurement time for 2-port full size CMT Planar VNA is 56 us per point (30 kHz IFBW) while 20 us is achievable with Cobalt series VNA.
Usually, automation of CMT instruments is achieved on the same computer which is running the VNA applications . Such functionality can be readily achieved using COM or ActiveX automation. Nothing is required for automation of the instrument other than the VNA application and the Programming Manual. However, in some instances it is desirable to execute the VNA application on one PC and to host the automation environment on a second Windows PC. This configuration requires use of Distributed COM or DCOM. The main challenge related to DCOM versus COM is getting Windows firewall and security settings, as well as LAN settings, to cooperate. It will probably be easiest to begin developing the application on the same machine that is running the VNA application (via a local COM interface) and then moving toward automation over the network via DCOM. Various programming languages, such as MATLAB, Python, VBA, C++, and LabView can be used for automation using CMT VNAs.
Advanced Measurement Techniques
CMT VNAs can be used extensively in advanced measurements, such as pulsed and high-power device measurements. All the CMT VNAs come with a trigger port and 10 MHz reference signal port for synchronization . An external trigger delay option is also available to avoid rising/falling edge and accurate measurement. Besides, measurement delay option is available for devices with slow time constant. A typical setup for pulse measurement using CMT VNA is shown in Figure 4.
Figure 4: Pulse measurement setup by CMT VNA.
It should be noted here that the length of the measurement window must be shorter than the pulsed signal for stable measurement results. Moreover, the external trigger delay allows a delay between the occurrence of a trigger event and initiation of the actual measurement as shown in the measurement timing diagram in Figure 5.
Figure 6: Measurement sequence for each point in the sweep
Finally, the measurement delay is applied between setup and sampling. The measurement sequence for each point in the sweep is shown in Figure 6.
Copper Mountain Technologies offers a USB-driven Planar 814/1 VNA that provides the user access to the reference and measurement receivers. The direct access to the receivers is essential for measuring high power devices that require higher input power than the VNA’s maximum output power. An external reflectometer along with the VNA is required to measure the forward reflection co-efficient of the DUT. A typical setup for the high-power setup by Planar 814/1 is shown in Figure 7.
The Planar 814/1 with direct receiver access can be used in load-pull and noise figure measurement as well where the measurement needs direct access to the receivers. In the Figure 7, two external directional couplers were employed to measure the forward reflection coefficient of the high power DUT. The first directional coupler is forward coupling and couples the reference power to the reference receiver. The second directional coupler is backward coupling and couples the reflected power from the DUT to the measurement port. Then the reflection coefficient is calculated from the measurements of the two receivers.
Figure 7: High power measurement setup with Planar 814/1
In this paper, measurement benefits and techniques using USB-driven VNAs have been discussed. The product family of USB-driven CMT VNAs have also been introduced. The measurement speed and the automation technique for USB-driven VNA have also been presented. Finally, some advance measurements, such as pulse and high-power device measurement by CMT VNAs have been demonstrated. It has been found out that the USB-driven VNA holds significant advantages over its counterpart in terms of size, cost, and ease of use.