Understanding the Inner Workings of Vector Network Analyzers
In this article, we explore how the signal source and receivers of a VNA enable its function.
Vector network analyzers (VNAs) are perhaps the most complicated and versatile piece of test equipment in the field of RF engineering. By measuring the forward and backward traveling waves, VNAs are able to characterize the response of a device under test (DUT). Figure 1 shows the basic block diagram of a typical VNA.
Figure 1. Basic VNA block diagram. Image used courtesy of David M. Pozar
A VNA uses an internal source to generate a known stimulus signal, which is then applied to the input port of the DUT. Some of the signal is reflected from the input port, while some of it passes through the DUT and reaches the output port. The VNA characterizes the performance of the DUT in terms of its reflection and transmission coefficients by measuring the magnitude and phase of both the incident and reflected waves at each port.
To understand what drives VNA performance, we need to familiarize ourselves with the VNA’s internal hardware. Previous articles in this series focused on the directional couplers employed in VNA ports—both their key role in VNA functionality and their impact on measurement precision. In this article, we’ll turn our attention to the VNA’s signal generators and receivers.
VNA Source Components
For basic S-parameter measurements, the VNA’s internal source needs to produce a single-tone sinusoidal wave. For more advanced measurements, we might need multi-tone inputs or modulated signals to characterize the DUT more completely. To enable different types of measurements, the frequency and power of the internal source must also be adjustable.
VNAs use a phase-locked loop (PLL) system, as illustrated by the simplified block diagram in Figure 2, to provide the required frequency stability and spectral purity.
Figure 2. Simplified block diagram of a PLL. Image used courtesy of Analog Devices
The PLL’s performance strongly depends on the characteristics of the tunable oscillator it employs. Two common options for building an RF/microwave tunable oscillator are:
- Voltage-controlled oscillators (VCOs).
- Yttrium iron garnet (YIG) tuned oscillators (YTOs).
As you can see, the oscillator in Figure 2 is a VCO. Most EEs have at least a passing familiarity with the operation of VCOs, so we’ll only go over them briefly before we move on to discuss YIG oscillators.
VCOs are based on either lumped LC or distributed microstrip resonators, and use varactor diodes to implement a tunable capacitor. They typically have a Q of a few tens to a few hundreds. Due to their low Q and high tuning sensitivity, wideband VCOs have higher phase noise than YIG-tuned oscillators.
Because of their low wideband phase noise and wide tuning range, YIG oscillators are the core of many modern broadband signal generators. Figure 3 shows a pair of YIG-tuned oscillators from Micro Lambda.
Figure 3. MLOS-series YIG-tuned oscillators. Image used courtesy of Micro Lambda
Yttrium iron garnet is a synthetic ferrimagnetic material that has unique magnetic and microwave properties. A YIG resonator takes the form of a small sphere—around 500 μm in diameter—manufactured from a single crystal of the material. The YIG sphere is usually mounted at the tip of a ceramic rod, as in Figure 4.
Figure 4. A YIG sphere mounted on a ceramic rod as part of an oscillator. Image used courtesy of VHF Communications
The U-shaped belt in Figure 4 is a coupling coil that surrounds the YIG ball, positioning it within the magnetic field of an electromagnet. The sphere’s resonant frequency is a linearly proportional function of the magnetic field strength, which can be adjusted by tuning the DC current going through the electromagnet. A relatively high Q—in the range of 4,000 at 10 GHz—is achievable using this type of resonator.
Advantages and Disadvantages of YTOs versus VCOs
YTOs have the following favorable characteristics:
- Low wideband phase noise.
- A very wide tuning range.
- A highly linear tuning curve.
Less favorably, YIG oscillators exhibit hysteresis effects that slow down their tuning speed. This poses a challenge for VNA applications that require the source to rapidly sweep across frequencies to collect the DUT’s frequency response. YIG oscillators are also large, power-hungry, and expensive compared to VCOs.
It’s worth noting that some companies have experimented with creating competitive alternatives to YIG-tuned oscillators. The PLL/VCO integrated circuits described in this Analog Devices application note are one example.
Spectral Purity vs. Phase Noise Requirement
Though the source’s phase noise affects all measurements, in some cases the spectral purity requirement might be relaxed—when characterizing a device’s linear response, for example. This is because the VNA knows the frequency of the stimulus signal. It can therefore tune to the appropriate frequency and make accurate measurements, even in the presence of undesirable frequency components.
Nonlinear measurements such as intermodulation distortion and frequency translation, however, are more likely to be affected by unwanted frequency components from the source.
Referring back to the block diagram in Figure 1, we can see that two receivers are incorporated at the DUT’s input port (Port 1) to measure the incident and reflected waves. The receiver for the reference channel processes the stimulus; the receiver for the measurement or test channel measures the unknown reflected signal.
A receiver is also present at the DUT’s output port (Port 2) to measure the signal emitted by the device. The VNA in Figure 1 also allows us to route the stimulus signal to Port 2, making it easier to measure both the output reflection coefficient and the DUT’s S12 transmission coefficient. Therefore, each of the VNA ports has a reference receiver and a measurement receiver behind it.
Since it’s difficult to determine the amplitude and phase angle of high-frequency signals, the receivers convert the input waves to equivalent low-frequency signals. These, in turn, are converted to their corresponding digital signals, which are then used to find the origin signals’ amplitude and phase information.
Interestingly, once equipped with these receivers, a VNA can be combined with one or more antennas to create a radar system. By applying imaging techniques, we can use such a radar system to detect invisible material defects without resorting to X-ray technology.
Heterodyne Receiver Architecture
VNA receivers commonly employ a heterodyne architecture. The term heterodyne derives from hetero (different) and dyne (to mix). Appropriately enough, these receivers mix two different-frequency signals: one from the input and one from a local oscillator.
Figure 5 shows a simplified block diagram of heterodyne reference and test channels. The input waves are labeled as VA and VB; the local oscillators are denoted by LO. A single digital signal processor (DSP) operates on the signals from both channels.
Figure 5. Simplified block diagram of a VNA’s reference and channels. Image used courtesy of Steve Arar
In Figure 5, each high-frequency input signal:
- Passes through a band-pass filter (BPF).
- Enters an RF mixer.
- Is mixed with a signal from the receiver’s local oscillator (LO).
- Leaves the RF mixer and passes through a low-pass filter (LPF).
- Passes through an analog-to-digital converter (ADC).
- Enters the DSP.
The band-pass filter performs image rejection for the RF mixer. The mixer then converts the RF input, which has the frequency fRF, to an intermediate frequency (fIF). This frequency is given by:
where fLO is the frequency of the local oscillator.
The RF mixers play a critical role in the dynamic range of the VNA. Applying a very large signal to the mixer can produce distortion, whereas a very small signal cannot be distinguished from noise. Therefore, the design of down-conversion mixers usually entails a crucial compromise between the noise figure and linearity of the system.
The intermediate-frequency (IF) low-pass filter represents the next block in the signal chain. This filter is used to limit the signal bandwidth, preventing aliasing in the ADC. It also keeps a large portion of the received noise out of the subsequent links of the signal processing chain.
Finally, the ADC digitizes the signal and delivers it to the DSP for further processing. The DSP determines the amplitude ratio and phase difference of the reference and test input signals. It then uses this information to characterize the DUT performance. To produce accurate measurements, the test and reference receivers must be well-matched.
The Digital Signal Processor
Figure 6 shows some additional details of the DSP functionality.
Figure 6. Simplified block diagram of a DSP used in vector network analysis. Image used courtesy of Rohde & Schwarz
As you can see in the figure above, this DSP includes a digital down-converter (DDC) to handle digital IF processing. Two digital multipliers are used here as a quadrature mixer to down-convert the IF signal to DC. If you want to learn more about the functionality of this part of the receiver, please refer to “Fundamentals of Vector Network Analysis” from Rohde & Schwarz.
In this article, we learned about the inner workings of VNAs by examining their signal sources and receivers. Future articles in this series will explain how to calibrate, analyze, and improve VNA performance. Until then, I hope you’ve found today’s discussion interesting and informative.
Featured image used courtesy of Adobe Stock