What is VNA?

Vector network analyzers (VNAs) are test instruments that measure electrical network parameters. They are essential for radio frequency (RF) and microwave component analysis of a wide variety of passive and active devices including filters, antennas and power amplifiers.

Network analyzers are ideal for transmission, reflection and impedance measurements as well as s-parameter measurements during design and production.

Network analyzers can be used to characterize radio frequency (RF) devices. Although initially only measuring S-parameters, network analyzers are now highly integrated and advanced in order to outperform the device under test.

Let's take a look at the basics of network analysis and then see what other advanced measurements a network analyzer can perform.

What is a network analyzer?

Network analyzers can be used to characterize radio frequency (RF) devices. Although initially only measuring S-parameters, in order to outperform the device under test, today'snetwork analyzerAlready highly integrated and very advanced.

RF circuits require unique test methods. Voltages and currents are difficult to measure directly at high frequencies, so when measuring high-frequency devices, they must be characterized by how they respond to RF signals. Network analyzers characterize devices by sending known signals to the device and then making constant ratio measurements of the input and output signals.

Early network analyzers only measured amplitude. These scalar network analyzers could measure return loss, gain, VSWR, and perform a number of other amplitude-based measurements.

Most network analyzers today are vector network analyzers-measuring both amplitude and phase. Vector network analyzers are an extremely versatile class of instruments that can characterize S-parameters, match complex impedances, and make time-domain measurements.

The high-level block diagram of this measurement example shows that there is a signal sent in the forward direction through the input of the device under test to the output. The measurement from the input to the output of the device is referred to as theForward measurementThe

The receiver side of the network analyzer measures the incident, reflected and transmitted signals in order to calculate the forward S-parameters.

Key Specifications of Vector Network Analyzers

Vector network analyzers are both signal generators and receivers, so they have a large number of very necessary technical specifications. In this section, you will learn about some of the key technical specifications of network analyzers.

Maximum frequency

The maximum frequency of a VNA is the highest frequency it can measure. The receiver side of a network analyzer has an analog-to-digital converter (ADC) that converts the input signals into digital format. These signals can then be analyzed and displayed. However, the ADC does not have the ability to convert signals in the RF range, so the incident signal must be downconverted to its operating frequency. This operating frequency is called the intermediate frequency (IF).

dynamic range

Dynamic range is the range of power over which the response of a component can be measured.

This figure shows two different ways of defining dynamic range. The system dynamic range is the value used for instrument specifications.

• The system dynamic range is the function of the instrument when no boost amplifier is used and the gain of the device under test is not taken into account. The maximum source power of an instrument is its maximum power level, Pref
• The receiver dynamic range is the dynamic range of the instrument when power amplification is used. Unlike using the source power as the maximum power level, this specification is based on the maximum power Pmax that can be measured at the receiving end of the instrument.

A trace measured by the bandpass filter S21, which shows the dynamic range of the instrument, is shown in the lower left-hand plot. The upper limit of the trace is relatively flat and the lower limit contains some noise. Let's look at what factors determine the shape of these boundaries.

The maximum power level of the dynamic range is determined by the upper limit of the source power level and the compression point of the receiver.

The mixers and amplifiers that make up a receiver can only handle so much power before they reach saturation, or before they reach maximum output. When a device is in the saturation region, there is no longer a linear relationship between its input and output.

The saturation of the amplifier can be seen in the right-hand diagram below. At input powers higher than 1W, the actual output (red) will deviate from the desired output (green). This phenomenon is called compression. The receiver cannot capture the signal from any device output above its compression point. This limitation of the input power constitutes the upper limit of the dynamic range.

output power

Output power reflects how much power the VNA's signal generator and tester can transmit into the device under test. It is expressed in dBm and is referenced to 50Ω impedance to match the characteristic impedance of most RF transmission lines.

High output power is useful for improving the signal-to-noise ratio of a measurement or determining the compression limits of the device under test.

Many active devices, such as amplifiers, require extremely challenging linear and nonlinear high power measurements beyond the power limits of the network analyzer.

trace noise

Trace noise is the superimposed noise you see forming on the response of the device under test due to random noise in the system. It can make the signal look less smooth and even a little jittery.

Trace noise can be eliminated by increasing the test power, decreasing the bandwidth of the receiver, or averaging.