VIDEO TRANSCRIPT:

What is a VNA? Unlike other RF test equipment, such as a spectrum analyzer where signals are characterized, a VNA comes into play when the components and sub assemblies of a larger system needs to be measured. A VNA is used to characterize passive devices such as cables, attenuators, filters, to active devices such as amplifiers, converters, etc. Also we can observe the linear and nonlinear behavior of a device where swept frequency or swept power technology can be used to take measurements.

Here in the image shown you’re looking at the measurements of BTS filter using Copper Mountain Technologies C1220 VNA. As you may notice in the test setup we make use of an external PC to drive the VNA. In the later sections of this webinar, we will be looking at why this is an advantage over traditional analyzers. Now let us see how a 2-port USB VNA works internally. The analyzer consists of a source oscillator, a local oscillator, source power attenuator, and a switch connecting the source signal to two directional couplers which are connected to port 1 and port 2 of the VNA. The incident and the reflected waves when the directional couplers are passed into the mixers where they are converted to first IF and then pass through the four channel receiver for further analysis.

The four channel receiver after filtering digitally encodes the signal and then supplies it to the signal processor. Either port of the analyzer can be a source of the test signal as well as a receiver of the signal transferred through the DUT. This combination of directional couplers, mixers, and a four channel receiver forms four similar signal receivers. That is two reference receivers and two measurement receivers for this two-port VNA. As seen in the block diagram an external PC controls the operation of the analyzer to the USB interface. In general, the main purpose of a VNA is to provide S-Parameter measurements of the device under test (DUT).

To perform these measurements the analyzer supplies the source signal of the assigned frequency from one of the ports to the DUT and then measures magnitude and phase of the signals transmitted and reflected by the DUT and finally compares the signal to the final compacitor results to the magnitude and phase of the source signal. The focus here is on accurate measurement of the ratios of reflected and transmitted signal to the incident signal.

So the instrument is always comparing the stimulated signal with the received signal. Also the reason why vector measurements are important is because it is essential to have magnitude and phase to completely characterize a device.