What Can You Do With a VNA?September 15, 2023
A Vector Network Analyzer (VNA) is a piece of test equipment that is used to make RF measurements of electronic devices. We know that RF signals can propagate on a transmission line in both directions simultaneously and a VNA can measure those signals separately. Being able to measure these incident and reflected signals allows the engineer to determine the characteristics of a Device Under Test (DUT). The block diagram in Figure 1 below shows the internal workings of a VNA.
Figure 1 – Vector Network Analyzer Block Diagram
The stimulus is switched back and forth so it can be passed out of Port 1 or out of Port 2. If the signal is leaving Port 1, then the incident port of the bridge on that side samples that outgoing signal, which is mixed down to a lower IF frequency and digitized. This is what is known as the reference signal R1. Any signal entering Port 1 will be sampled by the reflection port of the bridge, mixed down to the same IF frequency, and digitized. This is the A signal.
Similarly, the bridge on Port 2 can measure signals entering it on the reflection port and digitize them to become the B signal. Signals leaving Port 2 will be sampled by the incident port of the bridge and become the R2 signal. Figure 2 shows the measurement of a DUT in one direction. Some of the signal passes through the DUT and some is reflected back. A similar measurement can be made in the reverse direction.
Figure 2 – VNA Reflections
S-parameters are derived from these receiver measurements. For instance, the uncalibrated values of S21 and S11 are the ratios B/R1 and A/R1 respectively, and S12 and S22 are derived from A/R2 and B/R2 respectively.
All raw S-parameter values are ratioed to either R1 or R2, depending on which port is sourcing. Because of this, the absolute magnitude of the stimulus signal is unimportant. The network analyzer software applies calibration to the raw data and displays the results in a format chosen by the user. Advanced analytical features such as Time Domain and Gating are available in all Copper Mountain Technologies (CMT) VNAs at no extra cost, except for the M series models.
What Can You Use a VNA For?
A VNA is a very versatile instrument. It can be used to measure the frequency response of a filter. Filters are used to pass certain frequencies in the passband and reject frequencies in the stopband. Figure 3 below shows a 4 GHz filter measurement. The superior dynamic range of a Copper Mountain Technologies VNA allows for the simultaneous measurement of the 1.1 dB insertion loss in the passband and the 110 dB attenuation in the stopband shown in the red S21 trace. The purple S11 trace shows the return loss of the filter which is better than 20 dB in the passband of the filter.
Figure 3 – Filter Response
A VNA can also be used to measure an amplifier like the one shown in Figure 4.
Figure 4 – Amplifier
Figure 5 – Amplifier S-Parameters
Figure 5 shows the input return loss in the upper left corner, the reverse isolation in the upper right, the gain in the lower left, and the output match in the lower right. The 10 dB return loss above 1 GHz is not ideal. Adding input matching components would help improve the return loss.
At any single frequency, the VNA can be set to sweep the stimulus output power, and the software can display output power vs input power or S21 vs input power. If S21 is viewed with a power sweep, the point where it falls off by 1 dB is the 1 dB compression point of the amplifier as shown in Figure 6.
Figure 6 – Amplifier 1 dB Compression
A VNA can also be used to measure the dielectric properties of materials, or the moisture content of soil or agricultural grains. Antennas and their feedlines can be measured to verify performance. Cables, both balanced and coaxial, can be measured to verify insertion loss, return loss and characteristic impedance.
Offset measurements are possible to verify the performance of a mixer or a block upconverter or downconverter. This capability comes standard on all CMT VNAs, except for the M series.
Yes! If the VNA is set to “zero span” then it will remain at a single frequency and can be used as a signal source in the lab. The Copper Mountain Technologies 4-Port Cobalt VNAs have an auxiliary source which can be used to provide the LO for a mixer conversion loss measurement.
Can I Use a VNA as a Spectrum Analyzer?
Sort of. If the stimulus of the VNA is turned off and a signal applied to one of the ports, then the A or B receivers (Port 1 or Port 2) will show a blip on screen at the signal frequency. The IF bandwidth must be set greater than to ensure that the VNA doesn’t skip over the signal. There will be a second blip at the signal frequency plus twice the VNA IF frequency. The input signal is mixed down and there is no image filter since one isn’t needed for normal VNA operation.
The shape of the blip on the screen will not be as sharp as what is seen on a true spectrum analyzer. This is because the IF filter in the VNA is optimized for measurement speed (low delay) and not sharpness. So, there are quite a few differences between VNA vs spectrum analyzer.
Vector network analyzers have many different uses, as shown here. Copper Mountain Technologies offers world-class equipment to meet the needs of the RF engineering community. Our team of experts look forward to working with you to meet your unique requirements.