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Continuous-Wave Propagation Channel-Sounding Measurement System - Testing, Verification, and Measurements
Chapters
Summary June 25th, 2021
This report describes the setup, validation and verification, and results from propagation measurements using a continuous-wave, radio frequency channel-sounding measurement system.
Transcript
Propagation measurements inform models and policy. Verifying and validating a measurement system prior to making these types of important measurements is critical. We have used this technique to validate other measurement systems so that the measurements are consistent.
This system is easy to assemble and use. Using this protocol will ensure that you understand your system before making measurements in a more complex environment. This protocol does not address electromagnetic interference or electromagnetic compatibility issues which can appear when equipment is placed near each other.
These are issues that can be overlooked by a new RF engineer and thus it is important to work with more experienced engineers. Some aspects of this setup cannot be fully explained in words. Pictures can be helpful in assembling the measurement components correctly.
Before assembling the system, use a VNA to measure the S parameters for the cables, attenuators, power splitters, directional couplers, and low-pass filters. To measure the transmitting system with VNA, assemble the type N cable that connects to the output of the power amplifier, the directional coupler, the bandpass filter, and the type N cable that will be connected to the antenna. Use the VNA to measure the component chain and record the S21 value, which will be a negative number.
To measure the receiving equipment with the VNA, assemble the type N cable that will be connected to the receiving antenna, the filter, the cable between the filter, the power splitter, and the type N cable that will be connected to the VSA. Use VNA to measure the S21 for the VSA side of the receiver, then connect the cable that connects the spectrum analyzer side to the VNA and measure the components again to obtain their S21 values. Before making any measurements, power on the vector signal generator, making sure that it is set to RF off, the power meter and the power amplifier.
Allow the instruments to warm up for an hour prior to any measurements. When the instruments have warmed up, configure the VSA in the VSA 89601B mode. After configuring the spectrum analyzer, press Enter on the spectrum analyzer to access the menus and hold the Shift button while selecting the System button to enable the external reference.
Use the soft keys to select more, port settings, external input, and reference, and select a continuous wave output. To configure the VSG, set the frequency to 1, 717 megahertz. Set the VSG output amplitude to minus four decibel milliwatts and the upper limit to the linear range of the power amplifier.
To calibrate the power meter, plug the head into the reference port and plug the end into a measurement port. Set the power meter frequency to 1, 770 megahertz and zero and calibrate the power meter, making sure that the power meter reading remains within 0.2 decibel of zero decibel milliwatts. Then unplug the power meter head from the reference port and connect it to the output of the attenuator.
To synchronize the rubidium oscillators, set the voltage, taking care not to exceed the maximum input voltage allowed on the rubidium synchronization port and set timeline to 100 milliseconds and the y-axis to IQ.To align the power supply frequencies, press the current voltage button on the power supply while watching the dot on the VSA screen. If the dot rotates back and forth, the frequencies are aligned. If the dot rotates consistently in one direction, change the voltage until the dot on the IQ plot begins to slow down while moving back and forth in a pendulum motion.
When the frequencies are aligned, set the timeline back to one second and the y-axis back to log magnitude. To calibrate the VSA, select utilities, calibration, and calibration, and set the RF button on the VSG to on. Then obtain 10 records of acquisition on the spectrum analyzer to verify that all of the parameters have been correctly set and that the spectrum analyzer signal level matches the VSA signal level.
To verify the lab, insert a variable attenuator between the transmitting and receiving sides of the system without attaching antennas and set the step attenuator attenuation to zero decibel and the number of records on the VSA to 120. Set the number of sweeps to 120 records and the output amplitude of the VSG to zero decibel milliwatts. Set the RF button on the VSG to on.
Set a peak marker to find the value of the signal strength. If a signal can be observed on the VSA, press Record to start the verification, then start an SA measurement in the instrument control software. In this representative analysis, a single sweep spectrum analyzer data capture consisting of 461 points over a 0.5 second sweep time was plotted and the GPS information was assigned to the mean value.
The in phase and quadrature magnitude data were then compared to the smooth mean power over a 0.5 second window for the entire dataset to approximate a 40 wavelength driving distance. Plotting of the VSA and spectrum analyzer alignment data as functions of the elapsed time can be used to predict terrain losses. The VSA data are corrected by adding system losses and removing system gains to obtain the measured basic transmission loss or gain along the drive route as illustrated.
In this analysis, the basic transmission gain was equal for the irregular terrain model basic and free space transmission gains confirming that there were no terrain interactions. When the irregular terrain model basic transmission gains equal the free space transmission gains, it can be assumed that all losses come from buildings, foliage or other interactions with the surrounding environment. It is very important to check all the components of the system prior to measurement.
Broken cables are very common. The system is also initially tested in the lab to understand the measurement and the calculation of path attenuation. Short outdoor measurements near the facility will help understand measurements taken in complex environments.
The paths between transmitting and receiving antenna should include paths both with and without obstructions. These obstructions can be trees, buildings, or other structures. This system has been used to study outdoor propagation in different environments such as forested areas, urban areas and rural areas.
We have provided attenuation estimates for customers based on the measurements. Models can then be developed knowing that the measurement is validated.
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