RF Measurements

ETE 437

Course Description:

Electronic measurement equipment and techniques for measurements at radio frequencies of such quantities as power, impedance, standing wave ratio, frequency, voltage and current, Smith Charts, impedance matching, Network Analyzer usage and measurements.

Velocity of Propagation of Electronic Signals

The objective of this lab is to measure the velocity of propagation of a wave through a RG-58 Coaxial Cable. In this lab we had to measure Frequency,Time Delay, Phase Shift, Lambda, # of wavelengths in the 25 foot cable. We did this using the oscilloscope and signal generator. We adjusted the Frequency then measured the Time Delay and Phase Shift to calculate the wavelength(lambda).Which with the data collected and measure we end up seeing the figure below with Phase Shift vs Frequency in Orange and Time Delay vs Frequency in Blue. As Frequency changes the Time Delay stays relatively constant. Though as Frequency increases we see that Phase Shift increases.

RF Cable and Connectors

We take a look at various transmission line forms associated with high frequency signals. First we examined coaxial cables and measured their physical characteristics Figure 1. Next measuring the physical size of waveguides we could determine the cutoff frequency of operation based on the longest cross-sectional dimension which is equal to lambda over 2. Where the approximate operation frequency will be greater than one and a half times the cutoff frequency as we see the data in Figure 2. Taking data points across the frequency range of 100Mhz to 1300MHz of a type n, BNC, and SMA cable, we examined the power loss of each cable determining that certain cables has less power loss at specific frequencies over that of others see data below.

Pulse Transients on a Transmission Line and PSPICE

This lab was to understand through measurements the effect load impedances have on a transmission line signals with a matched and unmatched source. Also the effect pulse timing has on transmission line signals. Given the basic outline below in figure 1. We take a matched system, our system and transmission line (coax cable) are matched impedance and we change the load. Below you see the initial voltage and reflected voltage of each system and their related gamma value. We also emulated each part on PSPICE as in the figures below.

Figure 1

 1a matched system with 50Ω load 1d matched system with short > load > Zo 1b matched system with Open load 1e matched system with short < load < Zo 1c matched system with Short load

PSPICE with transmission lines

Lab Slotted line

Using a Slotted line we can determine the VSWR, First a probe serves as the antenna to detect the RF signals, it performs the non-contacting “through the air” detection of the voltage,the resonant circuit is a cavity tuned by the tuning stub which is the equivalent of a parallel LC circuit to “peak” the signal, a 1N21B diode in the detector output path provides the same function as the crystal diode converting the AM high frequency signal into a rectified voltage and current. We use this output to determine the VSWR by taking measurements at different milimiller positions which because our frequency is 833.33MHz it gives us a lambda value of 0.36 meters thus 1 mm -1 degree of phase. We than record the voltage for every 10mm position on the line giving us a full 360 degrees of measure measurements as we can see in the data below.

Sinusoidal delay line

We observed delay in a transmission line, through emulating a transmission cable based on its R,L,G,C components. Where R is the heat loss of a cable represented by a resistor, L is the magnetic field represented by an inductor, G the resistance of the dielectric material represent by a resistor, and finally C - the distance of the dielectric material as a capacitor. We used a board that has 18 LC filters which represents 180 degrees of phase shift 10 degrees at each filter. We sent a signal down the “line” and measured its phase according to its filter number as can be seen in the data below, also taking screenshots of the oscilloscope periodically.

 Channel 2 point Tap Voltage (rms)Vrms 82.8mV Phase Shift degrees Tap 1 (10 degrees) 87.8 9 Tap 2 (20 degrees) 88.4 20 Tap 3 (30 degrees) 89.7 30 Tap 4 (40 degrees) 91.4 42.8 Tap 5 (50 degrees) 93.3 51.5 Tap 6 (60 degrees) 95.0 60.1 Tap 7 (70 degrees) 96.5 70 Tap 8 (80 degrees) 97.1 82.1 Tap 9 (90 degrees) 97.1 90 Tap 10 (100 degrees) 95.2 97.2 Tap 11 (110 degrees) 93.4 105 Tap 12 (120 degrees) 90 114 Tap 13 (130 degrees) 87.9 125 Tap 14 (140 degrees) 84.8 136.1 Tap 15 (150 degrees) 82.3 147 Tap 16 (160 degrees) 80.8 160 Tap 17 (170 degrees) 80.5 171 Load Tap 18 (180 degrees) 81.2 179

 Channel 2 point Tap Voltage (rms)V1rms = 170mV Phase Shift degrees Tap 1 167 0 Tap 2 161 2 Tap 3 151 2 Tap 4 136 2 Tap 5 116 2 Tap 6 94.7 3 Tap 7 69.5 2 Tap 8 43.9 7 Tap 9 19.3 5 Tap 10 11.7 132 Tap 11 37.2 170 Tap 12 66.1 176 Tap 13 93.3 178 Tap 14 117 180 Tap 15 137 180 Tap 16 153 180 Tap 17 164 180 Load Tap 18 170 180

 Channel 2 point Tap Voltage (rms)V1rms =0 V Phase Shift degrees Tap 1 28.2 110 Tap 2 57.2 95 Tap 3 86.7 90-170 Tap 4 113 90 Tap 5 136 150 Tap 6 155 150 Tap 7 169 140 Tap 8 178 150 Tap 9 181 140 Tap 10 180 150 Tap 11 172 140 Tap 12 158 140 Tap 13 140 140 Tap 14 117 140 Tap 15 90.8 140 Tap 16 60.8 150-70 Tap 17 150 0 Load Tap 18 0 0