Time is money

Dec 1, 2007 12:00 PM, By Rob Barden

The effects of multipath depend on the relative phase of the signals. If the different signals are in phase, they combine and the signal strength increases. If the signals are out of phase, they partially cancel each other, decreasing the signal strength. RF signals arriving on different paths have different distances that they travel, which is what causes the phase differences.

The good news is that if you follow a consistent procedure, you can minimize the effects of multipath. The relative power, sensitivity and squelch levels should not vary by more than 5 or 6 dB. If they do, then that indicates there may be a problem. This leads to the next step, which is direct-connect testing.

Once you have completed the over-the-air test, it's time to get into the radio system to verify operation. A simple divide-and-conquer approach works well. By breaking the system in half, you can isolate the location of the problem quickly.

The most likely point to begin direct-connect tests is at the output of the amplifier. Using a radio test set, connect to the output of the amplifier and key up the radio to verify the transmitter is on frequency and outputting the proper power, and that the modulation is correct. A simple whistle test can show peak deviation in an FM system.

While connected to the radio, you also can perform a quick receiver sensitivity test. By connecting to the antenna input of the radio, you can generate RF with a 1 kHz modulating tone into the radio. By monitoring the speaker audio path, you can view SINAD and distortion of the audio. Then, by lowering the RF level, you can view the SINAD reading and correlate the receiver's sensitivity. With today's technology, a good radio should exhibit better than 12 dB SINAD at levels below -115 dBm.

If the radio is working properly but you still have a problem, you now must turn your attention to the cable and antenna using a technique called frequency domain reflectometry (FDR), which uses an RF frequency sweep to measure VSWR and distance to fault (DTF). A graph of return loss vs. distance shows the location of any fault that might be in the cable that leads to the antenna. (See Figure 1.) If there is no fault in the cable, then the only thing that will be shown is the location of the cable's end.

Once you know that the cable is good, you then can look at the VSWR vs. the frequency of the cable and antenna. You will sweep the antenna across the frequencies on which the radio is operating. You then can look at the graph of VSWR vs. frequency and determine whether the antenna is working correctly. (See Figure 2.)

By using the proper equipment, by knowing where to start and by using divide-and-conquer testing techniques, you can quickly isolate troubles in radio RF systems.

Rob Barden is director of product marketing for Aeroflex's radio test set line. He is a 24-year veteran of the test and measurement industry and graduated from Devry Institute in 1983.