Wireless Communication Hobby Projects

Posted on Feb 5, 2014

Another method of testing the received signal strength on a Symphony based wireless network is with a homebrew signal meter. The National LMX2240 Intermediate Frequency Receiver has a certain pin called `RSSI Out`. RSSI stands for relative signal strength indicator and supplies a voltage output that is an indication of the received signal strength. O

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n a weakly received signal the voltage output of the RSSI is small and on a strong signal it`s large. Unfortunately, this output is only on during the receive portion and off during transmit. This makes the RSSI output look like a series of spikes. To average out these spikes you can use a very common piece of test equipment on any good technician`s bench: a modulation meter. What You say Modulation meters convert received audio signals into a DC voltage indication of modulation, if you input the RSSI spikes into the meter (and readjust its ballistics) you will get a poor man`s signal meter. The meter I use is based on the Mark Weiss modulation meter. You can also make some attenuator pads to help simulate real life signal loss when testing your wireless network links. If you wish, you can order commercial inline attenuators from Mini-Circuits that will handle 2 Watts of power, DC - 18 GHz, and have built in SMA connectors for around $37 a piece. You may also combine attenuator pads to increase the overall attenuation. It`s also possible to use a long length of high-loss cable, such as RG-174, as an attenuator. Higher wattage resistors will handle higher RF power, though the frequency response will suffer due to parasitic reactances. Lower wattage resistors will handle higher power if applied in small durations (milliseconds), like which is commonly found in most wireless network applications. Here is a schematic for a...

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