Overview:

A time difference of arrival RDF unit utilizes the perceived frequency modulation of an incoming rf signal by switching between two antennas and comparing the timing of the antenna switching to the polarity of the tone generated in the receiver audio. This can be used to drive a meter needle or LEDs to give a left / right indication.

RDF Unit:

Circuit Block Diagram:

The 555 timer generates a 500Hz square wave output to the antenna switching unit through a low pass filter which prevents the RF picked up by the antenna from interfering with the circuitry.

The antenna unit uses MPN 3404 PIN diodes to switch between the two antennas. If the antennas are equidistant from the transmitter there is no perceived frequency modulation, however if one antenna is closer to the transmitter then the other there will be a perceived frequency modulation at the point when the antennas are switched due to the phase difference of the RF signal at the antennas.

The RF output from the antenna unit passes through a high pass filter to remove the 500Hz square wave and the signal is feed to the RF input connector on the radio.

When the antennas are not equidistant from the transmitter the perceived frequency modulation generates a 500Hz tone in the receiver audio (DF tone). The RDF unit passes the receiver audio through through a low pass filter to remove any audio above 500Hz. The DF tone is then amplified and processed by a CD4066 quad bilateral switch. The CD4066's control lines are connected to the 555 timer's output. The CD4066 switched the polarity of the 50uA meter such that the amplified DF tone drives the meter in one direction(left/right) based on the direction of the transmitter.

View the system my RDF unit was designed from at WB2HOL's home page.

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Copyright © 1998 - 2005 by Andrew Seltzman. All rights reserved.