Direction Finder Sensitivity

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Doppler Systems Blog

In our specifications we call out a sensitivity specification of -123 dBm.  A detailed explanation of how we define sensitivity is given in our technical application note entitled RF Sensitivity of Series 7000 (MPT) DF but I thought I would hit the high points of the application note as a way of providing an overview of what we mean by sensitivity. 

Our direction finding processor allows the user to set a sensitivity threshold.  The higher the setting of this threshold the more likely the device is to calculate a bearing.  So why not set it to the maximum?  Because a high sensitivity threshold increases the probability of a false alarm, basically calculating a bearing from noise.  So the object is to set the sensitivity threshold to a value that gives the highest probability of detecting a bearing with the lowest probability of false alarm.  Fortunately, as described in the application note, we have determined that a value of about 2000 gives optimal performance.  We allow the user to adjust this value in case they are pursuing a signal that they know is transmitting but is in the noise.  Under this condition the sensitivity can be increased to provide bearing readings on a very weak signal.  Some of our customers prefer to increase this setting to a value of 4000 to allow the operator greater latitude in determining if there is a signal on frequency or not.

There are two ways we can use to define sensitivity.  The first definition is the level of received signal that results in bearing measurements with a standard deviation of less than 2 degrees.  A weak signal will have considerable noise associated with the signal.  This noise causes the bearing readings to fluctuate.  Averaging the bearings reduces the noise.  The weaker the signal the larger the standard deviation.  For our direction finder a received signal strength of -123 dBm results in a 2 degree RMS bearing error.

Alternatively, the sensitivity is defined as the signal level that has a 90% probability of producing a bearing angle measurement.  This value depends on the threshold setting of the direction finder and is between -123 dBm and -128 dBm for threshold settings between 1000 and 4000.  In other words if you have the threshold set at 4000 you have a 90% chance of measuring the bearing of a signal with a signal strength of -128 dBm. 

For all the gory details of how we made our measurements and our results take a look at the application note.  If you have any questions feel free to call or e-mail us.

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