Re: Receiver sensitivity parameter


mediumwavedx
 

This can be a confusing subject, so please allow me to offer this explanation.

The common unit used in measuring received field strength is volts per meter, or usually, millivolts/meter 'mV/m' (one-thousandth of a volt per meter).

Volts or millivolts per meter expresses the voltage that would be induced in a one meter long wire placed parallel to the lines of flux of the received signal (remember the electrical flux from a mediumwave tower is vertical, the magnetic horizontal). This induced voltage results from the movement of the flux across the wire.

There is some confusion which persists about the term dBµ, which is related to mV/m.

dBµ, a contraction of dBµV/m, using the Greek letter µ ['mu', meaning micro, or one-millionth], is commonly and usually written nowadays as dBu [letter 'u'] and is the figure used in recent years by the FCC for measuring electric field intensity of AM broadcast stations at prescribed distances. dBu is electric field intensity and directly related to mV/m, always measured in decibels above (or below) one microvolt/meter. The decibel measurement is a logarithmic ratio as you may know.

dBµ ['mu' again] is also sometimes used erroneously as a shortened form of dBµV, or voltage expressed in dB above (or below) one microvolt into a specific load impedance, commonly 50 ohms. Important! Here we have voltage measured across a specific load impedance like a tuned circuit or reactance or resistor.

The modern DSP receivers like the Tecsun PL-380, 310, etc. which employ the Silicon Labs chips, measure and display dBµV as received at the tuned front end across a load, not dBu-dBµV/m. They call it the RSSI indicator, and erroneously mark it as dBµ on the display [Greek 'mu' again]). It is not the same measurement as dBu-dBµV/m, which is derived from volts or millivolts/meter.

You cannot convert dBuV as shown on the DSP radios to mV/m or dBu! The value is not interchangeable.

The FCC offers a conversion calculator to convert from dBu [letter 'u'] to mV/m and back.

http://transition.fcc.gov/mb/audio/bickel/findvalues.html

Or, you can figure it yourself by using the following formula:
dBu = 20 * Log(mV/m * 1000)
To reverse the computation, converting dBu back to mV/m:
mV/m = (10 ^ (dBu / 20)) / 1000

(Log is base 10)

Bill
RADIO-TIMETRAVELLER

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