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Author Topic: Db level  (Read 3367 times)
WA8JNM
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« on: October 05, 2011, 03:40:46 PM »

Would someone enlighten me with the answer to a very basic question that has confused me for an inordinate length of time? When we reference signal strength, the minus sign throws me off. What is the greater (louder) signal strength, -80 db or -110 db?

Thanks,

Dave
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KG4NEL
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« Reply #1 on: October 05, 2011, 03:46:06 PM »

Think of it as points on a line of numbers... -110 is "more negative" than -80, so it's less when compared to the 0dB reference.
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WB6RXG
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« Reply #2 on: October 05, 2011, 03:52:01 PM »

Signal Strength is usually specified in dbm or decibels referenced to a milliwatt.  A 0dBm signal would be 1 milliwatt.

This makes the -80dBm signal the stronger signal over -120dbm.

73,
Stuart
wb6rxg

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W8JX
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« Reply #3 on: October 05, 2011, 04:30:08 PM »

Also consider this, most wireless routers and access points will only hear down to -80 to -90db tops. And generally need -70db or stronger to have a reliable connection and better than -60 to get any real speed.  Ham gear can hear considerably below this level.
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All posted wireless using Win 8.1 RT, a Android tablet using 4G/LTE/WiFi or Sprint Note 3.
WB6BYU
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« Reply #4 on: October 05, 2011, 04:46:26 PM »

Numbers in dB represent a ratio, not an absolute level.  Somewhere you have to specify what the
reference level is that you are comparing to.

Very common is dBm, or dB relative to 1 milliwatt.  A 1 microwatt signal is -30dBm, that is, 30dB
weaker than 1 milliwatt.  (Each power of 10 is 10dB.)

Similarly, 1 watt is +30dBm, or 30dB stronger than 1 milliwatt.


So the larger the magnitude of the negative number, the weaker the signal is relative to the reference.

But you always have to have a reference when talking in dB.
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W8JI
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« Reply #5 on: October 05, 2011, 05:11:19 PM »

reference.

But you always have to have a reference when talking in dB.

Weeeelll, you should state the reference point, but a lot of people don't give a reference.

That's so the ratio they give can mean anything they damn well want it to mean.  :-)
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KA4POL
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« Reply #6 on: October 05, 2011, 10:06:55 PM »

Just take a look: http://en.wikipedia.org/wiki/Decibel
There you find lots of information including all the whens, wheres and whats  Wink
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WA8JNM
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« Reply #7 on: October 05, 2011, 11:31:45 PM »

That's very helpful, guys. Thanks. So if I injected one milliwatt into my receiver's antenna terminals, the resulting signal strength is defined to be 0 dbm.  And, the m in dbm states my reference point, correct? So -80 dbm is stronger than -120 dbm. But both are weaker than my injected 1 milliwatt, and hence the minus sign in both. Right?
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G3RZP
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« Reply #8 on: October 06, 2011, 12:33:44 AM »

Yes. Thermal noise is -174dBm in a 1Hz bandwidth: for every ten times that bandwidth, thermal noise rises by 10 dB. So in a 1 kHz bandwidth, thermal noise is -144dBm.

Which is why the WiFi needs so much signal - it has a wide bandwidth.
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RFRY
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« Reply #9 on: October 06, 2011, 03:45:36 AM »

A little development of this topic...

Most receivers state their input sensitivities in units of µV needed at their antenna input terminals.  That is a measure of voltage, not power (such as dBmW).

That value of voltage is not the same as the E-field intensity of the arriving r-f wave, which typically is expressed in units of µV/m, and describes the r-f voltage existing between two points in space separated by a linear distance of one meter.

An S meter calibrated to read S9 with a receiver input voltage of 50 µV does not necessarily mean that the E-field intensity of the arriving radio wave is 50 µV/m when the meter reads S9.  The S meter is measuring the voltage at the receiver input terminals, and the voltage there depends on the characteristics of the receiving antenna system, and its physical orientation and polarization with respect to the arriving EM wave.

As an example, a 144 MHz EM wave arriving at a matched, center-fed, 1/2-wave dipole oriented for maximum output voltage would need to have an E-field value of about 148 µV/m in order to produce a reading of S9 on a receiver with a 75 ohm input impedance that is calibrated to show S9 when the voltage at its input terminals is 50 µV.

R. Fry
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G3RZP
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« Reply #10 on: October 06, 2011, 04:54:15 AM »

The dBm business seemed to come from the electronic warfare/radar people. Originally, manufacturers specified input voltage - often without saying what SNR it represented! For example, -107dBm receiver sensitivity for a given (S+N)/N should be a 1 microvolt input signal, but it assumes an input impedance of 50 ohms, which may or may not be the case. An older approach was to specify the EMF rather than the PD, but marketing types didn't like that, since a 1 microvolt PD was 2 microvolts EMF if matched, (and maybe more if it wasn't) so the EMF voltage always looked worse.
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AA4PB
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« Reply #11 on: October 06, 2011, 05:23:31 AM »

That's very helpful, guys. Thanks. So if I injected one milliwatt into my receiver's antenna terminals, the resulting signal strength is defined to be 0 dbm.  And, the m in dbm states my reference point, correct? So -80 dbm is stronger than -120 dbm. But both are weaker than my injected 1 milliwatt, and hence the minus sign in both. Right?

That's correct. So given that 3dB represents a doubling of power,
 -3 dBm = 0.5mW
  0 dBm = 1mW
+3 dBm = 2mW

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RFRY
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« Reply #12 on: October 06, 2011, 05:39:07 AM »

That's very helpful, guys. Thanks. So if I injected one milliwatt into my receiver's antenna terminals, the resulting signal strength is defined to be 0 dbm.  And, the m in dbm states my reference point, correct? So -80 dbm is stronger than -120 dbm. But both are weaker than my injected 1 milliwatt, and hence the minus sign in both. Right?

Just a word of caution:  injecting 1 mW into the r-f input connector of a receiver with a 75-ohm input Z would produce an S meter reading of about 75 dB above S9, if the receiver was calibrated for S9 with a 50 µV input, and the meter was accurate.

Some receivers may be damaged by that level and above.
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WB6BYU
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« Reply #13 on: October 06, 2011, 07:59:41 AM »

Also note that the abbreviation is dB, with the "B" capitalized.  That's because a decibel is
(as the prefix would suggest) 1/10 of a Bel, named for Alexander Graham Bell.  It's the same
reason we capitalize the H in kHz (for Hertz):  it is the abbreviation of a person's name.
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RFRY
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« Reply #14 on: October 06, 2011, 08:15:22 AM »

Also note that the abbreviation is dB, with the "B" capitalized.  That's because a decibel is
(as the prefix would suggest) 1/10 of a Bel, named for Alexander Graham Bell.  It's the same
reason we capitalize the H in kHz (for Hertz):  it is the abbreviation of a person's name.

However as you correctly wrote, if the word is spelled out, it is written decibel, not deciBel.  Likewise it is kilohertz, not kiloHertz, milliwatt not milliWatt, etc etc.

To be picky, dBm really means nothing in a literal sense, although its meaning is assumed by convention to describe a power stated in decibels with respect to one milliwatt.   But "dBm" could just as well apply to any basic unit of measure such as a volt (for example decibels with respect to one millivolt).

The unambiguous abbreviation for decibels with respect to one milliwatt would be dBmW or dB(mW).

RF

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