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Author Topic: Converting microvolts to DBm  (Read 3990 times)
AF5CC
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Posts: 796




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« on: September 03, 2013, 07:40:09 PM »

In terms of receiver sensitivity, I understand DBm and how to compare radios using that measurement.  I am less clear about how microvolts fit into the picture.  For example, I am looking at 2 different radios in terms of the sensitivity on 70cm FM.  QST measures one at .16 microvolts, and the other at .21 microvolts.  How much of a difference is that really in sensivity?  Is there a way to convert both of these measurements to DBm so I could get a better idea of how much these 2 radios differ on UHF FM?

73 John AF5CC
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KB3HG
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« Reply #1 on: September 03, 2013, 08:28:48 PM »

These ought to help you


http://www.hawaiirepeaters.net/dBm-to-Microvolts.pdf

and

http://en.wikipedia.org/wiki/Decibel
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AC5UP
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« Reply #2 on: September 03, 2013, 08:34:54 PM »

Mister Google knows all..............  If a tabular format is your style:     http://www.moseleysb.com/mb/mv2dbm.html

Otherwise, here's a calculator exerciser:     http://www.microvolt.com/table.html

As for the difference between .16 uV and .21 uV, extremely unlikely that would make the difference between Q / No-Q.  It's a small fraction of a dB proportionately and given the quieting curve on FM the more significant measure would be how much signal is needed for a useable s/n ratio.

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AF5CC
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« Reply #3 on: September 03, 2013, 09:03:14 PM »

Thanks for the links.  It seems that the difference between the .16 microvolt MDS (the Icom 706MKIIG and Yaesu FT100D) and the .21 microvolt MDS (Yaesu FT857D) would be about 2.5 DB difference.  Not a lot when listening to the local repeater, but I am looking at this more for the FM satellites.  Having used all three before, I get the subjective impression that the FT857D is rather deaf on SO50, compared to the other 2 radios.

73 John AF5CC
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G3RZP
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« Reply #4 on: September 04, 2013, 01:35:42 AM »

dBm is a measuring technique that derived from radar and electronic warfare. It assumes a 50 ohm matched system, so -107dBm is 1 microvolt in a 50 ohm resistive load. Not many receivers  really have 50 + j0 ohm input impedance, and in any case, if looking for lowest noise, you want to noise match the receiver input impedance - which is not always the lowest SWR match.

Microvolts of input are an older way of measuring sensitivity. Even then, there's a 6dB difference between microvolts EMF (the open circuit source voltage) and microvolts PD (the actual volts across the antenna terminals). The US always tended to use microvolts PD which gives a 6dB more sensitive receiver number, while the Europeans used microvolts EMF - that's an easier number to handle when using an artificial aerial to simulate the real antenna source impedance. An example is in marine radio at 2 MHz, where the antenna impedance is 10 ohms in series with 250pF. Obviously, the volts at the receiver input tends to go up with frequency in that case, and a dBm measurement is pretty meaningless, while a PD measurement needs the actual PD measuring, which is not always that easy.
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WX7G
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« Reply #5 on: September 04, 2013, 09:14:57 AM »

How many decibels is 0.21 uV referrened to 0.16 uV?

    20LOG(0.21/0.16) = 2.4 dB
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