maemo.org - Talk - View Single Post - N900 Radio Transmitter: ways to amplify the radio signal

	Flandry	2010-01-01 , 15:57
	Posts: 1,559 \| Thanked: 1,786 times \| Joined on Oct 2009 @ Boston	#69

Originally Posted by j.s

Bug https://bugs.maemo.org/show_bug.cgi?id=7425 entered. I am in region 2. /sys/class/i2c-adapter/i2c-2/2-0063/region_bottom_frequency is 87500

If you have a region_bottom_frequency less than 88100 and 88.1MHz is the lowest frequency available to you in the FM transmitter GUI and you want to set a freq below 88.1, please vote for bug 7425.

I voted on this bug report, but then noticed it has been closed:

Originally Posted by Andre Klapper (maemo.org) 2009-12-29 13:00:00 GMT+3

The reason for 88.1MHz lower limit of the FM Transmitter is to use the common band with FCC countries. Frequencies under 88.1MHz are removed since the allowed output power under 88MHz is lower than on 88.1-107.9MHz band in FCC countries.

This will not be changed.

While that seems likely due to the overlap with channel 6, the amount of power output by our phone transmitters is surely orders of magnitude less than the licensed stations i would expect such a regulation would apply to.

What i was able to find with a short search is that the real reason is that
the unlicensed broadcast band only goes down to 88.0 MHz, which means a center frequency of 88.1 MHz. I would assume that's the legal realm in which the FM transmitter on the N900 is working. The power of said broadcast is expressly limited to

250 µV/m at a distance of 3 meters from the antenna. This is equivalent to 0.01 microwatts.

Any transmission over that power level is "punishable by law and confiscation of all broadcast equipment".

Originally Posted by fnordianslip

Voltage expressions in dB use the "20 log rule" whereas power expressions in dB use the "10 log rule", hence the figure of +6dB for a doubled voltage level, and +3dB for a doubled power level.

Thanks; i didn't know that about voltage dB units.

Edit. Actually, you said voltage and current both increase linearly, and they actually do, as according to Ohms law R=V/I, so V and I rise linearly, but the power goes up exponentially, as P=VI.

Yeah i was thinking of Ohm's law and assuming that dB was referring to power, not voltage. Is the value in the /proc/ a voltage amplification? Funny to call it "power_level" if so.

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