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

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- - N900 Radio Transmitter: ways to amplify the radio signal (https://talk.maemo.org/showthread.php?t=38582)

Re: N900 Radio Transmitter: ways to amplify the radio signal

Quote:

Originally Posted by YoDude (Post 448462)

Is it persistent?

No.

Quote:

Originally Posted by YoDude (Post 448462)

I
Must the transmitter be powered up?

Yes.

Re: N900 Radio Transmitter: ways to amplify the radio signal

it's maybe an idiot question but... can we put a higher value than 118 ? actually it is strength enough for me but i was just wandering ^^

Re: N900 Radio Transmitter: ways to amplify the radio signal

or to better put it?

legal issues asides? what's the highest transmit value that is safe for the radio transmitter (as in not burning it) :)

Re: N900 Radio Transmitter: ways to amplify the radio signal

yeah, I'd like to know

Re: N900 Radio Transmitter: ways to amplify the radio signal

The datasheet linked in an earlier post shows the figure of 118 dbuV typical for the programmable TXO output voltage. No minimum and maximum figures are specified.

Until someone finds application notes or other detailed documentation, I wouldn't consider going above 118, and would advise sticking with the initial figure obtained by cating the file without a charger plugged in (so as not to advise dangerously).

I've ran mine for a couple of hours at 118 and noticed no issues, but am generally using 112dBuV. IIRC an increase of 6dB will double the voltage.

Re: N900 Radio Transmitter: ways to amplify the radio signal

Ok ı'm running at 122 now, the device is still alive and running and seems stable, but I wouldn't use it like that for extended periods of time... I just felt naughty, and tbh it wouldn't be of use to me anyway with high noise so I had to do it.

Re: N900 Radio Transmitter: ways to amplify the radio signal

Quote:

Originally Posted by Kurare (Post 446738)

One thing to remember with the numbers is that the chip may have some kind of built-in Tx power management by monitoring the actual power pushed to the air vs. power reflected back from the antenna. As the antenna itself seems to be a just a metal sliver with carefully formed shape suited for antenna use and the contact is a standard metal-gold-spring-loaded touch-gold-nickel-copper contact, it is subject to variance in quality from device to device and also WILL vary as the device ages and gets dropped, dusted, humid, squeezed, dipped in beer and so on. The Tx power can quite likely be controlled to some degree by the chip itself, though the problem here is not frying the gain stage - even the maximum output is but a candle compared to the supernova-like relative luminosity of the "real" transmitter stations - but power usage that should always be minimized.

One nasty thing I noticed about the transmitter is that there's quite little fitering from the main power net of the phone to the transmitter. I noticed this when I was trying it with the not-that-good-but-adequate minihifi set I use as computer speakers and not that low sound level as I wanted to hear if the static changed as I played with the settings - the bundled charger distorted the transmitted signal to a horrible shriek that made my cat go from sleepy rest to VERY puffy in about 3,8 µs and resulted in eight bleeding deep puncture wounds on my shoulder, lots of swearing and bodily pain due to the shock from the shriek and the cat wanting to suddenly delocalize itself, which in turn resulted my computer chair toppling over and me lying on the floor, bleeding and wondering what the hell had just happened - and the shrieking just wouldn't stop until got up from the floor and pulled the charger cable - it was closer than the power button for the speaker set.

Perhaps this is why the SW is not sure whether to allow the FM transmitter to work while a power-carrying cable is plugged in? :D

When they said to cat that /proc/ interface, they meant the shell command "cat". ;)

Haha just kidding and thanks for the tragic but comedic tale. I hope you have recovered!

Quote:

Originally Posted by fnordianslip (Post 448866)

I've ran mine for a couple of hours at 118 and noticed no issues, but am generally using 112dBuV. IIRC an increase of 6dB will double the voltage.

Thanks for testing this. My electronics knowledge is fairly rudimentary, so i'm curious how that works. Since 3dB is basically a doubling in output power, does that mean that the voltage and current both increase linearly in a transmitter? I suppose either one could cause a failure.

BTW when i turn off my radio, i still get "112" as the power_level. Isn't it supposed to go away or to 0 when turned off?

Re: N900 Radio Transmitter: ways to amplify the radio signal

Well, power in Watts = voltage squared divided by the impedance (or resistance), so it isn't linear.

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.

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.

Re: N900 Radio Transmitter: ways to amplify the radio signal

Quote:

Originally Posted by j.s (Post 444581)

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:

Quote:

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

Quote:

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".

Quote:

Originally Posted by fnordianslip (Post 448886)

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.

Quote:

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. :D

Re: N900 Radio Transmitter: ways to amplify the radio signal

Quote:

Originally Posted by Flandry (Post 448917)

Is the value in the /proc/ a voltage amplification? Funny to call it "power_level" if so. :D

It is a voltage value, described as " Programmable Transmit Output Voltage, TXO", in units of dBμV, according to the linked datasheet.