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Author Topic: RF amplifier modification to a low power FM transm  (Read 1793 times)
SIRIUSMOJO
Member

Posts: 2




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« on: November 01, 2003, 07:49:00 PM »

excuse me if i am in the wrong place I just had nowhere else to go with this technical question. I've asked everyone who I can think of and done some research, but still can't seem to get an answer.
 
 
 
i have an iTrip low power FM transmitter (www.griffintechnology.com) for my Apple iPod and I am trying to increase the power just a few milliamps to get better reception from it.
 
I got the following information from the Ramsey FM10a FAQ and was wondering if it could be applied to the iTrip i have.
 
this is for a ramsey FM10a

please excuse the diagram i couldnt get it to display properly....

C1 is the cap that goes to the antenna currently

i plan on putting C1 to the transitor which then goes to the ground, 12 volt power supply & 220ohm resistor and the final wire goes to the 9k resistor and C2 then the antenna
 
it is a amp mod to increase the power
 
could i take this exactly as is and port it to my iTrip by just connecting it between the last cap and the antenna ....

what would the value of capacitor C2 be.... should i use the same as is used on the ramsey FM10a or do i need to figure out what the value of the last capacitor (C1) is on the iTrip
 


Ramsey FM-10 70mw output amplifier
----------------------------------
Provides almost 9db gain to bring the output power of the Ramsey FM-10 Stereo
transmitter from 8mw to 70mw.  Not the best design, but all parts can be
found at Radio Shack!  Much better designes are soon to be available at the
archive site.

                          +12v
                          |
                          /
                          \ R1 *220 ohms(1/2 watt)
                          /
                          \
                 R2 9k    |       C2
               -/\/\/\/-----------||-----> output
              |         /
              |        /
              |   |  /
          C1  |   |/         <----------mps2222a (276-2009)
    in  --||------|\              -or-  2n4401
          ^       |  -> --
          |              |
          |              gnd
  currently on board

* you can also use 2 440 ohm 1/4 watt resistors run in parallel

I built this thing right on the underside of the FM-10 kit, C1 is the
cap that currently goes to the RCA ant jack, the 9k and the 220 ohm
resistor have to be bought, note that if you cannot find 220 ohms you
can make one by using 2 440 ohm resistors in parallel, and that a 10k
will work in place of the 9k but yealds poorer performance (-5%).

The mps2222a is from Radio Shack part number 276-2009, use this part! if
you substitute it for a 2n2222a you will get only half the gain.  Be
very careful to get the leads in the correct orientation!

I have found that a 2n4401 can be used in place of the mps2222a with a
little better performance, about 5mw more.  I think the 2n4401 can be
found at Radio Shack too.

C2 is of the same value of C1, I took the one that goes to the on board
antenna pad.

Important! the value for R2 that seems to be optimal is 220 ohms, but it
is very close to the sat point, If the amp. seems noisy (interferes with the
TV etc.) back this value off to 240 ohms.  If you lower this value below 205
ohms the power meter may read higher power but this will not be true, the
transistor will be spewing all kinds of junk and the power meter will
mistake this for higher output (in reality the signal we want will drop
considerably.)

Well that's it, effective range with a good antenna should be a little
over double.
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W9GB
Member

Posts: 2611




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« Reply #1 on: November 02, 2003, 05:35:59 PM »

You are operating under Part 15 of the FCC regulations at this power level (of the original kit design).  Details of Part 15 can be found at FCC web site

An increase power (or a change in the permitted antenna) would place you outside of those regulations - "harmful interference" and into the area of FCC licensed commercial broadcasting.  You are then subject to federal penalties under these regulations.

What harm can it do?  Well if your power increase disrupts a neighbor's listening on their FM or stereo system - you are the culprit - and held responsible.

G. Beat
w9gb
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SIRIUSMOJO
Member

Posts: 2




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« Reply #2 on: November 02, 2003, 09:59:50 PM »

this is not for use in the US
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W9GB
Member

Posts: 2611




Ignore
« Reply #3 on: November 02, 2003, 11:49:58 PM »

OK,  have you taken apart the Apple iTrip product to "reverse engineer" ? - since you probably do not have a schematic.

I see that this was for car usage - to play through automobile receiver.  [www.griffintechnology.com]

How far are you trying to get?  A properly cut antenna (cheap FM twin lead style dipole) - shoudl imporve signal greatly

gb
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ANULLRETENTIVE
Member

Posts: 60




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« Reply #4 on: November 04, 2003, 04:16:33 AM »

Forget your schematic. Go to minicircuits.com and look at the ERA-5 mmic. Don't forget to add a few poles of Low Pass filtering as this device is spec'd to over 4 Ghz. You wouldn't want any harmonics amplified.
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GREG418
Member

Posts: 1




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« Reply #5 on: July 03, 2004, 09:11:41 PM »

Hello,

minicircuits.com's ERA-5 specifications present a typical amplification configuration -- their "Biasing MMIC Amplifiers" application note ( http://www.minicircuits.com/appnote/an60010.pdf ) also presents one, with an additional bypass capacitor.

How would the blocking and bypass capacitors' values be calculated for an optimum amplification of a FM range transmission?

Also, how could a 'simple' low pass filter be built, and integrated to the circuit?

Thanks in advance!

Greg
Quebec City
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