# Universal FM Amplifier for Digital Tuned PLL Transmitter

Joined Feb 27, 2010
249
I just bought this transmitter- http://electronics-diy.com/product_details.php?pid=302&name=Wireless%20PLL%20Stereo%20FM%20TX%20with%20LCD%20%28fully%20assembled%29 which is nice because the price. it does digital tuning, it has memory banks for set freq, and its PLL stabilized.

also, the output is claimed to be 50mw, which in my case will usually be fine, but i also want the option to turn on an amp to make the output to 500mw or more.

i found this schematic for a Class C RF amplifier. i understand most of this, but i have a couple questions because the parts i found arent exactly the same as whats shown. I spend a few hours last night scouring through datasheets to try and get parts that match up nearly perfectly to whats shown, i think

The MIDDLE row is what im after- 100MHz/24dBm boost.

1.- the variable capacitor i found is 50v 11pf
2.- the other capacitors are basic ceramic type
3.- the FD700 diode is hard to find. mouser has a seemingly similar one- this is my biggest worry.
4.- this will be tuned to amplify around the 100mhz range, so i do not need C3.

Do all my points seem OK? i just dont know enough about RF amps to know if im doing this right. i could post the list of all my acutal parts, but i dont expect someone to compare all the datasheets and everything (what a PITA!)

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#### ispymalik

Joined Apr 7, 2008
18
When it comes to RF, Impedance matching is the name of the game.
The values of the L's and C's are made to match the input and output impedance of the joining networks.

i seethe RFC at the top of the amp but not much matching networks. The output of the amp looks like a standard T-Low pass filter.

So, if you do build it and it might help some but it will be attenuated an/or may reflect to much power in the lines and ruin some other component.

P.s. I'm learning too, actually in my first class of RF Amplifier design.

sidenote: of coarse, if freq. is low enough to use lump-matter discipline then nvm about comments

Joined Feb 27, 2010
249
i think its supposed to be just fine for regular FM broadcast (88mhz-108mhz). i read somewhere that linear amplifiers arent needed because the frequency range desired is so small. does that sound about right? (you might be learning, but it already sounds like you know more then i do)

#### ispymalik

Joined Apr 7, 2008
18
It does sound right actually.
if the Freq. is low enough with respect to the size of the components used you can avoid the wave propagation side of it.

Have you tried to model it in PSpice?

i would actually try to do it too but i have research to do on... FM communication systems for my other class.
(why else would i be at the Radio and Communications board, im learning )

Joined Feb 27, 2010
249
yeah i didnt even know there was a subforum for this stuff, hence why this thread was moved from where i started it.

i guess i could try pspice simulation, but this diagram isnt my own design i just found it on a website that claims this is all you need for a low-power fm transmitter amp. my only concern is the parts i found at mouser arent exactly the same thing as what the diagram shows (mainly, that switching diode right at the input).

as far as i can tell, as long as the diode has the same or higher switching rate, and the forward capacitance is in the same range it should be OK... i just dont know the reason why you use this type of diode here so im not perfectly sure that what i found is compatible.

#### skeptic

Joined Mar 7, 2010
51
Linear amplifiers are needed for AM and SSB transmissions. Since with FM, information is not contained in the amplitude, class C amplifiers will work.

However if you plan to operate the power amplifier in the United States, you should be aware of some FCC part 15 regulations.

15.204
http://ecfr.gpoaccess.gov/cgi/t/tex...&view=text&node=47:1.0.1.1.14.3.241.4&idno=47

15.209
http://ecfr.gpoaccess.gov/cgi/t/tex...&view=text&node=47:1.0.1.1.14.3.241.7&idno=47

15.239
http://ecfr.gpoaccess.gov/cgi/t/tex...view=text&node=47:1.0.1.1.14.3.241.25&idno=47

Joined Feb 27, 2010
249
i live on mars, the fcc hasnt found me here yet.

about the RF inductors- are 20% tolerance ones OK? because all the small axial ones i find in that range (the 1uf for example) are all 20% ones...

also, the resistors... i can double up a couple of spare ones i have laying around right? i normally just guess and test a few in parallel until i get my desired rating... so i dont think i need to buy special resistors...

otherwise, this is all the parts i think-

Mouser #: 434-22-100
Mfr. #: MICC/N-100J-00 Desc.: RF Inductors BR/BK/BK/GD 10uH RoHS: RoHS Compliant

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Availability
5 Ships Now

$0.20$1.00 Mouser #: 652-79FR47M-TR-RC
Mfr. #: 79FR47M-TR-RC Desc.: RF Inductors RF CHOKE .47uH 20% AXIAL RoHS: RoHS Compliant

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Availability
5 Ships Now

$0.20$1.00 Mouser #: 434-22-R47
Mfr. #: MICC/N-R47K-00 Desc.: RF Inductors YW/PR/SL/SL .47uH RoHS: RoHS Compliant

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Availability
5 Ships Now

$0.22$1.10 Mouser #: 810-CS80-E2GA102MY
Mfr. #: CS80-E2GA102MYNS Desc.: Ceramic Disc Capacitors X1Y2 1000pF 20% 250VAC RoHS: RoHS Compliant By Exemption

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Availability
10 Ships Now

$0.07$0.70 Mouser #: 771-BAS516-T/R
Mfr. #: BAS516,115 Desc.: Diodes (General Purpose, Power, Switching) SW 75V 250MA HS RoHS: RoHS Compliant

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Availability
10 Ships Now

$0.06$0.60 Mouser #: 140-50N5-680J-TB-RC
Mfr. #: 140-50N5-680J-TB-RC Desc.: Ceramic Disc Capacitors 50V 68pF NPO 5% Tol RoHS: RoHS Compliant

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Availability
20 Ships Now

$0.07$1.40 Mouser #: 81-TZ03T110F169B00
Mfr. #: TZ03T110F169B00 Desc.: Trimmer / Variable Capacitors 3.0 - 11pF 100V RoHS: RoHS Compliant By Exemption

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Availability
6 Ships Now

$0.32$1.92 Mouser #: 434-22-R82
Mfr. #: MICC/N-R82K-00 Desc.: RF Inductors GY/RD/SL/SL .82uH RoHS: RoHS Compliant

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Availability
10 Ships Now

$0.22$2.20 Mouser #: 610-2N5109
Mfr. #: 2N5109 Desc.: Bipolar Transistors NPN Wide Bd AM RoHS: RoHS Compliant

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Availability
3 Ships Now

$1.87$5.61

#### retched

Joined Dec 5, 2009
5,208
i live on mars, the fcc hasnt found me here yet.
Good, do me a favor...We have a rover thats stuck up there, could you grab a friend or two and pull it out of the ditch we have it stuck in? Thanks! (We will send a 1w amp to you on next mission!)

Joined Feb 27, 2010
249
sorry, currently with the earth's position to mars it takes about 35min to get your responses. add that to a day of being lazy (thats how martians roll) and you can see why for the delay. the rover is unstuck, we took off the batteries and solar panels and just plugged in a long extension cord we had laying around. that should allow you to roam at will for as long as you want. ill be looking forward for to the amp... 1w wouldnt be enough for us to reach you directly however we could still talk to eachother all over mars. there is, of course, very little interference out here.

#### retched

Joined Dec 5, 2009
5,208
Must be nice..

As for your amp, are you gonna stick with 1 frequency? or are you gonna try to boost them all?