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FM TX impact of increasing voltage

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axeman22

New Member
Hi,

I have built the FM TX in the cct attached. I'd like to know what would be the net effect of raising the supply rail to 12 or 13.2 Vdc - would it increase output power, blow components or ..?

also, is there any way to calculate the output power if the Vcc is at 9 Vdc ..?

Many thanks,

Ian
 

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audioguru

Well-Known Member
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It is a simple circuit with poor performance:
1) It does not have a voltage regulator so its frequency will change as the battery voltage runs down.
2) It does not have pre-emphasis (treble boost) like all FM radio stations have so it will sound muffled on an FM radio like your stereo with its treble tone control turned all the way down.
3) It is mono, not stereo.

With a 9V supply, its output power is about 12mW.
Increasing the battery voltage to 13.2V might cause Q3 to overheat and the output power and range will increase a little.
 

axeman22

New Member
It is a simple circuit with poor performance:
1) It does not have a voltage regulator so its frequency will change as the battery voltage runs down.
2) It does not have pre-emphasis (treble boost) like all FM radio stations have so it will sound muffled on an FM radio like your stereo with its treble tone control turned all the way down.
3) It is mono, not stereo.

With a 9V supply, its output power is about 12mW.
Increasing the battery voltage to 13.2V might cause Q3 to overheat and the output power and range will increase a little.
Why thankyou Sir.. excellent reply.

I wonder..:

1. how did you calculate the power - I'm assuming you mean RF power. interested to see the forumla/logic if you will.

2. I'd like to have it run at 13Volts so and the increased power output would be good. Can I just increase the package of Q3 - ie. a higher heat dissipation one like TO-220 etc ? - or would the power gain be so small it'd not be worth it, better to reg the thing to 9V and be done with it perhaps?

3. Mono is fine as I'm feeding mono in, just the output from a PICAXE project which sends a Morse code Ident.

4. Is there a simple way I can make the signal a bit nicer i.e. the treble issue you suggest.. it doesn't sound too bad really.

yep! you're right.. I've had it running for ages with the CW ident going continuous.. battery voltage is dropping and yes freq is dropping also!
 

audioguru

Well-Known Member
Most Helpful Member
1. how did you calculate the power - I'm assuming you mean RF power. interested to see the forumla/logic if you will.
The antenna is about 75 ohms. All these simple FM transmitters have an output power of only about 5mW to 20mW when they have a 9V supply.

2. I'd like to have it run at 13Volts so and the increased power output would be good. Can I just increase the package of Q3 - ie. a higher heat dissipation one like TO-220 etc ? - or would the power gain be so small it'd not be worth it, better to reg the thing to 9V and be done with it perhaps?
I have never seen a TO-220 transistor that works above only a few MHz, certainly not as high as 100MHz.

Is there a simple way I can make the signal a bit nicer i.e. the treble issue you suggest.. it doesn't sound too bad really.
You don't need the treble boosted with Morse Code beeps.

yep! you're right.. I've had it running for ages with the CW ident going continuous.. battery voltage is dropping and yes freq is dropping also!
My FM transmitter is powered from a 9V alkaline battery that quickly drops to less than 7V where an ordinary 7805 regulator does not work. So I use a "low dropout" 5V regulator that still regulates when the battery drops to less than 5.5V.
 
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