Big Problem with small FM transmitter - Electronic Circuits Projects Diagrams Free

PeTkO · 14th March 2008, 07:50 PM

First I want to apologize everybody for my bed englesh, because, I'm Serbian!

I bulid simple FM transmitter.Shematic is here: http://www.electro-tech-online.com/a...f?d=1101705490

Transmitter is work fine.But, range of visibility is very small, cca 20cm.When I move antenna 30-40cm from reciver, signal is very bad.Antenna is telescopic, length about 40cm.

Power supply for this transmitter is 9V battery.

Please help me!

mneary · 14th March 2008, 11:36 PM

You must have a very good receiver to achieve even marginal performance. Even with the best receiver, 1/4 mile is extremely optimistic for this power level. The range is reduced further if anyone else is using this channel or one near this frequency.

mvs sarma · 15th March 2008, 04:55 AM

See the FM Tx Mod 4 schematic moderated by Audioguru. it works fine. see HERE

A revised PCB layout for Toner Transfer method, and component overlay are attached in PDF format. Perhaps one has take a LASER print at 1:1 ratio to be able to use it.

I have added a series resistor and an electrolytic cap or the supply to Electret MIC circuit.
If not needed they could be avoided by a shorting link for the resistor and leave the capacitor.

audioguru · 15th March 2008, 02:52 PM

Hi Sarma,
Thanks for the pcb design.

Hi PeTco,
Somebody on another electronics chat website also complained about the circuit you found. I also built it to see and mine also didn't work. So I improved it in a few steps:
1) I added an RF amplifier transistor to increase its range and to isolate its antenna from its oscillator so it doesn't change its frequency when something gets near or moves away.
2) I added a low-dropout voltage regulator so that the frequency doesn't change as the battery voltage runs down and so that the preamp transistor remains properly biased (it didn't in the original circuit).
3) I added pre-emphasis (treble boost) like FM radio stations have so that the sound quality is good.

It overloads my Sony Walkman and clock radios which makes it difficult to tune because its signal is all over the dial. Luckily my Sony Walkman radio has a local-distant switch that attenuates strong local signals so I could tune it to an empty spot on the FM dial.

Its frequency is so high that it won't work if it is built on a breadboard. I used stripboard with very short traces but a pcb is also good.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Latest
i have problem with small IC,s	imgemini	General Electronics Chat	3	7th August 2004 10:50 PM
Big thermocouple ADC problem	Oznog	General Electronics Chat	9	4th May 2004 08:42 PM
3V IR transmitter problem!	brodin	General Electronics Chat	27	29th April 2004 10:34 PM
FM Transmitter problem	Zarniwoop	Electronic Projects Design/Ideas/Reviews	2	19th October 2003 09:31 AM
How To Make Very Small Transmitter ?	Bertone	General Electronics Chat	3	4th July 2003 05:58 AM

14th March 2008, 07:50 PM	(permalink)
PeTkO	Big Problem with small FM transmitter First I want to apologize everybody for my bed englesh, because, I'm Serbian! I bulid simple FM transmitter.Shematic is here: http://www.electro-tech-online.com/a...f?d=1101705490 Transmitter is work fine.But, range of visibility is very small, cca 20cm.When I move antenna 30-40cm from reciver, signal is very bad.Antenna is telescopic, length about 40cm. Power supply for this transmitter is 9V battery. Please help me!

14th March 2008, 11:36 PM	(permalink)
mneary	You must have a very good receiver to achieve even marginal performance. Even with the best receiver, 1/4 mile is extremely optimistic for this power level. The range is reduced further if anyone else is using this channel or one near this frequency.

15th March 2008, 02:52 PM	(permalink)
audioguru	Hi Sarma, Thanks for the pcb design. Hi PeTco, Somebody on another electronics chat website also complained about the circuit you found. I also built it to see and mine also didn't work. So I improved it in a few steps: 1) I added an RF amplifier transistor to increase its range and to isolate its antenna from its oscillator so it doesn't change its frequency when something gets near or moves away. 2) I added a low-dropout voltage regulator so that the frequency doesn't change as the battery voltage runs down and so that the preamp transistor remains properly biased (it didn't in the original circuit). 3) I added pre-emphasis (treble boost) like FM radio stations have so that the sound quality is good. It overloads my Sony Walkman and clock radios which makes it difficult to tune because its signal is all over the dial. Luckily my Sony Walkman radio has a local-distant switch that attenuates strong local signals so I could tune it to an empty spot on the FM dial. Its frequency is so high that it won't work if it is built on a breadboard. I used stripboard with very short traces but a pcb is also good. __________________ Uncle $crooge