Jump to content
Electronics-Lab.com Community

Audioguru FM Tx


Recommended Posts

Hi Sssaaa,
The circuit you posted uses a crystal in an odd oscillator circuit. Since a crystal cannot be frequency modulated very much then the transmitter must be narrow-band for a ham radio receiver. It also doesn't have pre-emphasis like radio station FM transmitters use.

250mW is probably its input power, not its output power.

Link to post
Share on other sites
  • Replies 122
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Posted Images

This Andy guy says his FM transmitter has an RF output of 250mW. The other guy says that my FM transmitter that has nearly the same output circuit has only 10mW output. I think it is much more.

I suspect that the output stage is operating in class-B or class-C and therefore has a pretty high output power which is about half the power supplied by the battery. The coupling capacitor charges by the base-emitter junction in the output transistor rectifying the signal from the oscillator. Therefore the base voltage is a little bit negative and the transistor swings widely between saturation and well into cutoff. The tuned circuit at the output helps the wide swing and filters out some of the harmonics.

Who are we going to believe?

Link to post
Share on other sites

I tested my transmitter with a new 9V alkaline battery and it went far. After about 1 hour the voltage was about 8V and the range was less.
It should have more power output with an 11V supply but I don't know how much. The output transistor might be OK or might get too hot and melt.

The drive level to the output transistor is fixed so there is a minimum supply voltage where increasing it won't increase the power output.

The 2N3904 is a general purpose audio transitor, therefore its gain at 100MHz is a max of only about 3. A real RF transistor would have much more gain, give a much higher output power, require an emitter resistor to limit the power and require a heatsink.

Link to post
Share on other sites
The 2N3904 is a general purpose audio transitor, therefore its gain at 100MHz is a max of only about 3. A real RF transistor would have much more gain, give a much higher output power, require an emitter resistor to limit the power and require a heatsink.

you know any transistor which i use in the place of 2N3904 which will increse the power?
Link to post
Share on other sites

This circuit was posted on the other board

For all those that were asking for more power, well here is a design. This uses the output of the last circuit I posted at about 100mW. The power transistor can be a 2N3866 or 2N4427 and both work well. The transistor runs quite cool and needs only a small push-on heat sink. The values in brackets are my settings to obtain maximum output. This is quite close to my initial design values. On both 0.7W and 1W out the 2nd harmonic is 20dB below the carrier. This may need more filtering not to cause interference. Mount the 3.3k resistor directly over the 1uH choke. Use responsibly!

post-9230-14279143096221_thumb.jpg

Link to post
Share on other sites

Yes that might be the reason.

I was searching for prices on the 2n3866 and 2n4427 and found Mouser have them at $1.60 and $2.00. Unable to find any sale of the Bfs22a. The only ones are the component brokers that wants you to ask them for a quote. I tried them before and boy they want you to spend minimum $200 or more. I hate that!

Even better from JDR! $1.00 http://www.jdr.com/interact/item.asp?itemno=2N3866

Also found some 2n3866A.  What will be the difference  from a 2n3866?

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
  • Create New...