# Fm transmitter

Discussion in 'The Projects Forum' started by Kaiser Ed Bayawa, Jun 25, 2015.

1. ### Kaiser Ed Bayawa Thread Starter Member

Mar 19, 2015
60
6
Hi everyone. How can I detect the frequency of my circuit fm transmitter?
Can you please give my formula to compute it or any method to find it. Anu suggestions will help. Thanks!
I attached a picture.

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2. ### Bordodynov Active Member

May 20, 2015
670
194
F=1/(sqrt(Csum*L)*2*Pi) , Csum=C4+C5+Ccb+Cant+CL.CL~1pF, Ccb~3-4pF, Cant~1-5pF, Pi=3.1415. sqrt(x)=x^0.5=x**0.5

Last edited: Jun 25, 2015
absf, AnalogKid and Kaiser Ed Bayawa like this.
3. ### DickCappels Moderator

Aug 21, 2008
2,768
669
Simple technique: Tune around the FM band and see if you can hear it -without an audio signal, the output of the FM receiver will become quiet when tuned to a transmitter. If you cannot find it in the band, tune to the top of the band and press a finger against L1 to drop the frequency of the transmitter. If that doesn't work, start compressing and expanding L1 with the receiver tuned to a quiet spot on the dial.

Just in case: This circuit is not a good candidate for being built on one of those plastic plug-in breadboards.

4. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
actually impossible. And with the emitter biassing resistors not adjustable pretty rubbish doubtful it will work at all.

The 2n3904 isnt ideal at this frequency so needs correct bias. I know because i have built similar circuits and substituted with 2n3904.

You also get hFE variation anyway, so putting a fixed resistor is not good practice anyway.

5. ### DickCappels Moderator

Aug 21, 2008
2,768
669

Here is a copy of the schematic that we can read.

Yep, the component values don't look optimum, but it is a very forgiving circuit. A bypass capacitor from the +9V side of L1 to ground near R7 would increase the chances of Q2 oscillating.

6. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
Not with 2n3904, I found these circuits have a very small operating point (with clear transmission at good level). Depending on the particular transistor probably not working at all with fixed R7, same for R5

I think the gain of 2n3904 isnt very high at 100 MHz + it has internal capacitances at that frequency + losses.

Probably also relating to the fact different people use different batches/manufacturers of these transistors so results may vary.

7. ### Kaiser Ed Bayawa Thread Starter Member

Mar 19, 2015
60
6
can you please suggest what is the best transistor to be use in my circuit? please
thank you

8. ### takao21203 Distinguished Member

Apr 28, 2012
3,578
463
2n2999 uhf pnp

I sell 10 for usd2, and 25 for 3 usd

you need to change to pnp however for these

9. ### DickCappels Moderator

Aug 21, 2008
2,768
669
No reason to think the 2N3904 won't work in spite of the discouragement. If it is readily available I would try the 2N3904 first. I have used the 2N3904, 2N708, MPSH34, and 2N4401, 2N2222 and probably others in similar circuits and not experienced disappointments.

10. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
How about a 2N2222 that was the old metal TO18 number, they're usually plastic TO92 now and sometimes marked PN2222.

If you want to put some power out - a 2N3866 might do.