I need help adjusting transmitt frq of a "coiless fm bug"

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Ohm8

New Member

• Jun 13, 2009

• #1

I have this really nifty no coil - no trimmer fm mic that i built from the somewhere on the web. It works great but, the circuit can only transmitt on a fixed frq. The transmitter works on 3 main components and a few others. They are IC-CD4069, BB109 varicap diode and a 10.7mhz ceramic xtal. PLEASE HELP. Can i maybe bias the diode. Many thanks. If u wish to view it the address is https://www.electronic-circuits-diagrams.com/radioimages/radiockt8.shtml

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jun 14, 2009

• #2

You can't, as it works off the ninth harmonic of the ceramic filter.

It'a also a VERY poor design, as it will transmit interference all over the place.

 

F

flat5

Member

• Jun 14, 2009

• #3

I would think you can very the freq. by loading the xtal with a parallel capacitor. A series coil would also bend the freq., no?
Adding a tuned circuit at the antenna would clean up the output some.

Edit: It's not a crystal actually but a filter but it might still be bendable. You might try to replace the filter with a crystal.

 

Last edited: Jun 14, 2009

Hero999

Banned

• Jun 14, 2009

• #4

I've seen that before.

I agree with Nigel, it's a very poor design because it will transmit at every multiple of 10.7MHz up to a couple of hundred 100MHz.

If you're going to add coils then why not just build a properly designed FM transmitter with coils?

 

Last edited: Jun 14, 2009

K

kchriste

New Member

Forum Supporter

• Jun 14, 2009

• #5

Or at least add another transistor stage with a tank circuit tuned to 96.3 Mhz. That way you'd get more range with very little frequency drift and it wouldn't be spraying all over the spectrum.

 

M

mneary

New Member

• Jun 14, 2009

• #6

It transmits on maybe fifteen different frequencies at the same time, is this not enough?

 

duffy

Well-Known Member

• Jun 16, 2009

• #7

All I can see on that page is a big ad that covers the content.