FM transmitter

[svetoslav belchev]

Hi.So I try several years to build a smal FM transmittet. I built mani
small transmitters, but no one of them can't work stable. For example
when I touch the device or move it, its frequency deviate (the
frequency is not stable).
The other problem is that there is many harmonics. Does anybody occupy
in these kind of electronics ? Thanx.

 

[svetoslav belchev]

Yes the circuit have only one transistor. I don't want to use PLL
sinteser because it need complex (difficult)circuit and components
like IC. The other variant is to use quartz cristal oscillator at
about 10-20 MHz, but it requires multipler and band filter on its
output. I need some simple decision.

 

[SioL]

"svetoslav belchev" <slav4o_1@abv.bg> wrote in message news:8924df43.0502121108.7d3bf52e@posting.google.com...

Yes the circuit have only one transistor. I don't want to use PLL
sinteser because it need complex (difficult)circuit and components
like IC. The other variant is to use quartz cristal oscillator at
about 10-20 MHz, but it requires multipler and band filter on its
output. I need some simple decision.

http://www.pcs-electronics.com/en/guide.php?sub=schematics

S

 

[svetoslav belchev]

Any siplier circuit ?
Did you think, that power amp with one transistor next to the
oscillator, may enchance frequency stability ?

 

[Andrew Holme]

svetoslav belchev wrote:

Hi.So I try several years to build a smal FM transmittet. I built mani
small transmitters, but no one of them can't work stable. For example
when I touch the device or move it, its frequency deviate (the
frequency is not stable).
The other problem is that there is many harmonics. Does anybody occupy
in these kind of electronics ? Thanx.

I presume you're building single tansistor free-running VHF oscillators.

You won't get decent stability from a free-running oscillator much above
5MHz. Even at 5MHz, you still need to take great care, use high quality
components - especially the inductor - and mount it in a screened metal box.
Temperature variation is a major cause of drift. Even at 5MHz, designers
need to select components that cause drift in opposite directions and
thereby partially cancel.

For stability at VHF: either use a PLL synthesizer; or a crystal oscillator
and frequency multiplier chain. Hand-capacity effects are more significant
at VHF because circuit capacitances are so small.