1. Home
  2. General Electronics
  3. Damping Higher Harmonics on a parallel LC-Oscillator? (1 port Low-, High-, Bandpasses?)

Damping Higher Harmonics on a parallel LC-Oscillator? (1 port Low-, High-, Bandpasses?)

Hi, I built a small FM Radio as shown on

formatting link
As it is it works pretty well, allthough I operate it without any adition antenna.

Of course there are pretty high (unwanted) peaks in the frequency-band at multiples of the operating frequency. Because I don't want to use an additional antenna, I can't filter the signal on its way from L1 to the antenna with a common "2-port Bandpass". Putting such a Bandpass (or Lowpass) between the DC-Source and the LC-Circuit did not seem to work well.

So how can I damp those harmonics on L1 Coil? I thought of some kind of frequency dependent impedance in parallel with the Coil.

I tried this with another LC-prallel Circuit which has an impedance of approx. 0 Ohms at 200 Mhz. This works as expected but seems a bit unsophisticated and the Impedance falls too slow as the frequency approaches the 200Mhz.

Unfortunately I have no Idea how to design a "higher order

1-port-Bandpass", or even more likeably a Highpass.

What would you recommend? What shoud I read? Any suggestions are welcome.

Thanks to all, and sorry for the lenght of the message.

Diego S.

Reply to
kb
Loading thread data ...

How high are the harmonics relative to the carrier?

Your layout looks pretty spread out. All the connections in the RF portions of the circuit should be as short as possible. It's impossible to tell by the photo whether you are using a ground plane. If not, you should be. The power connections should be close to the RF section and bypassed with a capacitor that has a very low reactance at 100 MHz. It won't hurt to try bypassing the power supply at other places too, particularly around TR2. Another thing you might try is moving the connection of C1 from TR1-R1 to the other side of R1 (ground). The use of a resonant antenna may also reduce harmonics.

jgreimer

kb wrote:

Reply to
Skeptic

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.