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Damping Higher Harmonics on a parallel LC-Oscillator? (1 port Low-, High-, Bandpasses?)
Hi,
I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
An idea:
Use shunt feed and a pi network. It is a low pass filter. Since TR1 is also the oscillator, you may need to add an amplifier. +V Choke | |-------Coil---- ANT | | | | = = | | | | gnd gnd | | | |/ Collector of TR1 B-| |\ Emitter "Diego Stutzer" wrote in message om... Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
An idea:
Use shunt feed and a pi network. It is a low pass filter. Since TR1 is also the oscillator, you may need to add an amplifier. +V Choke | |-------Coil---- ANT | | | | = = | | | | gnd gnd | | | |/ Collector of TR1 B-| |\ Emitter "Diego Stutzer" wrote in message om... Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
Thanks,
but i dont want to use an antenna. Or did you mean the dc-input of the LC-transistor-oscillator by "antenna"? On Mon, 16 Feb 2004 16:08:15 -0600 "Steve Nosko" wrote: An idea: Use shunt feed and a pi network. It is a low pass filter. Since TR1 is also the oscillator, you may need to add an amplifier. +V Choke | |-------Coil---- ANT | | | | = = | | | | gnd gnd | | | |/ Collector of TR1 B-| |\ Emitter "Diego Stutzer" wrote in message om... Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
Thanks,
but i dont want to use an antenna. Or did you mean the dc-input of the LC-transistor-oscillator by "antenna"? On Mon, 16 Feb 2004 16:08:15 -0600 "Steve Nosko" wrote: An idea: Use shunt feed and a pi network. It is a low pass filter. Since TR1 is also the oscillator, you may need to add an amplifier. +V Choke | |-------Coil---- ANT | | | | = = | | | | gnd gnd | | | |/ Collector of TR1 B-| |\ Emitter "Diego Stutzer" wrote in message om... Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
I am confused. The circuit on the link you provided shows an antenna. You
need to get some sort of low pass characteristic between your transmitter and the antenna. That is what I was trying to suggest. Perhaps a capacitive tap feed to the ant instead of the inductive tap. Again, with only the osc, this is not easy: Original ckt +V | |------------0 |-----| Ant |/ C = O----------0 /|C6 I | L L1 |-----| | |/ Collector B-| TR1 |\ Emitter ***************************************** +V | New circuit | |--------|-------------------0 C = Ca ant O L1 |--------COIL--|----0 I |/ L2 | L = C6 =Cb - Low pass pi net | /| | |--------| gnd | |/ Collector B-| TR1 |\ Emitter I estimate Ca & Cb to be approx 100-150 pf. L2 resonates with (1/2)*(Ca series Cb) If you are not using an antenna and just radiating from the circuit, then your only hope is to either box it up or reduce the TR1 current to reduce the harmonis content (increase R1... 200, 500, 1000, 2000,...) Steve "kb" wrote in message ... Thanks, but i dont want to use an antenna. Or did you mean the dc-input of the LC-transistor-oscillator by "antenna"? On Mon, 16 Feb 2004 16:08:15 -0600 "Steve Nosko" wrote: An idea: Use shunt feed and a pi network. It is a low pass filter. Since TR1 is also the oscillator, you may need to add an amplifier. +V Choke | |-------Coil---- ANT | | | | = = | | | | gnd gnd | | | |/ Collector of TR1 B-| |\ Emitter "Diego Stutzer" wrote in message om... Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
I am confused. The circuit on the link you provided shows an antenna. You
need to get some sort of low pass characteristic between your transmitter and the antenna. That is what I was trying to suggest. Perhaps a capacitive tap feed to the ant instead of the inductive tap. Again, with only the osc, this is not easy: Original ckt +V | |------------0 |-----| Ant |/ C = O----------0 /|C6 I | L L1 |-----| | |/ Collector B-| TR1 |\ Emitter ***************************************** +V | New circuit | |--------|-------------------0 C = Ca ant O L1 |--------COIL--|----0 I |/ L2 | L = C6 =Cb - Low pass pi net | /| | |--------| gnd | |/ Collector B-| TR1 |\ Emitter I estimate Ca & Cb to be approx 100-150 pf. L2 resonates with (1/2)*(Ca series Cb) If you are not using an antenna and just radiating from the circuit, then your only hope is to either box it up or reduce the TR1 current to reduce the harmonis content (increase R1... 200, 500, 1000, 2000,...) Steve "kb" wrote in message ... Thanks, but i dont want to use an antenna. Or did you mean the dc-input of the LC-transistor-oscillator by "antenna"? On Mon, 16 Feb 2004 16:08:15 -0600 "Steve Nosko" wrote: An idea: Use shunt feed and a pi network. It is a low pass filter. Since TR1 is also the oscillator, you may need to add an amplifier. +V Choke | |-------Coil---- ANT | | | | = = | | | | gnd gnd | | | |/ Collector of TR1 B-| |\ Emitter "Diego Stutzer" wrote in message om... Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
Unfortunately, the question is something like how to convert a VW to a
Mercedes -- you pull the radiator cap from the VW, drive it off, drive the Mercedes under, and put the radiator cap onto the Mercedes. Harmonics are caused by distortion of the generated wave. This distortion is caused by overdriving the oscillator, which is necessary unless some other method of limiting is employed. An oscillator is an amplifier with gain greater than one whose output signal is fed back to the input. When it starts, the signal amplitude increases until something limits it. In a simple oscillator like this one, the limiting factor is the smashing of the waveform as the transistor gets overdriven each cycle. Distortion is inevitable and necessary. So, you can either reduce the distortion or you can try to filter it. Filtering presents a couple of problems. One is that you're apparently radiating enough signal (and harmonics) without an antenna. That means that even if you had a perfect filter someplace, you still have enough energy being radiated from the portion of the circuit that's ahead of the filter. If you simply try to trap or reduce the harmonics at the collector, you might stifle the oscillation by cleaning up the waveform and eliminating the smashing. In general, less feedback will reduce the waveform distortion. The tradeoff is that the oscillator will start more slowly, and if the feedback becomes too little at some temperature or battery voltage, it'll stop altogether. But it you'd like to try this, fiddle with the values of C1, C2, and C4. I believe that decreasing C1 or C2, or increasing C4, will reduce the feedback. If you want try improving the collector waveform, increase the Q of the tank by decreasing L1 and increasing C6 to compensate. Or you can try putting a series LC network (with fairly high L/C ratio) from the collector to ground, tuned to the second or third harmonic, to act as a trap for one particular harmonic. Any of these schemes could make the oscillator stop, though. A professional approach would be to add a buffer stage, and build the oscillator and buffer in a shielded box, with good filtering on any lines going in or out except the output signal conductor. Then, a filter at the output would be effective, so one could be designed to take care of the harmonics to any extent desired. Roy Lewallen, W7EL Diego Stutzer wrote: Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
Unfortunately, the question is something like how to convert a VW to a
Mercedes -- you pull the radiator cap from the VW, drive it off, drive the Mercedes under, and put the radiator cap onto the Mercedes. Harmonics are caused by distortion of the generated wave. This distortion is caused by overdriving the oscillator, which is necessary unless some other method of limiting is employed. An oscillator is an amplifier with gain greater than one whose output signal is fed back to the input. When it starts, the signal amplitude increases until something limits it. In a simple oscillator like this one, the limiting factor is the smashing of the waveform as the transistor gets overdriven each cycle. Distortion is inevitable and necessary. So, you can either reduce the distortion or you can try to filter it. Filtering presents a couple of problems. One is that you're apparently radiating enough signal (and harmonics) without an antenna. That means that even if you had a perfect filter someplace, you still have enough energy being radiated from the portion of the circuit that's ahead of the filter. If you simply try to trap or reduce the harmonics at the collector, you might stifle the oscillation by cleaning up the waveform and eliminating the smashing. In general, less feedback will reduce the waveform distortion. The tradeoff is that the oscillator will start more slowly, and if the feedback becomes too little at some temperature or battery voltage, it'll stop altogether. But it you'd like to try this, fiddle with the values of C1, C2, and C4. I believe that decreasing C1 or C2, or increasing C4, will reduce the feedback. If you want try improving the collector waveform, increase the Q of the tank by decreasing L1 and increasing C6 to compensate. Or you can try putting a series LC network (with fairly high L/C ratio) from the collector to ground, tuned to the second or third harmonic, to act as a trap for one particular harmonic. Any of these schemes could make the oscillator stop, though. A professional approach would be to add a buffer stage, and build the oscillator and buffer in a shielded box, with good filtering on any lines going in or out except the output signal conductor. Then, a filter at the output would be effective, so one could be designed to take care of the harmonics to any extent desired. Roy Lewallen, W7EL Diego Stutzer wrote: Hi, I built a small FM Radio as shown on http://members.aol.com/torkraemer/uk...ukwsender.html. 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. |
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