RF Low Pass Filter for FM Transmitters (88-108mhz)
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- เผยแพร่เมื่อ 16 ก.ย. 2024
- This circuit was designed to be used with any FM Transmitter to greatly reduce or eliminate unwanted harmonics in your transmitted radio frequency. The schematic for 88-108mhz is shown in this video. For other custom frequency ranges, please refer to the online calculator link shown below. Failure to suppress these harmonics, can result in interference in other bands such as aircraft and military, and possibly result in the FCC knocking on your door. In the past I uploaded a video for a 4W and 1W FM transmitter. Both videos did not show a low pass filter in the schematic. To add one(highly suggested), you would simply add this circuit to the RF output of either one of those circuits. ALWAYS transmit on a frequency which is not in use!
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Link to low pass filter schematic shown in this video:
www.gareth.net....
Link to a low pass filter calculator fro custom frequency ranges:
www.m0ukd.com/C...
Link to online air coil inductor calculator:
www.circuits.dk...
4W FM Transmitter video/schematic link(Easy/Very Good Transmitter!):
• How To Make A Powerful...
Very good 1W RF amp for boosting small transmitter output:
• 1W FM RF Amplifier Cir...
Very well done. Thank you for another and very useful video.
Thank You!
Just what I needed!! Thank you so much! Is it possible to replace the Capacitors with Trimmers so you can adjust the cut-off frequency? And if so which ones would I need to replace? I want the cut-off to be as near as possible to 108MHz but without reducing power near 108MHz of course :)
You can use trimmers, but I don't see the need for them if this is properly made. Thanks for watching!
I build this filter using trimmers and I measured the capacity first and set the trimmers at 27 and 56 picofarrad and marked that setting on the trimmers. After assembly I tested the filter and guess what? I only had to adjust them slightly. I had to reduce the capacitance a little because of the capacitance of the islands on the circuit board, which gets added to the components. A further interesting discovery I made, having used trimmers instead of capacitors, and after having done some online research, is that this LPF can also work as an antenna matcher. By tuning the circuit I was able to get my SWR reading down to 1.05 to 1 (!!). So it can help to make your antenna system more resonant. I hope other people reading this benefit from this info. Anyway building this filter was great learning experience. :)
Oh and btw, I read in the comments that this is called a Chebyshev filter but actually, with these component values it's a Butterworth filter. I found that out using the "Elsie" software.
VERY WELL DEAR FRIEND
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Thank you sir ! Help full video
You're welcome! Be sure to rate thumbs up and share. Thanks
You do not need to use flux for everything.
The tin is applied in the place where welded. It is not applied on the soldering iron and then transported.
Very good work anyway.
+Guillermo Sempron WE DON'T WELD, WE SOLDER.
I believe he applied the solder to the iron so that he doesnt overheat the board. And with the flux you can see it soldered just fine. There is almost always more than one way to handle a task.
Applying the solder to the iron, tins the soldering iron, and a little extra solder helps transfer heat to the board. The flux allows the solder to flow nicely.
Hey...Thanks for the video. I would like ask if this filter will work for a 1000 watt transmitter?
I'm not an expert on these filters, but I think you should be able to use this, just as long as you make the copper pads that the coils are soldered to larger(space them out more), and use heavier copper(or silver plated solid copper wire) when making the coils(maybe 12 ga). Be sure to use an inductor calculator online to adjust the turns(more turns for the same inductance) when using heavier wire.
Thanks For watching. Be sure to share, rate a thumbs up, and watch my other videos as well.
Oh ok. Thank you electronicsNmore
Kwame Baafuor Hi, I would not recommend that circuit for high power rf use. Its for wattages up to about 50 watts but could be used higher if the capacitors were paralelled (to make a higher voltage cap). For high power designs you need proper high voltage caps like j101 metal clad mica capacitors (500v rated). Great video though
MrKillaHurts
Your information is good except where you advise on parallel capacitors.
Capacitors connected in parallel will add their capacitance together.
A parallel circuit is the most convenient way to increase the total storage of electric charge. The total voltage rating does not change. Every capacitor will 'see' the same voltage. They all must be rated for at least the voltage of your power supply. Conversely, you must not apply more voltage than the lowest voltage rating among the parallel capacitors.
See www.coilgun.info/theorycapacitors/capacitors2.htm for that exact wording.
The 1000 watt transmitter (assuming a 50 Ohm load) will have a little under 224 volts of RF across the capacitors, so a 500 volt capacitor is a safe bet. The current the coils will carry will be about 4.5 amps.
Hi, just on the question about the filter for high power. See my comment below.
I have an fm transmitter I use to have musci around my property ( 3acres) I run just enough power to cover most of my land. I transmit @ 87.7mhz as there are no local stations on that frequency. However I have checked out the harmonics on the transmitter and I Have a fairly strong harmonic on about 91.5 mhz where there is a local station. Do you think this filter can be modified to use with my transmitter so that it doesnt pass anything above say 88mhz?
Also want to be certain theres no harmonics going out on the airbands.
A "harmonic" would be at a multiple of your transmit frequency (eg. 177.4MHz, 263.1MHz, etc.) One of the characteristics of a Chebbyshev filter is that is has a pretty-steep cut-off slope. It can filter-out other spurious emissions above your transmit frequency. But this filter is probably not sharp enough to deal with cutting off spurious products out of the bands immediately above the FM broadcast band if you are transmitting at the upper end. You especially want to keep from interfering with the aircraft band (108-136MHz).
Great video!
Thank You. Be sure to rate Thumbs Up, share, and check out my other videos as well.
sebe que esta bueno pero no entiendo ingles ..... el video ..... adonde esta la entrada y la salida de la señal en ese circuito esa es mi duda
Hola Rodrigo. Por favor, consulte el enlace del video "Acerca de" sección
¡ Gracias por mirar! Por favor tarifa pulgares arriba y compartir con los demás.
Hola Rodigo, es indistinto la entrada de la salida, es lo mismo
For me the 56pF Capacitors make no sense. I found the same circuit diagram with an inductance value, the 27pF capacitor at the end forms with a 74nH a low pass filter at 112Mhz but the 56pF at the beginning has a cutoff frequency of 78Mhz!! That would bolck out my complete signal.
I didn't design the circuit, but for me it worked just fine.
@@electronicsNmore Too bad. Because these circuits also seem to be dependent on the input and output impedance and I have no idea what it's doing when I connect this to my self made transmitter.
RF at 90 deg screws with the signal, I'd check the waveform on the scope...
Works perfectly. I have no issues. Sound quality is excellent, and the signal on my frequency counter is nice and stable. I cannot ask for more. I used this circuit in both my 1W and 4W transmitter. The link to the schematic also states to place the coils perpendicular to one another for better results.
Check out this link I just found. Scroll all the way down and you will see the EXACT same low pass filter layout as I did. The person that designed that 40W unit knows more than me on this subject, so mine must be fine.
radio.xtreamlab.net/40w-no-tune-amp.html
electronicsNmore The coils are mounted at 90 degrees to reduce the coupling between the coils. Also I believe it helps if some coils are wound clockwise and counterclockwise.
This circuit will indeed work using the technique you have employed. However if you want a real "brick wall" cut off with the best attenuation, you can do better. Since the cut off is 125MHz, a little extra lead length on the coils won't hurt to get them further apart. Getting them away from each other will improve the harmonic rejection by reducing the coupling between them. Winding the coil in the centre in the opposite direction may help but not too much if they are spaced appropriately and 90 degrees from one another. Also, using multiple smaller value caps in parallel instead of one is better. eg: 8.2pf + 8.2pf + 10pf in place of the 27pf and don't solder them all to the same spot of the ground. This gives a lower inductance path to ground for the harmonics and improves the power handling capability of the filter. For the 56p I would use 15p + 15p + 15p + 12p (57p total). I also wouldn't bother cutting the board but rely on the mechanical rigidity of the multiple capacitors and just solder it lead to lead above the board, it acting as a ground plane. If you want very high power handling, use the same technique as well as much thicker wire for the coils (adjusting the diameter of the turns to maintain the inductance) and use 2 or more metal clad mica capacitors in parallel at each circuit location. :) I hope this was of interest
can ı add this filter my 500 mw (1/2 W) fm transmitter ?
I don't see why not. It should work fine. Be sure to also look over my "FM Transmitter" video playlist below for other videos of interest to you, and most importantly share. Thank you!
th-cam.com/video/KeRkWYJRxTI/w-d-xo.html
you make a coil in the video 3 turn but the coil is 5 turn in the diagram (sorry my bad english)
The circuit was modified for the frequencies I was using.
@@electronicsNmore my transmitter is FM (88-108) ı make a diagram ?
Re-up circuit, please!
Video description area.
electronicsNmore link off
All fixed! Be sure to rate thumbs up and share. Thanks
im not smart enough to do this. know where i can order one? :/
+bryce f.
Hi Bryce. I'm sure you are smart enough, the problem is you may lack the skills required. :-) You can look online for an RF Low Pass Filter.
Thanks For Watching!
+electronicsNmore my reason of using something like this is called the raspberry pi for FM radio. only problem is it's square wave so the harmonics splatter all over vhf frequencies and most likely 10 m band if even possible.
Try looking on e-bay. I've seen a few boards like this.
For the many Cheap Chinese and home brew FM BCB transmitters, this LPF is not sharp enough. I have seen typical spurious emissions from such FM BCB transmitters At the bottom of the Airband at 110 MHz..
I have seen modulated signal images + 6MHz above and below the transmit frequency AND again about 30MHz above transmit frequency!!! ( from E-bay Chinese Transmiters as lose as 1 watt)
That puts your modulated signal right into the Aircraft Band ! And don’t think low power or being away from an airport will make it OK...Aircraft at Flight Levels of 24,000 feet receive signals hundreds of miles away.. if your on a terminal control frequency, of which there are many-not even fo your area- your signal will be blocking air communication. Not a good thing.
These cheap or home brew FM transmitters really need a tight BAND PASS filter , or Cavity filter to be safe to use. Period.
1 second ago
Yes, I agree with the Band Filter. Thanks for watching!