LC Filter and FPV

LC filter is one of the most basic filters in electroincs, which comprises of just one inductor and one capacitor in its simplest form. This circuit is widely used by FPV hobbyists to suppress noise in power source created by ESC and motors. 

What Is LC Filter And How Does It Improve FPV Experience

When we break down a simple LC filter, we have an inductor and a capacitor.

An inductor resists changes to the current flowing through it, while a capacitor resists changes to the voltage across it. See this post for a more sophisticated version of power filter.

simple-lc-filter-back

Apart from the video transmitter power, antenna gain, the FPV range and quality also depend on how noise your power supply is. If you have an independent power source, e.g. a separate battery for your video transmitter, you generally should not have noise issue.

But generally speaking when you share the main battery with the ESC/Motor, the power supply becomes noisy. It might cause horizontal white lines/ripples appearing across the screen when we are flying FPV.

When the multirotor is airborne, the motor speed is constantly changing in order to stay balanced in the air. The change of motor speed means change of current drawn by the motors, and fluctuations in voltage level – eventually this becomes noise.

Most common video transmitter requires 12V power supply, which can be powered from the 3S battery directly. If you are running 4S Lipo battery. To have a stable 12V we normally use a voltage regulator. Voltage regulators have some degree of power filtering, but we can further enhance it’s filtering by coupling a LC filter.

IMAG1241

IMAG1242

Building a LC Filter

The value of capacitor and inductor affects the ability of noise suppression at certain frequency, so if you are really going to use a LC filter, you normally need to identify what frequency noise you want to filter out. But I have been hearing a lot people just use any random capacitor and inductors and they do just fine, so maybe it’s not a big deal after all.

lc-filter-circuit

When choosing capacitor, ensure the capacitor voltage rating supports the voltage of your application. In my opinion 100uF to 2000uF is a good range.

As for the inductor, you can make one yourself with a Ferrite ring or just use a rod/pencil. The length and diameter should be large enough to get half a dozen winds of wire through it or around it. However try to use Ferrite rings and really, they are very cheap. Or you can just hack one out of an old PC mother board or radio power supply. Same for the capacitor: a cheap electrolytic type (polarized or non-polarised) hacked from an old radio, or PC part is going to do just fine. But make sure you use low ESR type of caps, they work better for filtering purposes.

simple-ferrite-ring

build-lc-filter

If you are making your own inductor, I find it works better with just the positive wire wound on the toroid. When winding the cable round the ferrite ring, do NOT loop any of the wires back to the point they originate from, i.e. what is behind the filter stays behind the filter. Try to place the capacitor on the side where the transmitter and camera are relative to the inductor.

Conclusion

LC filters are great because they are so simple and cheap to make/buy. They are likely to help eliminate/attenuate “noise” even when they are not well designed. I always put one in regardless.

27 thoughts on “LC Filter and FPV

  1. Cereal_killer

    Hi Oscar, I just swapped out some Afros for little bee’s and the video interferrance is HORRIBLE now! I had been using a small lc I built as described here (DIY ferrite ring w/ cap). I actually had a bad one of those same blue 1×2″ buck regulators you show above (the big black FET on it blew but the output cap & 330uH L were good so I physically cut the board and used it as an LC filter). This works pretty well and really makes a large difference but I’m still getting some annoying lines in the video (I can live with them but I’ve already spent time on it so why not make it perfect [if possible] ya know)

    I’m running my vtx off direct 4S battery voltage, do you think my next step should be a different value inductor or cap (right now it’s a 330uH / 220F) or to run the vtx off UBCC at 12v?

    Reply
    1. Oscar Post author

      Yea the littlebee ESC have very good active braking and it causes some serious power noise.
      you will need something better for filtering. consider buying a proper PDB with voltage regulator and filtering, like the DemonRC Core

      Reply
  2. CB1

    Seems that way.
    They are both Foxeer HS1177 cams from Surveilzone. I may play with them more to see if I can get them any better.
    I have a RMRC-PICO-WIDE-V2 cam on my Hellbender122 that is excellent as far as being focused. Just seems odd to me that I can’t get the HS1177 cams any clearer. I would say that they both are the same as far as clarity.
    Thanks for the reply!

    Reply
    1. Oscar Post author

      Try the Runcam Swift, i found it to give me better performance (if you haven’t seen my test/review yet)

      Reply
  3. CB1

    Hello Oscar,

    I have two racing quads, both are on 4S running HS1177 camera, Aomway 200mw VTXs and directly connected to my PDB (shared power with the ESCs). I am not experiencing any noise in my video feed, however the video image just doesn’t seem sharp enough (a little blurry). I have focused both cameras as good as I can using a focus image and through my HD TV as well as adjusted the internal camera settings from some videos that I have seen on YouTube that offer some suggested settings for the HS1177. But the image is still a little blurry, in my opinion.
    Would a LC filter like this help my issue or should I look at something else to resolve this? http://www.ebay.com/itm/FPV-LC-Power-Filter-2A-LC-FILTER-for-RC-FPV-Aerial-Photography-US-STOCK-Y0M6-/282018491043?hash=item41a99caaa3:g:zgIAAOSwZ1lWgKVK
    Thanks for any advise!

    Reply
    1. Oscar Post author

      from what you described, i don’t think it’s your power, it sounds more like your cameras.
      LC filter won’t sharpen your image unfortunately, if you are not getting any noise then LC filter won’t do anything for you.

      Reply
  4. Matt

    I think you will find the parts in a pc power supply. The motherboard doesn’t have the parts you are looking for :)

    Reply
    1. Nivaldo

      Hi Florian0311, I have the same doubt!? The filter is for the vTx or Cam? Today I tried the LC Filter just on the vTx and the lines don’t disappear.
      Tomorrow I’ll test it putting in the fpv cam!

      Reply
      1. t.goat

        If your vtx has 5v out, it’s better to put lcfilter before vtx, then power camera from vtx. That way it’s getting double filtered.

  5. Hannes

    Hi Oscar,

    I’m currently planning a built with littlebee ESCs which are quite notorious for voltage spikes that take out electronics like FC, VTX, or the camera. Would it be sufficient to solder a capacitor to the Vin and GND of the polulu step-down on the PDB or should I use a different combination of LC-Filter and BEC?

    Reply
  6. Romain

    Hi Oscar,
    I am suffering white stripes across my FPV video. I tried changing VTX, VTX antenna, camera, FPV goggles… nothing changed.
    So I tried to add a BEC 5V for the FPV camera power supply and the type of LC filter your describe here for the VTX power supply. Stripes are still there.
    https://youtu.be/ED97Aa0-SbY

    Any idea on what could be the cause and potential solutions ?

    Cheers

    Reply
    1. Oscar Post author

      do you get this before take off? if you don’t then it’s probably the power, if you always get this, it’s could you your signal.. check your VRX as well?

      Reply
      1. Romain

        Thanks for you reply. I do get this only after take off… But if this is due to power issue, the 5V BEC and LC filter should have fixed it, right ?

  7. Dmitri

    Hi Oscar,

    Thank you for your posts, they really help.

    I wanted to ask for your advice on filtering noise with 2-battery setup. I power FPV (vTx, camera and minimOSD V2 board) from a separate 3S, 12V battery # 2 (minimOSD is connected thru 12V-to-5V linear regulator.) From APM (powered by main 3S/12V battery # 1) I get telemetry data (Rx) and ground form battery # 1. I connected grounds of both batteries. Everything works except that I am still getting some horizontal lines on video. My question is if, by connecting grounds or two batteries, I am adding noise from battery # 1 and, if so, how I can reduce it in 2-battery setup by using LC filter or ferrite ring (and where to insert them to.)

    Thank you!
    Dmitri

    Reply
    1. Oscar Post author

      Using a separate battery for FPV gear you shouldn’t get any noise… there could be something else is wrong in your setup…

      Reply
  8. Levi

    Hi Oscar,
    I used a Schaffner Common Mode Choke(RN202-2-02) for an inducer, similar to a very popular FPV LC Common Mode filter Choke(RN202-1-02), yet I went with the model that can handle 2A, instead of 1. I set it all up, and when I connected I noticed that I had a video signal, but it was just black, with some static on a repeating, regular time interval. The difference between the two inducers is the mH and the mOhm, but I can’t figure out why my new inducer does not work. Any thoughts?

    Reply
  9. bncache

    Hi Oscar
    I just wanted to suggest smaller lighter coils to make the filter lighter.

    With leads like these:
    no.mouser.com/Passive-Components/Inductors/_/N-5gb4?P=1z0z819&Keyword=passive&FS=True
    and even smaller, leadless like these:
    no.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=47&storeId=10169&categoryId=700000005466&pageSize=25&beginIndex=1&showResults=true&pf=110137774,110178564
    Just make sure they are specced to handle the current drawn by the camera combo.

    bncache

    Reply
  10. Norbertas

    Here you can calculate what do you need for your LC filter:
    circuitcalculator.com/lcfilter.htm
    I’m filtering 1200-1500HZ freq. so for 1000uF (1000E-6H) capacitor i need to do only few turns on ferrite ring (~20uH or 20E-6H). When i do more turns on ferrite video gets very dark and loosing colors.

    Reply
  11. Gabor

    Nice post!
    I would like to do this in my FPV project. ~3.7 volt 1S lipo, the FPS setup works fine with it,but when I connect the quadcopter and give throttle,noise appears! how shall I start? what do I need for this config to make it work?

    Thank you!

    Reply
    1. Oscar Post author

      how much lift can you quadcopter carry? Get a 100uH – 200uH inductor, and a 50uF-100uF capacitor, solder them together shown in the pictures.

      Reply

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