Best LC filter

Discussion in 'Wireless & RF Design' started by Synaps3, May 12, 2014.

  1. Synaps3

    Thread Starter Member

    Jun 5, 2013
    I'm working on a transmitter and found some interesting results with the filters. I have two transmitters. One transmitter has a simple adjustable LC filter on the output and the other has a 7 component (butterworth I think) filter. When I use an amplifier, the 7 filter appears to have more harmonics than the simple LC filter. I tried adding a second 7 component filter to the transmitter and it didn't seem to improve. I don't have a spectrum analyzer so all of my testing is based on SWR. How can I build a filter that is as good as the adjustable LC without needing adjustment for a small frequency change?
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    You mentioned an amplifier. Was this a higher power unit to boost your transmission power?

    A diagram would help.
    Last edited: May 13, 2014
  3. Papabravo


    Feb 24, 2006
    What kind of filters are they? What does their characteristic look like? IIRC filters don't produce harmonics.
  4. dougc314


    Dec 20, 2013
    How are you determining harmonics? With a receiver? Without a Spectrum Analyzer you are very crippled.

    Normally designing a filter for harmonic suppression first you need to be able to specify how much suppression you need, and at what frequencies. A spectrum analyzer helps with that, plus verification as well. Presumably the first harmonic to be rejected is the second, but perhaps your amplifier mostly generates odd harmonics much more so than even. Any how once you know how much rejection you need, typically you will (at least I will!) look up in the tables of a filter design book what filter type and how many poles you need. (Butterworth, Chebychev, and Elliptic (or Caur) being the big three. Of course with the advent of computers there are other ways, but I'm a bit old school and can usually look the answer up in about a minute.

    Actually designing the filter can be very complicated, you have implied that at the frequencies you are working at you can use lumped filters (L-C). If you are really serious about doing this you will need to read up on this topic, I doubt the online resources are up to it. One of the classics on this topic is Arthur B Williams Electronic Design Handbook. I think the later editions also have Fred Taylor as a co-author. There are many others out there, but Williams has worked examples on both specifying and designing, without going overboard on the math. You can't get away from math when designing filters, but it can be minimized.

    Another thing to consider is the choice between a band-pass or a low-pass filter. Most people might think that harmonic suppression filters need to be low pass filters, but in reality many transmitter systems are banded, (TX only over a fairly narrow band, say with a ratio of less then 2:1), and a band-pass filter can be more efficient (as far as number of parts) than a low-pass. This might explain while your tuned LC works better than a fixed set of parts. Presumably the tuned circuit is narrow band and may have more harmonic rejection then a fixed frequency low-pass filter, especially if you are transmitting on the low end of a band. Without more information about your system though I am just guessing here.
  5. Papabravo


    Feb 24, 2006
    Look at the dates on the posts. I think the OP is long gone.