December 27, 2011

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Homebrew Filter Selection Techniques

In this our 6th week, we discussed something near to the hearts of all homebrewers ... filters.  We all use them and we all need them, yet sometimes they seem like a big mystery. 

This week in the "Chat With The Designers" online session, we reviewed a lot of core technical information based on the desired characteristics:  input/output impedance, filter type, roll-off rate, ripple, # poles, and more  ... 


Next week we'll discuss ways to measure the performance of an LPF.

~George N2APB & Joe N2CX

(Listen to MP3 recording

George N2APB, Joe N2CX, Al VE3RRD, Bob K2UT, Charles Wc5mc, Chris W0ANM, Dave, AD7JT, Frank N3PUU, Fred WA8PGE, Jim AF9A, Jim KK6MC, Jim WA7DUY, John AB5ZR, John KJ6CVB, John K5JHF, John NU3E, Joe KC2VGL, Ken VA3KMD, Larry W2HVV, Mike N4NLN, Guy N7UN, Paul AK1P, Ray K2ULR, Rick K3IND, Rick W1DSP, Terry WB4JFI, Walt VE7WRS

Online chat discussion log ...
<20:44:09>"George - N2APB": Great reference on LPF design (and other types) is in the ARRL Handbook.  I'm currently looking at the 2008 edition and this is in Chapter 12.  (Check the index for "Low Pass Filter" in the rear of the publication)
<20:54:51>"Fred - WA8PGE": here's a pretty nifty toroid calculator, tells you how much wire you need too..
<20:54:58>"Fred - WA8PGE":
<20:55:38>"George - N2APB": That's indeed a great reference Fred.
<20:58:05>"George - N2APB": Q-Dope ... source for this?
<21:00:58>"John - k5jhf": Thank you guys
<21:01:40>"Joe - N2CX": Qdope is a General Cement (?) product available from various electronic suppliers.  i think A good source might be Antique Radio in Arizona.
<21:03:31>"Paul - AK1P": Try <>
<21:04:05>"George - N2APB": AADE Filter Designer     <>
<21:06:02>"George - N2APB": Tonne Software Elsie Student Version    <> also in ARRL Handbook disc
<21:06:35>"George - N2APB": Tonne Software SCVFilter    <>     also in ARRL Handbook disc
<21:20:25>"John - k5jhf": roger that
<21:23:31>"Larry-W2HHV": Topic suggestion: Test equipment most used for QRP breadboarding?  Which items should one buy versus build?

Mini-Tutorial Notes ...

Impetus – Lowpass filters for SDR power amp
Need to assure that end amplifier meets FCC reqm'ts
  Part 97.307 43 dB suppression
  5W - .25mW 10W - .5 mW, 20w – 1 mw
Give ability to check as-built to software-defined filters
Need multiple filters to cover HF range
  One or two bands per filter
  Simple design one filter
  Sophisticated filter reduces number needed
    One “ordinary” and one WARC band per filter

First of all what do you need?
Low pass filter
    Passes RF below some Pass frequency
    Attenuates RF above the Reject frequency
    There is a rolloff slope in between
    Still some loss in passband
      How much? .5 dB is about 6%
    How much rejection needed?
      FCC Part 97.307 says -43 dB
    Generally need different lowpass filter for each ham band
For 40 meter band pass all below 7.3 MHz
Attenuate all above 14 MHz (2nd harmonic)

Easiest to use filter design software
    AADE Filter Designer     <>
    Tonne Software Elsie Student Version    <>   also in ARRL Handbook disc
    Tonne Software SCVFilter    <>    also in ARRL Handbook disc
    AADE Filter Design package … free at


Designing the filter

Softrock RXTX 6.3 LPF on the TXPA Board

    Cauer (elliptical) design often best choice for optimum rollof

    Enter passband freq, cutoff freq, passband atten (ripple) and stopband atten

    Get basic design

    Observe filter plot

    Save values and plot and keep track of what you have done

    Some programs allow toroid winding info

    Goto program selection that lets you enter custom values

        Plug in standard value caps and toroid

        View resulting plot

        Save info

    Iterate until passband attenuation and stopband attenuation are optimized

    Build and test!

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