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RF and AF Filters

Introduction

An RF filter is an electrical circuit configuration (network) designed to have specific characteristics with respect to the transmission or attenuation of various frequencies that may be applied to it.

There are three general types of RF filters:

  • A high-pass filter similarly has a cut-off frequency, above which there is little or no loss in transmission, but below which there is considerable attenuation. Its behavior is the opposite of that of the low-pass filter.
  • A low-pass filter is one that will permit all frequencies below a specified one called the cut-off frequency to be transmitted with little or no loss, but that will attenuate all frequencies above the cut-off frequency. High-pass and low-pass filters can be difficult to construct properly. Whenever possible, many amateurs simply buy them.
  • A band-pass filter is one that will transmit a selected band of frequencies with substantially no loss, but that will attenuate all frequencies either higher or lower than the desired band.

Articles

Second-Harmonic-Optimized Low-Pass Filters  QST February 1999, pp 44-46
Sometimes we need a little more output filtering than traditional designs offer. This filter can give you that extra boost.

Bandstop Filters for Attenuating High-Level Broadcast-Band Signals  QEX November 1995, pp. 3-12

A No-Tune 2-Meter Bandpass Filter  QST May 2000, pp. 54-55 A homebrew solution to annoying intermod

Narrow Band-Pass Filters for HF  QEX Sept/Oct 2000, pp. 13-17  The ability to attenuate frequencies above and below a particular band can be very useful to both the contester and the casual operator.

Receiver Band-Pass Filters Having Maximum Attenuation in Adjacent Bands  QEX July/Aug 1999, pp. 27-33

Clean Up Your Signals with Band-Pass Filters - Part 1 QST May 1998, pp. 44-48
Clean Up Your Signals with Band-Pass Filters - Part 2 QST June 1998, pp. 39-42
 

A Diode-Switched Band-Pass Filter  QST January 1991, pp. 24-26 How to use high-speed silicon diodes as RF switches for selecting two or more filters.

Band-Pass Filters for HF Transceivers
QST September 1988, pp. 17-19, 23

Free Software

 PCFILT & S/FILSYN filter design software is now available as a free download from www.alkeng.com

 http://www.tonnesoftware.com/ has a collection of filter design programs. Free student versions are available for download.

Measured Toroidal Inductor Q Values

The hard part of designing RF filters is getting good inductance and Q data.  Zak Lau has written an QEX article with five pages of data in Table 1.

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