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Author Topic: Help With Icom 730 Filters  (Read 785 times)
KB8MCZ
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« on: October 28, 2003, 06:37:44 PM »

I have a Icom-730 and want to know what the differance is about the IC-EX203 CW audio filter 150Hz/6db and the FL-45 CW Narroww Crystal Filter, what do these two do?

Thanks, Dave
kb8mcz@charter.net
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W9GB
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« Reply #1 on: November 03, 2003, 09:15:24 PM »

The IC-EX203 CW audio filter 150Hz/6db and the FL-45 Narrow (500 Hz - CW) Crystal Filter are 2 completely different filters.  How are they different? - you need to understand how a superheterodyne receiver works (which is the type of receiver used in your IC-730).

The Superheterodyne receiver was invented by Edwin Armstrong.  Read about him sometime - a genius and patriot in giving this invention (and patent) to the U.S. government during World War I.

In the superheterodyne, the frequency of the incoming signal (that you select - such as 7.10 MHz) is heterodyned to a new radio frequency, the intermediate-frequency (IF).  The signal is amplified through the IF section, then detected though a product detector (CW, SSB, AM) and finally translated to audio frequencies that you can hear in your speaker.

A simple means to improving the selectivity is to use piezoelectric quatrz crystals as a selective filter in the IF.  Using a FL-45 crystal filter provides discrimination against adjacent signals and reduces the broadband noise.  Any signals outside the 500 Hz are attenuated (looks like a bell shaped curve)

The audio filter only operates at the audio section at your speaker or headphones.  While useful in some instances -- it is easier to remove adjacent signals or noise before they get to this section.

Read the Receiver or Receiving Systems section of ANY ARRL Handbook (local library, used and new at any hamfest) for further study.

It would also help if you operated any receiver and listened to CW signals with a standard SSB 2.4 kHz bandwidth (with or without filter) selection and then use a CW filter at 500 Hz.  it make ALL the difference in the world - I never had a CW filter option or selection on my novice receiver in 1970's -- and I used any number of audio filters -- but the difference is day and night for copying CW !

Greg
w9gb
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W3UHF
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« Reply #2 on: November 23, 2003, 09:53:12 PM »

Dave,

The IC-FL45 is a CW Narrow filter for the 9 Mhz IF, and has a bandwidth of 500 Hz. The IC-EX203 is an active Audio Filter which uses an IC op-amp (and supporting circuitry) as the AF filtering element. The EX-203 has a bandwidth of 300 Hz.

The advantage, of course, goes to the IF filter, as close-in QRM will be attenuated by the filter's stopband rejection, whereas with the AF filter it is not. While you won't necessarily hear the close-in QRM with just the AF filter, the receiver front end will, and you will see it on the S-Meter. The front end may be de-sensed by strong adjacent signals using just the AF filter, and AGC action could cause a weaker station you are working to be buried.

Using both the IF filter and the AF filter is a great combo - I have both in my IC-730, making CW very enjoyable.

73, Frank W3UHF
www.icomclassic.com
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