706: Filters

© 2003-2011 by Harold Melton, KV5R. All Rights Reserved. Rev.06/11/05

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Icom 706MkIIG Filters: Make a narrow CW filter work in SSB for the digital modes

If you run CW or narrowband digital modes like PSK-31, you need a narrow filter. I recommend the FL-232, 350Hz CW-RTTY filter. It is available from all Icom dealers. Look for the best price online. I have seen them as low as $79.95 (Universal Radio), and HRO sold me one for the rock-bottom price of $78. Many vendors sell them for $95-$105. Don’t pay too much!

Pop the filter in, then go into setup and tell the radio it’s an FL-223 (not 232). This will allow you to select the filter in SSB mode, as well as CW and RTTY. This will allow you to select Narrow on SSB and have about a 450Hz wide bandwidth on your waterfall.

How it acts:
  1. 1. In CW and CW-Reverse, it acts as expected, setting itself at 600 Hz. The IF shift will move it around about 600 Hz.
  2. 2. In SSB, it’ll act like a RTTY filter. In USB, it centers at about 2150 Hz. The IF shift will pull it down to about 1000 Hz for PSK use. But in LSB, it positions itself low. The IF shift will move it down through zero to the other sideband, or up to about 2k.
  3. 3. Though called a 350 Hz filter (+-6db), my waterfall shows it to be about 500 Hz wide on moderate signals. Kicking in the attenuator or rolling down the RF gain will narrow it just a bit. As with all narrow filters, it works best when lightly loaded, so keep the signal level fairly low when running digital modes.
  4. 4. When running PSK at 1000 Hz with a strong signal, an harmonic image can be seen (barely) at 2000 Hz. It’s not a problem at all (it may be generated in my computer, not the radio).
How to use:
  1. Run setup (Lock+Power) and select FL-223 for the slot in which the FL-232 is installed. That will make the radio think it’s a 223 (a narrow SSB filter) so that you can then select it while in SSB.
  2. For the usual computer soundcard digital modes (PSK, FSK, MFSK, etc), set USB, start with no filter, set the signal at 1000 Hz, select the narrow filter (Fil N), and crank the IF shift all the way right and that’ll center it at 1000 Hz in USB..
  3. Or, simply leave the IF centered (filter at 2150) and then tune your desired signals there. However, if you set your PSK software to copy at 2150, it will also transmit at 2150 and this may confuse other users who are accustomed to using 1000 Hz only.
  4. When a strong signal overloads your receiver, just shift the IF until your desired weak signal is right on the edge and the filter will wipe out the nearby strong signal. If the offending signal is splattering, complain!

Disclaimer: The author assumes absolutely no responsibility, under any circumstances, for what the reader may do with this information. Building and connecting circuits, and performing adjustments or modifications, may damage your radio, void your warranty, and/or cause it to operate in violation of FCC rules and Type Acceptance, etc, unless you are very careful.


3 thoughts on “706: Filters
  1. I am using both wide and narrow filters i just shut off the one im not wanting to use just remember you have to identify which one your using in i think steps 1 and 2 by an astric? check your manual min works great either way just got to change setup[ cat use both at same time.

  2. I just acquired a 706MkIIg with both optional filters installed (FL-232 and FL-223). How would you recommend installing them? Which filter in which slot and what to tell the radio?

    I intend to use the radio for digital modes. Thank you and 73!

    • Well, I donno except what I wrote above. I think you won’t be able to use the actual 223 (ssb narrow) filter, but just use the 232 and tell the radio it’s a 223 so you can select it in SSB mode and use it as a 350Hz filter for SSB digital modes. Doesn’t matter which slot as long as you pick the same one in setup. The radio design expects a CW-N filter and a SSB-N filter, but for digi you use a CW filter but tell the radio it’s an SSB filter model so it gets activated when you select Narrow while in SSB mode.
      Hope this makes sense! 🙂
      73, –kv5r

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