K3 1500 Khz Roofing Filter

[Roy Morris-6]

I would like to hear from anyone who is using the #727 Inrad 1500 Khz roofing filter.  I wonder if this filter adds to razor sharp clear SSB reception when properly set in the DSP bandwidth.   What DSP bandwidth setting should be used to insert this filter?  Any comments will be appreciated.  Thanks.  Roy Morris  W4WFB  
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[David Gilbert]

Hi, Roy.

I've had the 1500 Hz 8-pole Inrad filter in my K3 for at least 18 months
now, and while I don't do a lot of SSB contesting I certainly appreciate
that filter when I do.  It works fine and gives just a bit extra margin
of protection against really loud adjacent stations.   What DSP
bandwidth you use is up to you.  Of course there isn't much point in
setting it much wider than 1.5/1.6/1.8 KHz or so because the filter will
chop it off anyway.  I have often set my DSP as narrow as 1.2 KHz or
even sometimes 1.0/1.1 KHz, but the key to decent intelligibility with
any of those really narrow bandwidths is to shift the center frequency
much lower than the K3 default 1.5Khz.  I typically set the center
frequency to about 1.1 KHz when I have the bandwidth at 1.5 KHz, and I
drop down to a center frequency about 0.9 KHz when I have the bandwidth
set to 1.1 KHz.

A few caveats to all of that:

a.  The optimum center frequency will probably be different depending on
the voice you are listening to.  I sometimes have to adjust the center
frequency depending upon the caller, occasionally as low as 700 Hz but
usually higher than normal.  A few voices seem difficult to copy at 1.5
KHz or less bandwidth no matter what center frequency I use.

b.  No filter on this earth is going to protect you from in-band
splatter.  Somebody 3 KHz away with a 5 KHz wide signal is going to mess
you up even if you're using the 1.5 KHz filter.  I'm always amazed at
the incredibly bad, almost unintelligible signals we have to put up with
during a major SSB contest, many of them coming from long time high
profile contesters.  I find it to be simply pathetic.  My voice won't
hold up during a SSB contest anymore, so I'm honestly tempted to spend
the next CQ WW DX SSB contest just recording audio samples (both good
and bad) identified by callsign.

c.  I bought the 1.5 KHz filters instead of buying the 1.8 KHz filters.  
If you already own the 1.8 KHZ filters I'm not sure it would be worth
the hassle to switch, especially considering the in-band splatter issue.
   The only thing you gain from the 1.5 KHz filters is 150 Hz additional
protection from strong signal overload either side of you since the DSP
can give you whatever narrow bandwidth you might want for moderate
signal strengths.

73,
Dave   AB7E




On 5/12/2010 1:08 PM, Roy Morris wrote:

> I would like to hear from anyone who is using the #727 Inrad 1500 Khz roofing filter.  I wonder if this filter adds to razor sharp clear SSB reception when properly set in the DSP bandwidth.   What DSP bandwidth setting should be used to insert this filter?  Any comments will be appreciated.  Thanks.  Roy Morris  W4WFB
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> Elecraft mailing list
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>    

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[Roy Morris-6]

Hello David,
Thank you for your response regarding the 1.5Khz INRAD filter.  I agree, I would not buy the 1.5 if I had the 1.8 or 2.1 filter.  Since I do not have a filter between 2.7 and .5, I am going to give the 1.5 serious consideration.  Thanks again for your reply.  Best 73,  Roy Morris  W4WFB
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