[Elecraft] roofing filter questions

[Art]

Dick
The conditions you describe are convincing evidence most if not all his 
noise is internally generated in the 950. As Jack points out, Where is 
the question, not Whether.

Since it is very unlikely the noise comes from outside the radio, no 
roofing filter will eliminate it. What a narrower roofing filter might 
possibly do is reduce the distance (in Hz) between the current dial freq 
and a station which causes unwanted interference in the radio. Then 
again, if the internal interference is generated in front of the filter, 
  a narrower filter may not help a bit. More testing required.

In all probability the fellow who suggested replacing the 950 has a good 
point. Its front end is similar to, and a good deal older than, many 
made today that suffer similar problems in the face of strong nearby 
(but outside the filter) signals. While the numbers posted at sites like 
ARRL and Sherwood Engineering tell the technical story well, they don't 
address what defects actually sound like, such as (probably) your 950.

I too operate almost 100% CW. A case in point is my two rigs, Icom 
IC-765 and TenTec Orion. Both delightful rigs but for different reasons. 
The (upconverted) IC-765 has the IR narrow roofing filter 4KHz. Strong 
signals inside the roofing filter, but outside of the 2nd IF filter 
produce interference products that I bet sound similar to or exactly 
like your 950. The Orion produces no such products regardless of filter.

BTW my shameless plug, the Orion is for sale because I have a K3 on 
order, ad on eham.net.

73 Art
k6xt at arrl dot net

Message: 2
Date: Tue, 13 May 2008 13:18:40 -0400
From: Jack Smith <jack.smith at cliftonlaboratories.com>
Subject: Re: [Elecraft] roofing filter questions

 > >
 > > Questions came up in a discussion with a friend of mine who is 
considering
 > > the INRAD roofing filter mod for his Kenwood TS-950SDX. He operates 
almost
 > > 100% CW, and doesn't hear any IMD products. His beef is noise from 
stations
 > > that are just outside the passband of his cascaded IF crystal 
filters. He's
 > > hoping the roofing filter will increase the selectivity of the radio.
 > >
 > >
 > >
 > >
 > > 73, Dick WC1M
 > >
 > >
 > >
 > >
Dick:

The question is the source of this noise - it could well be reciprocal
mixing from the 1st LO in which case no post-mixer filtering is going to
fix the problem. This is exactly how reciprocal mixing appears to the user.

Until the noise origin is pinned down, it's not possible to state with
certainty that a narrower high IF  filter will do anything.

Jack K8ZOA
www.cliftonlaboratories.com