ultimate rejection

[Merlarts]

Sometime in 1977 I acquired a TS 820; this radio was supposed to be hot  
ticket; well made, highly sensitive, with fancy dual gate MOSFETs, and the  
crummiest filter performance imaginable.  
 
It was'nt the fault of the filter.  It was a reasonable 8 pole design  which
probably produced pretty good numbers in the lab.  But in the radio  it was
awful.  
 
The filter sat on a sort of Faraday shield, and the input and output talked  
to each other across that latticework of copper strips.  
 
Going first for the simple, dumb approach I began putting little RF  
"barriers" across the middle of the filter.  They consisted of a piece of  printed
circuit material, connected to ground and soldered all along its  length.  It
worked great!  Everything about the filter got  better:  opposite sideband
rejection, ultimate rejection etc.  
 
Back in the not so sophisticated days filters always had some sort of  
metallic barrier keeping the input from "seeing" the output.  In the Drake  TR4C,
which claimed a shape factor of 1.66, the radio was designed so that  the
sideband switch was sitting right next to a piece of the chassis which  separated
the input from the output.  
 
Enough reminiscing;  I just spent about a hour with my KSB2, making  sure
that the solder connections near the 7 pole filter were filed down, or  clipped
as much as possible, so that they don't act as little "antennas",  thus messing
up the characteristics of the filter.  It worked!  The  "audio image" that
one hears when tuning past zero beat got a lot weaker!  
 
Next, I went to local hobby shop and bought some tin.  I made little  tin
"houses" which I placed around the matching toroids, ground them  carefully.  It
worked too.  Next I added a little tin "fence" which  went on the bottom the
board, shielding the input of the filter from the  output.  It worked.  Three
times is a charm!
 
My point:  it ain't rocket science, but the old ideas of keeping RF  away
from places where it shouldn't be work just as well as they ever did.  
 
I don't have means of measuring this stuff accurately.  Let's just say  that
when tuning across really loud forty meter broadcast stations the filter  
sounds A LOT tighter then it did.  
 
Has anybody else tried this stuff.  There must be hundreds of you old  guys
who have messed with such things.  
 
So, until Elecraft finds a way to put a bulletproof 8 pole filter in its  
magnificent radio it's time to improvise a little.  
 
73,
 
Merlin W3ICT
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[Stewart Baker]

> Next, I went to local hobby shop and bought some tin.  I made little  tin
> "houses" which I placed around the matching toroids, ground them  carefully.
> It
> worked too.  Next I added a little tin "fence" which  went on the bottom the
> board, shielding the input of the filter from the  output.  It worked.  Three
> times is a charm!
> My point:  it ain't rocket science, but the old ideas of keeping RF  away
> from places where it shouldn't be work just as well as they ever did.
> I don't have means of measuring this stuff accurately.  Let's just say  that
> when tuning across really loud forty meter broadcast stations the filter
> sounds A LOT tighter then it did.
> Has anybody else tried this stuff.  There must be hundreds of you old  guys
> who have messed with such things.
> So, until Elecraft finds a way to put a bulletproof 8 pole filter in its
> magnificent radio it's time to improvise a little.
> 73,
> Merlin W3ICT
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Hi Merlin,

I would like to try your approach, Any chance of a photograph, to show how you
have positioned the shielding ?

73
Stewart G3RXQ

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[Geoffrey Mackenzie-Kennedy-2]

Morning Merlin,

>From an old guy (hmmmpf) who has messed around with RF circuits for ages,
for what they are worth a few comments. Your idea to keep RF away from where
it should'nt be is a basic rule in good RF design, and will always be I
would think. Some digital designs get hung up because this rule is not
observed,
and there is a lot of RF about in some digital circuits.

By adding your fences, houses, barriers and getting rid of little "antennas"
is rocket science, and certainly not "dumb" !!  I have built many many HF
crystal filters for the homebrew gear, 10, 12 and 14 pole, and the isolation
of various parts from other parts is critical if a stopband attenuation
around 100db is to got, together with a good 6db / 80db shape factor and
very small passband ripple. In this playground, 6db / 80db is used more
often than 6db / 60db in design work. With VHF crystal filters using
crystals in an overtone mode (I have hombrew filters at 130Mhz and lower)
life really gets interesting - even the filter's box wants to reduce input /
output isolation. The "box" problem can be there at HF, so both the layout
and the enclosure of the filter must be thought about. But modifying an
existing filter's mechanical / electrical design can be more difficult than
starting from scratch.

I have not played with K2's filter, but apart from the input / output
isolation etc, I would suspect that the Varicaps and their bias circuitry
could also spoil "isolation" especially as pcb traces are used, but by how
much ? I have not done the sums, but I also have a hunch that the varicaps
could be one of the things that limits K2's strong signal performance. But
to be FAIR, it is my understanding that K2 started life as a QRP rig, highly
portable, and it is very nice to have the variable bandwidth filter in a
single conversion receiver. The KSB2's filter layout is compromised by lack
of space - it is much better to have the crystals in a straight line.

All said IMHO you have done the right sort of things. RF circuits that are
supposed to be separate, must not gossip.

73,

Geoff   GM4ESD


----- Original Message -----
From: <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, December 07, 2004 4:57 AM
Subject: [Elecraft] ultimate rejection
>
> Going first for the simple, dumb approach I began putting little RF
> "barriers" across the middle of the filter.  They consisted of a piece of
printed
> circuit material, connected to ground and soldered all along its  length.
It
> worked great!  Everything about the filter got  better:  opposite sideband
>
>
> Enough reminiscing;  I just spent about a hour with my KSB2, making  sure
> that the solder connections near the 7 pole filter were filed down, or
clipped
> as much as possible, so that they don't act as little "antennas",  thus
messing
> up the characteristics of the filter.  It worked!  The  "audio image" that
> one hears when tuning past zero beat got a lot weaker!
>
> Next, I went to local hobby shop and bought some tin.  I made little  tin
> "houses" which I placed around the matching toroids, ground them
carefully.  It
> worked too.  Next I added a little tin "fence" which  went on the bottom
the
> board, shielding the input of the filter from the  output.  It worked.
Three
> times is a charm!
>
> My point:  it ain't rocket science, but the old ideas of keeping RF  away
> from places where it shouldn't be work just as well as they ever did.
>
>

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[Don Wilhelm-3]

Merlin,

We have known for some time that the presence of the KSB2 filter reduces the
ultimate rejection of the CW filter too.  Perhaps you have created a 'cure'.
Could you put a picture somewhere that we can see it?  If not, I may be able
to put it up on my website for all to see. (attachments to the reflector
will be stripped off)

73,
Don W3FPR

----- Original Message -----

> ...
> Enough reminiscing;  I just spent about a hour with my KSB2, making  sure
> that the solder connections near the 7 pole filter were filed down, or
> clipped
> as much as possible, so that they don't act as little "antennas",  thus
> messing
> up the characteristics of the filter.  It worked!  The  "audio image" that
> one hears when tuning past zero beat got a lot weaker!
>
> Next, I went to local hobby shop and bought some tin.  I made little  tin
> "houses" which I placed around the matching toroids, ground them
> carefully.  It
> worked too.  Next I added a little tin "fence" which  went on the bottom
> the
> board, shielding the input of the filter from the  output.  It worked.
> Three
> times is a charm!
>
> My point:  it ain't rocket science, but the old ideas of keeping RF  away
> from places where it shouldn't be work just as well as they ever did.
>
> I don't have means of measuring this stuff accurately.  Let's just say
> that
> when tuning across really loud forty meter broadcast stations the filter
> sounds A LOT tighter then it did.
>
> Has anybody else tried this stuff.  There must be hundreds of you old
> guys
> who have messed with such things.
>
> So, until Elecraft finds a way to put a bulletproof 8 pole filter in its
> magnificent radio it's time to improvise a little.
>
> 73,
>
> Merlin W3ICT

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[John Huffman]

Merlin -

Like others, I would like to see pictures and any other information on this
project.

I have found that I get distorted audio when transmitting SSB if the data cable
between the K2 and KPA100 happens to fall near the KSB2.  I even get some RF on
my CW signal.  I believe your solution would be much more effective than my
efforts to shove the cable out of the way.

73 de NA8M
John

> Enough reminiscing;  I just spent about a hour with my KSB2, making  sure
> that the solder connections near the 7 pole filter were filed down, or
> clipped
> as much as possible, so that they don't act as little "antennas",  thus
> messing
> up the characteristics of the filter.  It worked!  The  "audio image" that
> one hears when tuning past zero beat got a lot weaker!
>
> Next, I went to local hobby shop and bought some tin.  I made little  tin
> "houses" which I placed around the matching toroids, ground them  carefully.
> It
> worked too.  Next I added a little tin "fence" which  went on the bottom the
> board, shielding the input of the filter from the  output.  It worked.  Three
> times is a charm!
>
> 73,
>
> Merlin W3ICT

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