Beware of the conclusions of the so-called K3 Filter Study.  I don't know
who wrote it since there is no name on it, but the 250 Hz filter does not
fall apart at -25 dB.  The results shown are exactly what you would expect
for a well-designed receiver with no signal at the input.  The author of
this study doesn't state the conditions of the K3 such as band, preamp
status and antenna connection, but I will assume no antenna connected with
the preamp on.

If you look at the first plot on page 3 labeled Study of Inrad 400 and 250
Hz Filters, you will see a broad shoulder at about 30 dB down that the
author is concerned about.  This shoulder drops off at 100 Hz and 1100 Hz
due to the DSP filtering.  So let's look at what is causing this.

There is no signal into the K3, so all of the noise is internally generated.
At 600 Hz, the noise is coming from the preamp and is then filtered by the
250 Hz and 400 Hz roofing filters.  The shoulder is caused by the internally
generated noise after the roofing filter.  The noise from the preamp is
dominate, so that is sets the noise figure.  However, if we made it even
more dominate, so that the shoulder was lower, we would reduce the dynamic
range of the K3.  Receiver design is a tradeoff so that you create both
minimum noise and maximum dynamic range.

If the author had coupled a noise diode to the input of this K3, he would
have found that the shoulder would disappear because he now had a noise
signal into the K3 much stronger than the preamp.  My advice would be to
disregard this report because the measurements were not made in such a way
as to accurately determine the response of the K3 filters.

-John
 KI6WX

> 
> Both you and Ed raise good points. No intent there -- I agree that
> the so-called 250 Hz filter is slightly more useful than the 400
> Hz. My real objective is to get Inrad to clean up their act!  BUT
> -- take a look at the data in this link. Lots of food for thought.
> There's something different about the K3 Sub-RX, and the Roofing
> filters seem to fall apart below about -25dB. Makes a guy wonder
> what's going on!
> 
> http://audiosystemsgroup.com/K3FilterStudy-250HzRoof.pdf
> 
> 73, Jim K9YC
> 
> 
> ______________________________________________________________

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