[Elecraft] [K3] Questions about ARRL review

[Geoffrey Mackenzie-Kennedy]

Toby Deinhardt wrote on Thursday, November 27, 2008 at 9:17 AM:

> you might want to read the description by PA3AKE, especially the chapters 
> on his development of his roofers:
>
> <http://www.xs4all.nl/~martein/pa3ake/hmode/index.html>
>
> He talks about, among other things, how important clean surfaces are for 
> good IMD performance in crystals filters.

Toby,

I'll second that, I am using the same 9MHz crystals that Martein uses.

> Not to forget that in a design such as the K3, it's the first 20dB to 30dB 
> of stop band attenuation which are really important.

Maybe, assuming that the IMD performance of the roofer and ALL stages which 
follow the roofer is good enough at those spacings.

> This said, I am playing with idea of rolling my own roofing filters for my 
> K3 for three reasons. Firstly, can a couple dBs be tickled out by using 
> extremely good crystals (expensive!)? Secondly, it should be educational. 
> And most importantly, HAMs just wanna have fun.

Something to bear in mind is that the passband insertion loss of a 
quadrature type roofer using good crystals is less than that of most roofing 
filters of the same bandwidth, which will increase signal levels at the IF's 
input all other things being equal. The difference can be as much as 6 - 8db 
in the case of a 500 Hz roofer. Also if you do build these filters, include 
a shield so that the input - output hybrids do not see one another and 
forget about using disc ceramic capacitors - bad for IMD!

73,
Geoff
GM4ESD