[RTTY] Crystal filter width preferences for RTTY contesting

[Kok Chen]

On Aug 25, 2009, at 10:59 PM, Jeff Blaine AC0C wrote:

> A common data point  seems to be the Inrad/Elecraft K3 filter @ 250  
> hz. Looking at the top of that filter, it is about -3db down at 240  
> hz and -6db at about 370.  Guys seem to have no negative comments  
> about that filter.

The skirts of the stock K3 filters are not especially tight.  But they  
don't need to be.  They are basically roofing filters to keep the A/D  
converter of the "real" software (DSP) filters from clipping.  Steep  
skirts are much easier, and more accurately, obtained by using DSP  
math than by using a limited number of poles from a crystal filter.

Jim K9YC has a nice write-up on filters, with curves for the FT-1000MP  
(stock and Inrad, and how filters can be cascaded in the MP) and K3.   
See here

http://audiosystemsgroup.com/FilterTestNotes.pdf

I'd stocked my FT-1000MP with some Inrad filters from the days when I  
was still depending on narrow I.F. filters.  (The Inrads are "tighter"  
than the equivalently spec'ed Yaesu filters.)  As a consequence, I  
still have the following compliment for my MP, though I seldom need  
the narrow filters anymore.   With RTTY, I'd used the 500 Hz (Yaesu)  
at the 8215 kHz I.F., cascaded with the 400 Hz (Inrad) at the 455 kHz  
I.F..

>          			455 kHz IF
> 		 		YF110CN          250 Hz CW	250
> 				IRC 703C          400 Hz CW	500
> 				Empty		-			2.0
> 				XF115S		Stock	        2.4
>
> 				8215 kHz IF
> 				IRC	708B        250 Hz CW       250
> 				XF501		Stock	        500	(soldered)
> 				IRC 709B          2100 Hz SSB    2.0
> 				XF262		Stock	        2.4	(soldered)
> 								
> 				2nd Rx
> 				YF115C		500 Hz CW

For my K3, I have the 400 Hz, 2.8 kHz and 6 kHz roofing filters.

73
Chen, W7AY