[Elecraft] K6SE Sprint Summary for Team Elecraft

[Charles Greene]

Mark,

True, the desire for general coverage is a fifth reason for having a 15KHz 
roofing filter.  However, the transceivers which do not have general 
coverage or FM, (which is all the rest except the Orion) still have a 15 
Khz roofing filter.  It must be one of the other three reasons.  The Ten 
Tec Omni-VI comes to mind.  At any rate, a narrow band roofing filter gives 
a big improvement in close in IMD rejection.

At 02:43 PM 9/11/2002 +0000, Mark J. Dulcey wrote:
>Charles Greene wrote:
>
> > While your concern for the xtal mods is well taken although I highly
> > doubt that the mod causes "ringing," it is a worthwhile mod.
>
>The mod has little effect on the early part of the filter slope (the first 
>30dB or so), so it should have little effect on ringing. What it helps is 
>the later part of the slope; the modified filter has somewhat steeper 
>skirts past that point and considerably better ultimate rejection.
>
> > Most ham rigs use a 15
> > kHz roofing filter as they have a FM mode and need it, or the designers
> > follow the lead of designers of FM mode rigs and continue to use 15 kHz
> > roofing filters, or else they haven't figured out how to do it yet, or
> > worse, don't care.
>
>I think the main reason for the use of wide roofing filters is the desire 
>for general coverage reception. The only way to get complete general 
>coverage is to use up-conversion (a first IF above the frequencies that 
>the radio receives); typically, this is in the neighborhood of 70 MHz. 
>Narrow filters for 70 MHz are not readily available, and are difficult to 
>build. (Not impossible; I have heard of a military design that used them. 
>But the filters cost about $500 each, back in the 70s; they'd be even more 
>expensive now.)
>
>The Orion can have narrow roofing filters because its main receiver 
>doesn't have general coverage. The secondary receiver in the Orion is more 
>typical of current transceiver design, using up-conversion and a wide 
>roofing filter; it does have general coverage.
>
> > Being a single conversion superhet helps the K2
> > reduce the number of birdies and may improve the IMD also.
>
>Yes, it does both of those things. It also, alas, means giving up the 
>convenience of passband tuning, though it can be simulated to some extent 
>with the variable bandwidth filters and BFO settings. And, as explained 
>above, it means no general coverage reception. So it goes.
>


73, Chas, W1CG
K2 #462