[Elecraft] K6SE Sprint Summary for Team Elecraft

[Mark J. Dulcey]

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.