> 13 kHz comes quite close to the 15 kHz DSP IF, so using the 13 kHz > roofing filter for AM transmit exposes the possibility of > transmission at an IF image as well as the desired frequency.
The IF Image is 30 KHz away (twice the IF), not 15 KHz. Although neither INRAD nor Elecraft will provide specifications (shape factor) for the FM filter, one should be able to assume it is considerably less than 2:1. For comparison, the INRAD 6 pole 7007.5 MHz filter has a 1:60 dB shape factor of 2.1 and the 7045.0/7065.0 MHz 20 KHz filters have 1:60 dB shape factors of 2.2. Converting to 6:60 dB would make the shape factors about 1.9 and adding two more poles would further sharpen the skirts. Still, even with a shape factor of 2 by the time one was out 20 KHz (40 KHz bandwidth point) the filter response should be down more than 100 dB ... somewhere about the level of stray coupling (feed through) in the IF. 20 KHz leaves plenty of "guard band" from even the highest sidebands with a 3.5 KHz maximum modulating frequency. If that's not enough, the filter could even be offset another 4 KHz "away" from the image during transmit without any impact on the transmit bandwidth (3500 Hz maximum modulation frequency). If I have time between now and the time I leave for Dayton, I'll set up one K3 to transmit AM (and ESSB) through the FM filter and make some spectrum analyzer captures (using the SQR-IQ, SpectraVue and the other K3). If there's a problem, it will be easy enough to see. 73, ... Joe, W4TV > -----Original Message----- > From: elecraft-boun...@mailman.qth.net > [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Don Wilhelm > Sent: Sunday, May 03, 2009 9:16 PM > To: Frank Ross W4NHJ > Cc: elecraft@mailman.qth.net > Subject: Re: [Elecraft] Shortwave filters for K3 > > > Frank, > > 13 kHz comes quite close to the 15 kHz DSP IF, so using the 13 kHz > roofing filter for AM transmit exposes the possibility of > transmission > at an IF image as well as the desired frequency. > > As I recall, that was a wish expressed by someone, but I also > recall the > Elecraft designers saying that it has a high probability of creating > spurs, but would be further evaluated sometime in the future. > If it has already been evaluated, then the answer is that it > really does > create spurs and cannot be used. If it has not yet been > evaluated, then > it is still on the list for future consideration. > > 73, > Don W3FPR > > Frank Ross W4NHJ wrote: > > Don, > > > > I thought I read some where that you can use the 13 kHz filter to > > transmit > > AM by using the transmit DSP to limit modulation bandwidth. > > > > Frank > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
