Few filters have vertical skirts. They roll off gradually; the slope of that rolloff is one of the measures of filter quality.
The filter width is often (but not always) described as the 6 dB down point on that rolloff curve, A filter described as 2.8 kHz wide will pass some energy through at frequencies outside its 2.8 kHz bandwidth. Graphs of representative Elecraft K3 filters are available at http://www.elecraft.com/K3/K3_filter_plots.htm For example, the representative 2.8 kHz 8-pole filter is 2.888 kHz at 6 dB down, and 4.488 kHz at 60 dB down. The 2.7 kHz 5-pole filter is wider at the 60 dB down point. Some older radios depended on filtering to remove the unwanted sideband and carrier. Current generation radios generate just the desired SSB signal, without the unwanted sideband and without the carrier. There are a number of amplifiers after the SSB signal generation and filtering that generate various levels of IMD products. It's up to the radio manufacturer to provide equipment that meets the FCC specified limits when the equipment is operated according to its owner-level instructions. It's up to us to operate the radio within these described limits in order to keep the unwanted signals within the FCC-prescribed limits and good amateur practice. When the instruction manual says "4-5 bars", that doesn't mean "turn it up as far as it will go". http://www.youtube.com/watch?v=XuzpsO4ErOQ None of this is specific to the K3 except the details of the filters. 73 de Dick, K6KR -----Original Message----- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of gold...@charter.net Sent: Sunday, July 15, 2012 6:13 AM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] [K3] Tunijng the Radio Hi Jack, So I think what you are telling me is that the IPA is after the DSP and that obvioulsy any inperfections in this circuit and the final amp are what creates the IMD products. Is that another reason that the tx filter is 2.7 or 2.8 wide at the skirts to act as a wall to prevent this from getting out on the air compared to using a 3.0 wide filter. Thanks Don ~73 Don KD8NNU On Sun, Jul 15, 2012 at 8:51 AM, Jack Smith wrote: > Don: > > Not so at all - whilst the low level SSB generated by the K3's DSP may > be close to theoretically perfect, intermodulation products produced > by the K3's IPA and final amplifier stages are present in quantity and > are the limiting factor. > > Jack K8ZOA > > > On 7/15/2012 8:02 AM, gold...@charter.net wrote: >> Being a SDR there is really no suppressed carrier as the carrier is >> never created only the sideband for the mode selected. So unless >> you >> have signficant splatter there should not be an IMD problem with a >> K3. >> >> Again assuming no splatter and a properly functioning radio. >> >> This is no analog radio and all TX is based on the DSP first and then >> the TX filter last. >> >> ~73 >> Don >> KD8NNU >> >> >> On Sun, Jul 15, 2012 at 7:34 AM, Geoffrey Mackenzie-Kennedy wrote: >> >>> Dave, >>> >>> To add to the replies which you have received, the width of your USB >>> signal should also include those intermodulation products which >>> appear at frequencies less than that of the suppressed carrier >>> frequency, i.e. in the LSB region. If you tuned your K3 to 14.225 >>> MHz, these IMD products would fall outside of your General Class >>> portion of the band and could create interference. To be safe I >>> would suggest that you do not operate your K3 in USB mode below >>> 14.228 kHz. >>> >>> In Europe the top end of our 80m allocation is 3.800 MHz, and some >>> European hams operate LSB on or very close to 3.800 MHz. This >>> practice has caused some non-amateur users operating just above >>> 3.800 >>> MHz to complain about interference from amateur IMD products. >>> Fortunately those complaints which I have heard are still at the >>> "muttering" stage. >>> >>> 73, >>> Geoff >>> LX2AO >>> >>> >>> On July 14, 2012 at 10:39 PM, David Guernsey wrote: >>>> If I tune my K3 to 14.225 MHz will I be in the General Class portion of the 20 meter band, or do I have to allow for the width of my USB signal when I transmit? ______________________________________________________________ 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
