Since it is meant for public consumption, maybe Wayne won't mind that I reproduce this, that Wes also cited (from the Elecraft website), written by Wayne, N6KR: Maybe we can agree that Wayne and Elecraft ought to know. And so end of thread. -----------------------------------
What "Roofing Filter" means to Elecraft There's been so much discussion about this topic that I'd thought I'd better try to clarify why we used the term when discussing the K3S. A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages. The narrower this filter is, the less exposure later stages will have. Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain. The term "roofing filter" has most often been used in relation to triple- or quadruple-conversion receivers. Such receivers have an IF above the highest RF band covered; it's typically something in the range of 30 to 70 MHz or higher. But "roofing" as a term should be interpreted as "protective," not "high in frequency." A roofing filter protects later stages, including amplifiers, mixers, narrower filters, and DSP subsystems, just as the roof on your house keeps rain out of all of the rooms. But a roofing filter can be equally at home at a low first IF, if that is how the radio is designed. It still provides the same protective function. When we released the K2 in 1999, we never described our 1st IF crystal filters as roofing filters. We had only one IF, so the receiver model was simpler; there were no narrow filters at later stages that required protection. But now, we find that the term is in widespread use. Average hams now think of roofing filter bandwidths as the standard of comparison between receivers. This is why manufacturers have jumped through hoops to try to provide the narrowest possible roofing filters. Many operators have an understanding (justified) that a roofing filter that is wider than the communications bandwidth will not best protect the receiver's later stages. So the term now seems appropriate to use even in a radio such as the K2, K3S, or Orion, all of which use low-frequency IFs (5 to 9 MHz). In recent years, the roofing filter has become the centerpiece of receiver redesign: Suppose that manufacturer "A" initially designed their receiver to use a 15- or 20-kHz roofing filter. Yes, this allows the receiver to handle NBFM and other wide modulation modes; it may also be selected to constrain the signal bandwidth ahead of a noise blanker or spectrum scope. But it comes at a price. If you're using CW mode, you'll have much narrower filters selected at the radio's 2nd and 3rd IFs. Yet the 1st IF roofing filter allows a broad swath of signals into the earlier stages. You don't need this energy in your passband. It can cause trouble. Manufacturer "A," realizing they have a problem with dynamic range at close spacing, then announces that they've had a breakthrough: they can now offer a 6-kHz, or more recently 3-kHz roofing filter. This will certainly improve the situation for SSB and AM operation, but it still opens the barn door in CW or DATA modes, because the bandwidth is a factor of 10 wider than needed for communications. So why don't they offer much narrower roofing filters that can be switched in for CW and data modes, or at times when adjacent-channel SSB QRM is very high? It's because they can't make filters any narrower at such a high IF. Enter the "down-conversion" rig (K2, K3S, Orion, etc.). By converting to a low first IF, the designer can easily create narrow filters that are compatible with the required communications bandwidth. This is why we are offering filters with bandwidths as low as 200 Hz. And yes, these are still "roofing" filters, because they limit exposure (bandwidth), thus protecting later stages (in the K3S case, the IF amp, 2nd mixer, and DSP). 73, Wayne N6KR CWops #1085 CWA Advisor levels II and III http://cwops.org/ -------------------------------------------- On Thu, 6/14/18, K9MA <k...@sdellington.us> wrote: Subject: Re: [Elecraft] Field Day rig experience To: elecraft@mailman.qth.net Date: Thursday, June 14, 2018, 8:28 PM There are at least two excellent reasons for the narrow crystal filters in the first IF of the K3(s). (Wayne can correct me if I'm wrong.) One, of course, is to reject the image of the second IF. However, the dynamic range of the ADC in the second IF, by itself, just isn't enough to provide the 140 or so dB we need. The combination of the ADC/DSP and the crystal filter does the trick, even though 8 MHz crystal filters aren't all that great. As I recall, there were some earlier DSP only receivers, but their dynamic range was poor. Crystal filters are expensive, but until we have fast ADC's linear to at least 24 bits, they're necessary to get that kind of dynamic range. I've often wondered if any other communication system requires the close in dynamic range we do. Why would anyone design a system that allowed signals 2 kHz apart to differ in strength by 140 dB? 73, Scott K9MA On 6/14/2018 20:33, WILLIE BABER wrote: > Wes, > > "A "Roofing filter" is simply a filter in the radio's first IF through which all signals must pass before they will be "seen" by later receiver stages. The narrower this filter is, the less exposure later stages will have. Thus a "narrow" roofing filter is desirable -- but "narrow" is relative, as I'll explain." > > What Elecraft said (above) is exactly what I said. Moreover, Elecraft's explanation is required because the term roofing filter is now applied to up-conversion in multiple conversion radios (with relatively wide first I-F filters compared to what is achievable at a low first I-F) which is what the term initially sought to rebuff in the first place, also my point. > > 73, Will, wj9b > > CWops #1085 > CWA Advisor levels II and III > http://cwops.org/ > > -------------------------------------------- > On Thu, 6/14/18, Wes Stewart <wes_n...@triconet.org> wrote: > > Subject: Re: [Elecraft] Field Day rig experience > To: elecraft@mailman.qth.net > Date: Thursday, June 14, 2018, 4:47 PM > > Will, > > First of all I have said before and will repeat > it, I detest the term "roofing > filter." That said, by the generally > accepted definition, you are wrong. See > Elecraft's take on this: > > http://www.elecraft.com/K3/Roofing_Filters.htm > > If you will think in > Wayne's terms, the post-mixer filter is a > "protective" > filter, not a > mode-specific filter. So the question becomes, how much > > protection is necessary? In > Elecraft's case, quite a lot, IMHO. With its QRP > DNA, Elecraft uses post crystal filter > circuitry that minimizes current > consumption. The trade off for this is the > need for a bank of pricey crystal > filters > to limit the frequencies that the circuitry is exposed > to. > > Now what if the > subsequent circuitry doesn't require this much > protection > because it is more robust? We > now have direct-sampling radios that can digitize > a whole ham band with good performance. If the > BW was limited to 10-15 kHz in an > up > conversion configuration they should be even better. The > limitation now > becomes LO phase noise, but > newer synthesizer designs overcome that obstacle. > Another thing to note is that IMD in crystal > filters is reported to be inversely > proportional to BW. So a wider filter might > actually be better from that > perspective. > Some Elecraft filters exhibit passive IMD BTW. > > Wes N7WS > > On 6/14/2018 8:01 AM, WILLIE BABER wrote: > > Hello Wes, > > > > I took a look. Both designs are using > the idea of "roofing filter" to refer to > up-conversion radios similar to the use of up-conversion > 3khz filters as roofing filters in Icom radios. > > > > "Roofing > filter" (a mode specific filter after the first mixer > including narrow cw filters) only makes sense in the > context of the history of superhet design and in > particular the use of one broad 15 khz first I-F (so that > all modes may pass through it) typical of all Japanese > radios until recently. Calling a 45 mhz filter at the > first I-F a "roofing filter" as noted in the info > you sent entirely misses the point of what roofing filter > means. Or, to put it another way, all Ten-Tec radios had > roofing filters in them (and were ssb and cw only) well > before the term roofing filter was coined! Which is why an > Omni C will out perform any wide (15 khz) first I-F Japanese > radio, even those built well after the 1980 vintage Omni > C. > > > > Unless mode > specific up-conversion crystal filters can be made and as > narrow as 200 hz (this is possible with down-conversion) > then "roofing filter" and up conversion > doesn't make sense historically or in reality. > > > > Actually, Icom says > that did it with 1.2khz filter at 64 mhz in the Icom 7851, > though I'm not convinced the filter is that narrow, and > 1.2khz is far from the 200hz filter that my K3 has in it > (however, the placement of this filter is why the 7851 is > among the best radios in Sherwood's chart, on cw). > > > > It is possible to > make very narrow and precise crystal filters as narrow as > the 200 hz inexpensively, and this is the point of having > multiple roofing filters at the first I-F. So, this is the > origin of the term roofing filter---in comparison to the > barn-door up conversion first I-F. > > > > 73, Will, wj9b > > > > CWops #1085 > > CWA > Advisor levels II and III > > http://cwops.org/ > > > > > -------------------------------------------- > > On Wed, 6/13/18, Wes Stewart<wes_n...@triconet.org> > wrote: > > > > Subject: > Re: [Elecraft] Field Day rig experience > > To:elecraft@mailman.qth.net > > Date: Wednesday, June 13, 2018, 3:08 > PM > > > > > Certainly not to disparage the > > K3(S) > architecture (I have two of them) there is > > nothing inherently wrong with an > up-conversion > > receiver, if modern > hardware is used. > > > > See:https://martein.home.xs4all.nl/pa3ake/hmode/g3sbi_intro.html > > > > and my friend > Cornell's, > > Star-10 > transceiver. > > >https://pdfs.semanticscholar.org/eb33/5c12858779a653d9b9b93ca20120aebb7616.pdf > > > > Wes N7WS > > > > > > On 6/13/2018 11:38 AM, WILLIE > BABER > > wrote: > > > > Robert is talking about the > > > crystal filters, also known as roofing filters now-days, > > that are typically placed after the > first mixer (I > > mistakenly typed > "ahead" but I meant > > > "after" as Robert notes), though there is a > post > > amp and NB before these filters > in K2 and K3. > > > > > > The idea is that a > > crystal filter right after the first > mixer gives high > > dynamic range > because high selectivity comes before the > > receiver has developed stages of gain > that otherwise could > > cause blocking > or IMD, especially when selectivity is > > postponed to the second mixer while > ignoring gain > > distribution in prior > stages of the receiver. This basic > > > idea was popularized in Solid State Design for the Radio > > Amateur, and it was applied to Ten-Tec > radios for decades > > (at a 9 mhz > I-F). > > > > > > > > > Roofing filter gets defined in > relationship to Japanese > > radios that > had up conversion 15 khz filters at the first > > I-F, and generally lower dynamic range > as a result, (but you > > got all modes, > general coverage, and optional crystal > > filters at the second I-F). > > > > > > Good > for everyone radios.... but with > > > lower dynamic range and phase noise from the early > > synthesizers. This is why Ten-Tec > radios were so popular > > among > contesters, especially Omni V and VI (modified with a > > narrow cw filter at the first I-F). > > > > > > 73, > Will, wj9b > > > > > > > > ______________________________________________________________ > > 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 > > Message delivered towlba...@bellsouth.net > > > ______________________________________________________________ > > 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 > > Message delivered towes_n...@triconet.org > > > ______________________________________________________________ -- Scott K9MA k...@sdellington.us ______________________________________________________________ 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 Message delivered to wlba...@bellsouth.net ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
