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 CWops #1085 CWA Advisor levels II and III http://cwops.org/ -------------------------------------------- On Wed, 6/13/18, Jim Brown <j...@audiosystemsgroup.com> wrote: Subject: Re: [Elecraft] Field Day rig experience To: elecraft@mailman.qth.net Date: Wednesday, June 13, 2018, 10:36 AM On 6/12/2018 4:50 PM, Robert G Strickland wrote: > A small nit... perhaps my ignorance... but, I think that the roofing > filter [by whatever name] comes after the first mixer, at the > so-called IF frequency. A month or so ago, as part of a project to measure input Z of receivers and preamps, I measured the 2nd RX of my K3 as 50 ohms and with a bandpass filter between the antenna input and the 50 ohm load. Clearly, in the K3, there is a "per band" bandpass filter ahead of the first RF stage. 73, Jim K9YC ______________________________________________________________ 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
