Tom, K6CT, wrote: The Feb 05 QST has review of InRad roofing filter for FT-1000MP series. The article states, "the Ten-Tec Orion and Elecraft K2 are ham band only designs and thus have their first IF and roofing filters in the HF region". The article describes how Icom 7800 has roofing filters at 15 and 6 kHz and that new Yaesu FT9000 DX will have 2 roofing filters at 15, 6 and 3 kHz.
The article further states, "Single conversion receivers, such as the K2, can set both the roofing filter and selectivity with a single filter following the mixer. Question, what roofing filter default are we using in the K2 and how do we change it? Or, is there any need to do so. I have noticed on ssb that I get more intermod than during contest ops using cw ------------------- As I understand it, the "roofing filter" is always the first filter in the receiver I.F. chain. In most superhet receivers, there are more than one filter in the I.F. chain. Since the first superhets appeared, the design of this first filter has been a subject of intense interest that profoundly affect the overall performance of the receiver. That hasn't changed, and improved after-market filters have been developed and marketed as replacements for the factory-supplied filters in many rigs. Inrad is one such company. This first I.F. or "roofing" filter is the "second line of defense" your receiver has against unwanted signals. The first line of defense is the input filter that is responsible for suppressing any mixer image responses. In the K2, the input filters are wide enough to pass an entire Ham band without attenuation, but they must effectively suppress image signals. In the K2, the image responses are 10 MHz away. Different input filters are switched in by relays as you change bands. After the mixer, the filters in the I.F. trim away nearby signals in the same band. In multi-conversion receiver designs, the first I.F. filter often is purposely made wide, covering several kHz. Basically it limits the upper frequency limit passed - that is to say it puts a "roof" on the passband. Subsequent mixers and filters then select part of the passband. This allows things like front panel "passband tuning" but at the expense of a lot of added complexity. Complexity not only adds cost, it adds more opportunity for overloading, noise and other bad effects that can compromise the overall receiver performance. As the QST article said, the K2 uses a single conversion design, which provides optimum performance at minimum cost by avoiding extra complexity and dropping features like passband tuning. Instead of a low pass "roofing filter" the K2 uses a passband filter at the I.F input that provides almost all of the selectivity to reject unwanted signals within the band. Since it's a bandpass filter that only passes a certain narrow range of frequencies, perhaps "roofing filter" isn't the right name, but that name seems to be sticking to the first I.F. filter in any receiver. In the K2, there is one "roofing" (passband) filter for CW and a different "roofing" (passband) filter for SSB, if you have the SSB option. As you know, the CW filter is an adjustable-bandwidth filter which you can adjust in CAL FIL. They are in the HF range - about 4.9 MHz as QST said. Since the K2 is a 'single conversion' design, that is the only intermediate frequency. You can adjust the passband of all the K2 "roofing" filters relative to the carrier frequency by adjusting the BFO frequencies in CAL FIL as well. The passband of the CW filters and the position of the filter passbands relative to the signal are the only adjustments provided. The K2 actually has another filter, the 2nd Xfil (second crystal filter). It is a wider, fixed filter at the output of the I.F. amplifier to "clean up" the broadband noise produced by the amplifier. There are no operator adjustments for this filter. Of course the K2 also offers additional filters but they do not work in the intermediate frequency range. There's the analog KAF2 and the digital KDSP2 filter modules. These filters provide additional selectivity and performance features by acting on the audio signal. If you are hearing intermod when the band is crowded with strong signals, 1) be sure the noise blanker is Off and 2) switch the preamp Off and perhaps the attenuator On. The very action that makes the noise blanker work also makes it a good mixer that will produce a lot of byproduct frequencies on the band when strong signals are present. The sensitivity of the K2 is not compromised by turning the preamp off on any frequencies below about 14 MHz unless you are using a very tiny, inefficient antenna (e.g. a short whip). On the lower frequencies with a decent antenna all the preamp does is provide extra overall gain that makes the S-meter jump more and the signal 'sound' louder but that gain can cause intermod if very strong signals over-drive it. The preamp is before the "roofing filter" so it is not protected from extremely strong signals anywhere within a few MHz of the Ham band. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
