Here are my thoughts on K3 crystal filter setup, inspired by a recent question posted to the Elecraft K3 Yahoo group by Eric Scace, K3NA.
Offset (FLx FRQ) ---------------- There are many considerations that go into the positioning of the K3's crystal filters with respect to the I.F. center frequency. We shift them upward if the filter is too wide to be centered at Fc/2 + 200 Hz, thus preserving the lower edge of the filter around 200 Hz. This usually happens only in CW mode, typically with lower sidetone pitches. The result is optimization for opposite-sideband rejection. You can argue for a different approach, but the K3's success in CW DXing and contesting suggests that this approach is just as valid as any other. I don't recommend trying to fool the firmware by adjusting the crystal filter offsets; I'd use the marked values. Changing them is likely to cause unwanted side-effects, since the filter passband is inverted for complimentary modes (CW/CW REV, LSB/USB, DATA/DATA-REV). The exception to this rule is when fine-tuning the offsets of 5-pole filters on the main and sub-receivers in order to provide best performance in diversity mode. These adjustments will rarely be larger than +/- 20 Hz anyway. Bandwidth (FLx BW) ------------------ It's OK (but not necessary) to fudge the bandwidth of specific filters (FLx BW). For example, INRAD's 8-pole "400 Hz" filter can be declared as 0.4, 0.45, or 0.5 kHz, depending on where you want this filter to be kicked in as WIDTH is rotated. The audible effect is subtle. Ed Muns, W0YK, has described this technique in detail elsewhere. Gain (FLx GN) ------------- Regarding filter loss compensation: I recommend simply sticking with what's on page 46 of the owner's manual (1 to 2 dB compensation for 400-500 Hz filters, 3 to 4 dB compensation for 200-250 Hz filters, and 0 for all others) unless you have a very specialized application and lots of time on your hands. There are three reasons for this. (1) The *perceived* loss of a crystal filter is a function of both actual loss and S/N ratio. If a narrower filter has a little more loss, you may not notice it. (2) Most signals are flattened slightly by AGC anyway. (3) Adding a lot of gain to filters can again cause side-effects, such as interaction with RX EQ settings and variations in gain between main and sub if the filter complements are different (e.g., during diversity use). In fact we recently issued new guidelines for factory-assembled K3s: all crystal filters' FLx GN numbers will be to 0 except for 400-500 Hz (1 dB) and 200-250 Hz (2 dB). Customers are free to optimize these further, of course -- but most won't need to. 73, Wayne N6KR --- http://www.elecraft.com ______________________________________________________________ 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
