All, My K3 (with one extra roofing filter for CW) is not here yet, so I am asking this question without direct experience.
It will be interesting to do an A/B comparison to see how the receiver sounds with/without the narrow crystal filter for a given bandwidth. I suspect this kind of test is of interest to many K3 owners. However, to be meaningful, it must be carried out properly. I want to ask the group: 1. How best to carry out such a test, and 2. What say those that have already tested. As to how to carry out the test, I think I read somewhere that the narrow filter can be disabled from the radio front panel. (Someone please correct me if I am wrong.) Now doing so, without changing the DSP bandwidth, is going to change the overall bandwidth unless the narrow crystal filter being disabled is already considerably wider than the DSP bandwidth. The case where the crystal filter is narrower than the DSP width setting is obviously going to result in less noise when the crystal filter is in, so that is not the case I am interested in. If the crystal filter is nominally the same width as the DSP filter, that is an interesting case. The composite width would then be narrower than the DSP alone (actually with say an SSB crystal filter). To make the comparison properly, when one takes out the narrow crystal filter (replaces it with an SSB crystal filter), one would probably need to change the DSP filter width to a narrower setting in order to get the same overall filter width. This wouldn't work out unless the required change in DSP width happens to match one of the available DSP width settings. If the crystal filter is moderately wider than the DSP setting, say using (vs not using) a 1 kHz crystal filter with a DSP setting of 500 Hz, that is an interesting situation that should be common in the K3. In this case maybe one can leave the DSP at 500 Hz, and just listen for any change. What kind of change am I talking about? If there is a signal within the crystal filter bandwidth that is strong enough to pump the hardware AGC, then I would expect to hear increased qsb to the down side. The point of the experiment would then be to assess how bothersome this effect is in a real situation. Is the narrow crystal filter worth it? While doing this, I would really like to see an indicator that shows the hardware AGC kicking in. Is there any way to see this on the K3, short of hanging a scope on the AGC line? On another line of thinking, reality tends to be complex, and, especially after the hardware AGC trigger level was raised (I think that is what was done??), maybe the composite of random noise and multiple strong signals in the crystal filter passband can produce some very short-lived signal peaks that either overload the ADC or by non-linear effects can generate additional audible noise in the case where the narrow crystal filter is not used? Whatever the cause, I am very interested in any noise increase that is actually audible in a real-life case with the K3, as a result of not having the ideal crystal filter for a given effective receiver bandwidth, especially for the case where NO ONE strong signal within any of the available crystal filters is strong enough to activate the hardware AGC. What has been observed by the group? 73, Erik K7TV _______________________________________________ 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
