K3 Stock 2.7 SSB filter vs. 2.8 optional filter - what to buy?
From Zerobeat
Q: What's the difference between the Stock 2.7 SSB filter vs. 2.8 optional filter?
A: .1
This would be a small difference even if the filter was used to determine the final receive bandwidth as on a non-dsp transceiver. Rather, it's a roofing filter on receive, and the final transmit filter on SSB. This makes it -far- less critical.
The difference is negligible, however since many will carry over the old ideology of putting the most expensive rock in their radio that they can find, Elecraft offers the 2.8 filter.
Rest assured, if the difference was anything more than marginal, Elecraft would have included the 2.8 filter as a stock item.
The second receiver contains its own 2.7 filter, free, at no extra charge.
Don WB8YQJ
The K3 FAQ states that only the first 30 dB of skirt selectivity of the roofing crystal filters is important - the DSP will take care of the rest.
Here's what is going on. The ADC/DSP system has about 100 dB blocking dynamic range versus about 140 dB for the receiver's analog front end. That's why there is an analog AGC system between the first mixer and the ADC - to keep very strong signals within the range that the ADC can handle. So long as any out-of-band signal is attenuated at least 40 dB, the analog front end will overload before the ADC is overdriven. And so long as the ADC is not overdriven the DSP can filter out the unwanted signal.
Actually, the analog front end will be pretty much toast even before before the interferer gets to the blocking point. Remember that blocking dynamic range is much greater than IMD dynamic range. That's probably why they (somewhat arbitrarily) picked the 30 dB number rather than 40 dB.
Of course, the 5-pole 2.7 kHz filter is actually a little narrower than the 8-pole 2.8 kHz filter at the -6 dB points. Let's assume the 8-pole filter has better attenuation beyond about -10 dB. For the unwanted signal to be a problem with a 5-pole filter but not with an 8-pole filter, it has to occur within the -10 to -30 dB points on the filter skirts, about 1.5 to 2 kHz from the center frequency. In that range, the 8-pole filter is only a few dB better than the 5-pole filter, let's say 0 to 10 dB better, depending on frequency. The interfering signal strength has to be within that 0-10 dB window, within the 1.5-2 kHz range, in order to cause a problem that the 8-pole filter could cure. A pretty rare case.
Al N1AL
