Mike, W9QS wrote:
I have a K3/10 on order. I have used my K2 for over a year and find
the filters to be very good. I operate 90% on the time on QRP CW.
I'm looking for opinions on which roofing filters I might include.
Mike,
If you operate CW at all, you'll almost surely want at least one narrow
filter. Either our 400 Hz or 500 Hz filter would be a great all-around
choice.
I'm *not* just trying to sell filters :) The reason you need a narrow
filter is that the K3's entire receiver architecture is based on the
premise that the 1st I.F. filter should be close to the communications
bandwidth in use. This is what sets the K3 apart from other
transceivers that use an up-conversion architecture, and thus very wide
front-end filtering (at least in relation to CW or data modes -- 3 to
15 kHz).
The K2 has a similar architecture to the K3. In that case, a single
crystal filter covers bandwidths from about 200 to 2000 Hz. The K3
builds on this concept, using multiple fixed-bandwidth filters with
much lower ripple, stronger signal handling in all stages, and a
synthesizer with extremely low phase noise. But both receivers use
low-noise 2nd IFs that really should be protected from out-of-band
signals. (In this case, out of band means "outside the crystal
filter.")
If you're a K2 owner, you're already use to the protection you get from
narrow filtering. You'll probably want that same kind of signal
handling performance in your K3 if you use it on Field Day, or during a
contest, or just when the band is open and signals are strong.
If you look at Sherwood's receiver performance chart
(http://www.sherweng.com/table.html), you'll see that the K2 has
respectable IMD dynamic range at 2 kHz: close to that of the Icom
IC-7800 (at 1/10th the price :) But the K3 is at the top of the chart
-- 15 to 20 dB better, depending on the filter bandwidth. To obtain
this benefit from having a K3, IMHO you really need narrow filtering.
73,
Wayne
N6KR
---
http://www.elecraft.com
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