[Elecraft] Re: Roofing Filter question (+ some K3 receiver philosophy)

[wayne burdick]

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

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