[Elecraft] Contemplating Purchase of K3

[Joe Subich, W4TV]

> I find that I nearly always have the IF set to 200-250 Hz for CW, and
> 400 Hz for RTTY. In other words, I rarely use the 400 Hz filter.

Whilst the 250 Hz filter works for strong signals on RTTY, the group
delay increases rapidly as one approaches the knee of the response.
That group delay has a significant impact on RTTY and causes a loss
of several dB snr on weak signals that can hardly afford the added
hit.

The main purpose of the *roofing filter* is to protect the analog to
digital converter from overload by multiple strong signals at the same
time (and to prevent the hardware AGC from activating).  The difference
in selectivity between the 400 Hz (435 Hz @ -6dB) and 250 Hz (370 Hz @
-6dB) - see  http://www.elecraft.com/K3/K3_filter_plots.htm is only
some 30 Hz on each side of the passband.  If a strong signal is close
enough to be in that 30 Hz a slight tweak of the shift control will
move him out.  Even then, it is entirely likely that any signal close
enough to cause problems in the 400 Hz filter will have phase noise,
splatter and/or key clicks that fall inside the passband of *either*
filter and can't be rejected in any case.

If one is looking for the most efficient combination of IF filters,
consider 400 Hz (or 500 Hz if RTTY is not a priority) with the 200 Hz
(5 pole) as the 200 Hz filter is tighter than the 250 Hz filter to at
least -50 dB on the skirts (and the 200 Hz filter is less expensive
than the 250 Hz filter).

73,

    ... Joe, W4TV


On 2015-03-07 1:43 PM, Jim Brown wrote:
> On Sat,3/7/2015 10:26 AM, Rick WA6NHC wrote:
>> I'm now wondering about changing to the 200 filters 5 pole or 250 8
>> pole (I'm adding the new synth, when the backorder is fulfilled). An
>> edge is an edge but I suspect the 3 db is a rather bare edge.
>
> I started with 400 Hz, then added 250 Hz. I've defined the 400 Hz filter
> as 450 Hz (so that it switches in at 450 Hz) and the 250 Hz filter as
> 350 Hz. Both are 8 pole filters. I find that I nearly always have the IF
> set to 200-250 Hz for CW, and 400 Hz for RTTY. In other words, I rarely
> use the 400 Hz filter.
>
> It should be noted that the 250 Hz filter actually measures closer to
> 325-350 Hz. Since they're the same price, I'd buy the 250 Hz filter if I
> was going to buy one at all.
>
> 73, Jim K9YC
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