[Elecraft] K3 crystal filter offset/bandwidth/gain setting guidelines

[wayne burdick]

Here are my thoughts on K3 crystal filter setup, inspired by a recent 
question posted to the Elecraft K3 Yahoo group by Eric Scace, K3NA.


Offset (FLx FRQ)
----------------

There are many considerations that go into the positioning of the K3's 
crystal filters with respect to the I.F. center frequency. We shift 
them upward if the filter is too wide to be centered at Fc/2 + 200 Hz, 
thus preserving the lower edge of the filter around 200 Hz. This 
usually happens only in CW mode, typically with lower sidetone pitches. 
The result is optimization for opposite-sideband rejection. You can 
argue for a different approach, but the K3's success in CW DXing and 
contesting suggests that this approach is just as valid as any other.

I don't recommend trying to fool the firmware by adjusting the crystal 
filter offsets; I'd use the marked values. Changing them is likely to 
cause unwanted side-effects, since the filter passband is inverted for 
complimentary modes (CW/CW REV, LSB/USB, DATA/DATA-REV). The exception 
to this rule is when fine-tuning the offsets of 5-pole filters on the 
main and sub-receivers in order to provide best performance in 
diversity mode. These adjustments will rarely be larger than +/- 20 Hz 
anyway.

Bandwidth (FLx BW)
------------------

It's OK (but not necessary) to fudge the bandwidth of specific filters 
(FLx BW). For example, INRAD's 8-pole "400 Hz" filter can be declared 
as 0.4, 0.45, or 0.5 kHz, depending on where you want this filter to be 
kicked in as WIDTH is rotated. The audible effect is subtle. Ed Muns, 
W0YK, has described this technique in detail elsewhere.

Gain (FLx GN)
-------------

Regarding filter loss compensation: I recommend simply sticking with 
what's on page 46 of the owner's manual (1 to 2 dB compensation for 
400-500 Hz filters, 3 to 4 dB compensation for 200-250 Hz filters, and 
0 for all others) unless you have a very specialized application and 
lots of time on your hands.

There are three reasons for this. (1) The *perceived* loss of a crystal 
filter is a function of both actual loss and S/N ratio. If a narrower 
filter has a little more loss, you may not notice it. (2) Most signals 
are flattened slightly by AGC anyway. (3) Adding a lot of gain to 
filters can again cause side-effects, such as interaction with RX EQ 
settings and variations in gain between main and sub if the filter 
complements are different (e.g., during diversity use).

In fact we recently issued new guidelines for factory-assembled K3s: 
all crystal filters' FLx GN numbers will be to 0 except for 400-500 Hz 
(1 dB) and 200-250 Hz (2 dB). Customers are free to optimize these 
further, of course -- but most won't need to.

73,
Wayne
N6KR

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