K3 - Diversity Reception, Matched Roofing Filters

[Walter V. Gilles]

I am finishing up my K3/100 kit (all 8 pole roofing filters, no sub rcvr), and already looking over the fence.

 

Am I correct in my understanding that for diversity reception with the KRX3, and using all 8 pole roofing filters (same 8 pole BW in same slots for main and sub boards), that matching of filters is unnecessary due to the inherent 8 pole filter zero offset?  Conversely, matching of filters for diversity only applies if one uses offset 5 pole filters?  Thanks.

 

Walter, N0DZJ

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[Bill W4ZV]

Walter V. Gilles wrote

Am I correct in my understanding that for diversity reception with the KRX3, and using all 8 pole roofing filters (same 8 pole BW in same slots for main and sub boards), that matching of filters is unnecessary due to the inherent 8 pole filter zero offset?  Conversely, matching of filters for diversity only applies if one uses offset 5 pole filters?  Thanks.
 
Walter, N0DZJ

Theoretically true, but 8-pole filters do not necessarily have zero offset.  Several folks have measured actual offsets of up to 80 Hz, which could be significant for a narrow CW filter (e.g. 80 out of the "250" 8-pole's actual BW of 370 is >20%).

73, Bill

[w0mu]

>From the Elecraft ordering page:

For same operating frequency diversity receive use with K3 and KRX3. Set the
K3 offset menu for both filters equal and to the nearest 10Hz offset
approximately half way between their marked offset frequencies. Only applies
to our 5 pole filters, 200 Hz, 500 Hz and 2.7 kHz, which have non-zero
offsets.

Note: Best if both filters to be matched are ordered at the same time. We
will try to accommodate matching to 5 pole filters already shipped (8 pole
filters all have a nominal 0 offset) but we can only guarantee an offset
match within 40 Hz if both filters are ordered at the same time, due to
batch to batch manufacturing frequency changes. The menu offset for both
filters that are within 40 Hz of each other can be set equally to the
midpoint between their offsets.  

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Walter V. Gilles
Sent: Saturday, January 31, 2009 12:51 PM
To: [hidden email]
Subject: [Elecraft] K3 - Diversity Reception, Matched Roofing Filters

I am finishing up my K3/100 kit (all 8 pole roofing filters, no sub rcvr),
and already looking over the fence.
 
Am I correct in my understanding that for diversity reception with the KRX3,
and using all 8 pole roofing filters (same 8 pole BW in same slots for main
and sub boards), that matching of filters is unnecessary due to the inherent
8 pole filter zero offset?  Conversely, matching of filters for diversity
only applies if one uses offset 5 pole filters?  Thanks.
 
Walter, N0DZJ


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[Don Wilhelm-4]

Walter,

Whatever the absolute offset required for the 8 pole filters, it should
not deter your building.  The offsets are entered into the menu after
the build is complete.

While the 8 pole filters have a 'nominal offset' of zero, you may find
that to be perfect for diversity reception that you should find the
absolute offset difference between the 2 filters of the same width and
use half the difference as the offset.  With the 8 pole filters, any
offset will be small - but remember that 'nominal' does not equal
'exact', and as far as diversity reception is concerned, you are dealing
with exact if there is to be no shift between the main and sub receivers.

I am not going to go into saying how to determine the exact offset
required, I have not done that because I don't have the sub RX and have
only the 5 pole filters - that meets my needs.  I leave that exercise to
others.

73,
Don W3FPR

Walter V. Gilles wrote:

> I am finishing up my K3/100 kit (all 8 pole roofing filters, no sub
> rcvr), and already looking over the fence.
>  
> Am I correct in my understanding that for diversity reception with the
> KRX3, and using all 8 pole roofing filters (same 8 pole BW in same
> slots for main and sub boards), that matching of filters is
> unnecessary due to the inherent 8 pole filter zero offset?  
> Conversely, matching of filters for diversity only applies if one uses
> offset 5 pole filters?  Thanks.
>  
> Walter, N0DZJ
>
>
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[Gary Smith]

> While the 8 pole filters have a 'nominal offset' of zero, you may find
> that to be perfect for diversity reception that you should find the
> absolute offset difference between the 2 filters of the same width and
> use half the difference as the offset.  With the 8 pole filters, any
> offset will be small - but remember that 'nominal' does not equal
> 'exact', and as far as diversity reception is concerned, you are dealing
> with exact if there is to be no shift between the main and sub receivers.

I also have the sub and a full compliment of 8 pole filters. The
narrowest is the 250 and the next up is the 1K.

I am wondering how I, without all those varieties of test gear can
determine if indeed there is a significant offset that exists with
the 8 pole filters in my K3 and if it exists to know how much it + or
- it is. Of course I would need to be able to test both main & sub to
know what the particulars are so I could properly adjust the ofset
settings.

I do find using diversity is much better for CW copy than without.

Gary
KA1J
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[Barry N1EU]

Gary Smith wrote

I am wondering how I, without all those varieties of test gear can
determine if indeed there is a significant offset that exists with
the 8 pole filters in my K3 and if it exists to know how much it + or
- it is.

Feed the K3's audio into your pc's sound card and run a spectrum analysis program.  Check for symmetry in the passband response around your sidetone frequency (cw filters).  Observe while varying the offset.  A small amount of asymmetry is no problem but it shouldn't be noticeably skewed to one side. (set K3 dsp width=filter width)

73,
Barry N1EU