Attenuation at narrow bandwidths NOT!

[Dan Barker]

> -----Original Message-----
> From: W3FPR - Don Wilhelm [mailto:[hidden email]]
> Sent: Tuesday, December 07, 2004 3:50 PM
> To: Dan Barker
> Subject: Re: Discrete Passband Tuning Revisited
>
>
> Dan,
>
> I must disagree - you must have a very good K2 CW filter - don't change

it.

I won't.

>
> There ARE K2s that have signifigant attentuation at bandwidths more narrow
> than 200 Hz.  One of mine is good to 150 Hz and the other is good only to
> 180 Hz.  I have built and worked on many K2s, and they are all a bit
> different in the filter attenuation aspect - depends on the precise
crystal
> mix, component tolerances, construction techniques, etc.
>

Interesting, let's see. See below.

> 73,
> Don W3FPR

Don, you got me thinking. Why would you say something like that? My K2
filters are EXCELLENT! But, I decided to test it out to prove my belief.

Test Setup:

Freq 7.040.

Mode CW Normal.

Set FL4 width to 0.00 (FL4 has the BFO for a "reasonable" left skirt. I
didn't want to change frequency, just bandwidth.

Broadband noise source on ANT2, Antenna selector to ANT2.

Spectrogram on Fixed Audio Out (top of AF pot).

Peak Hold on, wait for the graph to "fill in".

Measure the peak, and the amplitude of the -6db points for bandwidth
calculation.

Note: BWs above 1,200 hz have significant ripple (not unexpected). However
these bandwidths are not particularly useful to me. I measured the peak as
above, and measured the -6db points on the far right hump. The ripple is at
most 10db, but I'd measure the -6db point of the next hump. Not that it
matters anyhow at 2.49! I imagine anyone wanting really wide CW reception is
going to use OP1 anyway.

Repeat measurements with FL4 bandwidth 0.10 (or more) higher. After 2.49,
use OP1. Finally, return FL4 bw to operating BW, in my case, 0.20. Verify
trace is still centered on my sidetone (it was).

Raw data available at ftp://ftp.visioncomm.com/k2filter.xls, anonymous, your
email address, bin, get.

As these numbers appear to powerfully demonstrate the extremely excellent
job done by the designers, I'll be happy to compare notes with any K2 owner
with less stellar performance. EVERY K2s filter plots should look this good.
I'm not sure what to compare tho, to find a discrepancy<g>. Wayne? Eric?
Ideas?.

As I normally S&P with the 0.90, and work heavy QRM at 0.20, the ZERO db
attenuation is what I notice. If I lose AF volume with FL4, then I AM OFF
FREQ. It's not my filter<g>.

Dan Barker / WG4S / K2 #2456

Summary:

Bandwidth   -6db Bandwidth
Setting  Peak  Measured
0.00      -43    109
0.10      -43    133
0.20      -43    158
0.30      -42    178
0.40      -42    200
0.50      -42    231
0.60      -42    263
0.70      -42    292
0.80      -42    352
0.90      -43    421
1.00      -42    450
1.25      -43    663
1.50      -43    897
1.75      -42   1231
2.00      -43   1574
2.25      -44   1894
2.49      -45   2103
OP1       -47   1784

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