[RTTY] Crystal filter width preferences for RTTY contesting

[Jeff Blaine AC0C]

Chen,

Thanks for pointing me to Jim's site on this filter comparison.  I have used 
his RFI white paper and RF choke design recommendations before.  He really 
is one of the masters.

>From a -6db standpoint, the advertised specs of the K3 250 is 370 hz and the 
K3 400 is 425.  I think Jim is seeing a bit more narrow of a spread.  But 
the comments on the combination effects are interesting.  The cascade effect 
of similar width filters essentially provides an improvement in the net 
shape factor.  It's a confirmation of the modern rig that some (like the 
Icom, for example) do not have exceptionally narrow roofing filters yet the 
DSP is able to provide quite high close-spaced DR3 performance.  Amazing 
really.

Let me summarize what I think your summary point are:

1. There is a technical argument for a roughly 320 hz bandwidth - to 
accommodate adequate sideband capture of a perfectly centered signal.
2. And then we need to consider some mis-tuning - and add a bit of width to 
accommodate that.

If we assume a +/- 25 hz mistuning range (another comment was the typical 
tuning error was perhaps 20 hz as a guess), then we come up with a 370 hz 
number.  Exactly the same as the 250 hz K3 performance.  I think this helps 
to explain why there is quite a bit of satisfaction with the K3 using this 
filter on RTTY.

Very interesting subject...

73/jeff/ac0c


--------------------------------------------------
From: "Kok Chen" <chen at mac.com>
Sent: Wednesday, August 26, 2009 1:31 PM
To: "RTTY Reflector" <rtty at contesting.com>
Cc: "Jeff Blaine AC0C" <keepwalking188 at yahoo.com>
Subject: Re: [RTTY] Crystal filter width preferences for RTTY contesting

>
> On Aug 25, 2009, at 10:59 PM, Jeff Blaine AC0C wrote:
>
>> A common data point  seems to be the Inrad/Elecraft K3 filter @ 250
>> hz. Looking at the top of that filter, it is about -3db down at 240
>> hz and -6db at about 370.  Guys seem to have no negative comments
>> about that filter.
>
> The skirts of the stock K3 filters are not especially tight.  But they
> don't need to be.  They are basically roofing filters to keep the A/D
> converter of the "real" software (DSP) filters from clipping.  Steep
> skirts are much easier, and more accurately, obtained by using DSP
> math than by using a limited number of poles from a crystal filter.
>
> Jim K9YC has a nice write-up on filters, with curves for the FT-1000MP
> (stock and Inrad, and how filters can be cascaded in the MP) and K3.
> See here
>
> http://audiosystemsgroup.com/FilterTestNotes.pdf
>
> I'd stocked my FT-1000MP with some Inrad filters from the days when I
> was still depending on narrow I.F. filters.  (The Inrads are "tighter"
> than the equivalently spec'ed Yaesu filters.)  As a consequence, I
> still have the following compliment for my MP, though I seldom need
> the narrow filters anymore.   With RTTY, I'd used the 500 Hz (Yaesu)
> at the 8215 kHz I.F., cascaded with the 400 Hz (Inrad) at the 455 kHz
> I.F..
>
>>          455 kHz IF
>> YF110CN          250 Hz CW 250
>> IRC 703C          400 Hz CW 500
>> Empty - 2.0
>> XF115S Stock         2.4
>>
>> 8215 kHz IF
>> IRC 708B        250 Hz CW       250
>> XF501 Stock         500 (soldered)
>> IRC 709B          2100 Hz SSB    2.0
>> XF262 Stock         2.4 (soldered)
>>
>> 2nd Rx
>> YF115C 500 Hz CW
>
> For my K3, I have the 400 Hz, 2.8 kHz and 6 kHz roofing filters.
>
> 73
> Chen, W7AY
>
>
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