[RTTY] 300hz or 500hz IF filter?

Fri Aug 23 18:48:07 EDT 2013

> In the RTTY case, it also depends the group delay characteristics -
> which is a metric never published by Inrad (that I've seen). On the
> Inrad filters I have measured, the group delay humps near the
> transition region are significant; which is the expected trade-off
> for the steep skirts - and that adds to the ISI problems as mentioned
> before.

Group delay information for the Collins 300 Hz and 500 Hz filters
is available here: 
<http://www.rockwellcollins.com/Capabilities_and_Markets/More/Precision_Manufacturing/Rockwell_Collins_Filters.aspx> 
  Follow the links for the "Low Cost
Series" filters.

Note that the 300 Hz filter group delay peaks at +/- 160Hz from center 
reaching more than 4500 usec where group delay in the 500 Hz filter
peaks at less than 2500 usec +/- 250 Hz from center.  The 300 Hz filter
shows delay of more than 1000 usec at just +80Hz while the 500 Hz filter
remains below 300 usec over nearly 300 Hz.

73,

    ... Joe, W4TV

On 8/23/2013 6:25 PM, Jeff Blaine wrote:
> Getting back to the original query - I think the answer to this question
> depends on the primary use.
>
> The guy says it's a portable rig.  But he does not go beyond that.  I
> don't know - but my guess with the data on hand is most portable rig
> applications don't suggest heavy duty contest activity or pile-up work.
> And if that's the case, then the 500 hz filter is the better choice as
> it's far easier to use in general RTTY and CW work for casual band
> operation.
>
> Now if the guy is setting up camp on some rare IOTA or likes to camp out
> on rare mountain peaks, then the narrower filter is probably the better
> choice.
>
> The choice depends on the application.  And to an extent, how hard it is
> to jump in and out of this filter setting on that rig (which I don't
> have experience with).  However, I would not call the 300 hz filter
> "generally better" because there is no "general" answer.
>
> In the RTTY case, it also depends the group delay characteristics -
> which is a metric never published by Inrad (that I've seen).  On the
> Inrad filters I have measured, the group delay humps near the transition
> region are significant; which is the expected trade-off for the steep
> skirts - and that adds to the ISI problems as mentioned before.
>
> 73/jeff/ac0c
> www.ac0c.com
> alpha-charlie-zero-charlie
>
> -----Original Message----- From: Ron Stailey
> Sent: Friday, August 23, 2013 4:32 PM
> To: rtty at contesting.com
> Subject: Re: [RTTY] 300hz or 500hz IF filter?
>
> I agree with Jim, 250Hz filters are great in a pile-up during a test.
> Several years ago someone told me I could order a 270Hz filter from
> Kenwood I was running two TS-850s at the time. I order two 270Hz
> filters and liked them better than the 250's. I still have them some
> place either here or in Texas not sure which but I do have them. Dunno
> if you can still get them from Kenwood or not. I think you will like
> it if you get or can find one..
>
> Also as Don said I would rather have a 300Hz than a 400 or 500Hz
> Filter
>
> 73, de Ron K5DJ
> ==========================
> Joe...
> I have been RTTY contesting for many (20+) years with some of the best in
> the business. We ALWAYS used 250 Hz filters during crowed band conditions.
>
> And we have the wallpaper to prove it.
>
> 400 and 500 Hz crystal filters (and DSP filtering) is just not tight
> enough.
>
> Period.
>
> 73
> Jim W7RY
>
>
> -----Original Message----- From: Joe Subich, W4TV
> Sent: Friday, August 23, 2013 8:05 AM
> To: rtty at contesting.com
> Subject: Re: [RTTY] 300hz or 500hz IF filter?
>
>
> No, the half bit makes the baud rate effectively 90.9 (the shortest
> element is now 11 ms) thus the calculation is:
>      (2 * 90.9) + (1.2 * 170) = 385.5 Hz.
> although the actual occupied bandwidth will be dependent on the
> information content (how often/how regularly transitions occur will
> effect the value of "K" in the previous formula).
>
>> Alternatively, observe RTTY signals on-air.
>
> And most FSK signals are 370 Hz wide or more depending on the care
> with which the manufacturer has designed the FSK circuits.  The only
> exception are later versions of the K3 firmware which generate very
> clean FSK using DSP.
>
> 73,
>
>     ... Joe, W4TV
>
>
> On 8/23/2013 8:15 AM, Kai wrote:
>> Absolutely incorrect. Consult ITU-R SM.1138:  BW = 2M + 2DK; D=shift/2;
>> M = Baud/2   K = 1.2 (typically)
>> BWrtty=2M+2DK = Baud + shift*1.2 =249.5 Hz
>>
>> If you consider the effect of the 33 ms (1.5 bit) stop bit, that effect
>> has a narrower spectrum which is contained entirely within the 249.5 Hz
>> BW of the 22 ms start and Baudot bits. The shortest element is still 22
>> ms.
>>
>> Alternatively, observe RTTY signals on-air.
>>
>> Kai, KE4PT
>>
>>
>>
>>
>> On 8/22/2013 10:34 PM, Joe Subich, W4TV wrote:
>>>
>>> On 8/22/2013 9:42 PM, Kai wrote:
>>>> The theoretical bandwidth of 170 Hz shift 45.45 baud RTTY is just
>>>> under 250 Hz.
>>>
>>> Absolutely incorrect as 250 Hz does not account for the necessary
>>> modulation sidebands or for the discontinuity (additional bandwidth)
>>> generated by the 1.5 bit stop.  Due of the half bit, the necessary
>>> bandwidth for 170 Hz shift RTTY approaches 170 + (2 * 90.9 * 1.2) or
>>> slightly over 370 Hz as the shortest element is now 11 ms.
>>>
>>> 73,
>>>
>>>    ... Joe, W4TV
>>>
>>>
>>> On 8/22/2013 9:42 PM, Kai wrote:
>>>> The theoretical bandwidth of 170 Hz shift 45.45 baud RTTY is just under
>>>> 250 Hz.
>>>> 73
>>>> Kai, KE4PT
>>>>
>>>> On 8/22/2013 6:54 PM, Joe Subich, W4TV wrote:
>>>>>
>>>>> The -6 dB bandwidth of the INRAD "300 Hz" filter is shown as 340 Hz
>>>>> which is slightly less than the theoretical 370 Hz required for 170 Hz
>>>>> shift 45.45 baud RTTY.
>>>>>
>>>>> That said, performance will be a trade off between improved
>>>>> selectivity and interference rejection - up to a point.  If the
>>>>> receiver can withstand AGC effects of close in interference, a 400
>>>>> to 500 Hz filter will generally provide better copy than a 300 Hz
>>>>> filter.  Note: no amount of selectivity is useful when signals
>>>>> overlap or the interfering signal includes distortion (spurious)
>>>>> products that overlap the desired signal.
>>>>>
>>>>> 73,
>>>>>
>>>>>    ... Joe, W4TV
>>>>>
>>>>>
>>>>> On 8/22/2013 5:38 PM, David VE3VID wrote:
>>>>>> Hello everyoneI would like to outfit my FT-857D portable rig with an
>>>>>> IF filter on its 455khz stage.  INRAD sells a suitable 500hz unit.
>>>>>> They also have a 300hz unit.   I am leery about the 300hz filter
>>>>>> being too narrow.
>>>>>> Any opinions?
>>>>>> 73Davidhttp://www.ve3vid.webs.com/
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