[RTTY] 300hz or 500hz IF filter?

[Kai]

The RTTY elements are either 22 ms (bits and start bit) or between 22 and 44 ms 
(1 to 2 stop bit lengths), usually 33 ms (1.5 bit lengths)..
I've never heard of a half bit length tone (11 ms) sent in isolation.

That means the spectrum will be dominated by the 1000/22 = 45.45 Hz component, 
and will have a fine underlying structure of 1000/33= 30.3 Hz component. 99% of 
the energy is contained withing 250 Hz.

I agree with the K3 comment - that is one cool radio.

-Kai KE4PT

On 8/23/2013 11:05 AM, Joe Subich, W4TV wrote:
>
> 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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