[RTTY] 300hz or 500hz IF filter?

[Kai]

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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