[RTTY] 300hz or 500hz IF filter?

[Jim W7RY]

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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