[RTTY] 300hz or 500hz IF filter?

Fri Aug 23 18:25:25 EDT 2013

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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