[RTTY] 300hz or 500hz IF filter?

Fri Aug 23 14:37:48 EDT 2013

Since David's original question regards choosing between two crystal IF filters in a superhet, I would be inclined to rely on anyone who has compared two filters you are interested in with your radio.   Keep in mind that a "300Hz" filter by one manufacturer is not the same as a "300Hz" filter by another.   Do what works best whith the constraints you have.  I don't really have much to add in that regard.

That said, I suspect many people would be well served to look at the dynamic range and IMD characteristics of their RX chains, as Chen has mentioned, before getting into radio surgery. You may find that improvements "for free" will eliminate the need for the filters that are theoretically "too narrow."  In fact, running with the AGC slow (or even off) works pretty well so long as you have the dynamic range and linearity through the whole receive chain.  

I kinda came to the same conclusion as Chen, although I might be a little more blunt:  the value of the narrow IF filter is to limit the damage done by nonlinearities (e.g., IMD products landing on the signal you want) in the RF and AF audio systems, as well as the limitation in dynamic range of the system as a whole (think AGC pumping messing with the demodulator).  If you can guarantee spur-free dynamic range through the whole RX chain you can run with IF filters as wide as you want.  A demodulator running directly off of a direct sampling SDR is a classic example of an "AGC-less" system that does away with all of the filters, conversion stages, audio amplifiers, etc.  It's like having the simplicity of FSK keying, only for the receiver!

Kai referenced the ITR-R SM.1138, whence the formula BW = 2M + 2DK comes.  The document is located here if anyone is interested in the source--I hadn't seen it before, but it's fairly easy to understand, even if you aren't "mathy":

http://www.itu.int/rec/R-REC-SM.1138/en

As I understand it, this document is used as a guideline for channel spacing in radio services.  My hunch is that the formula takes into account the fact that most FSK circuits assume a relatively high SNR and are really concerned with making sure that adjecent channel intereferrence isn't too likely to be the limiting factor in the channel.  They really don't care much about those FSK signals in the noise since they aren't reliable anyway, and so they really don't care about those high-order keying sidebands that are 45dB down from the adjancent channel.  It's probably a rough guide for packing toghether signals with high SNRs. As Annex (1) on the page iii page clearly states, it's just a rule of thumb...but probably a useful one.  I'm sure others on the know more about the history of this guideline than I.  Just remember that the ITU's values in spectrum management may not totally reflect ours on 20m during CQWW :-)

We know that how "wide" 45.45b RTTY signal appears depends on much more than just the shift and the keying rate: keying waveform, rise/fall shapes, possibly IMD induced by the shaping or IF filter, propagation effects.  The SNR at the receiver that determines how much of this can be heard.  We care about those -50dB keying sidebands or whatever else when we are trying to work longpath JA's on 40m near a local KW transmitter next door.  The "eye of the beholder" does matter when talking about bandwidth and interferrence.  Knowing that, be careful when arguing, "Modulation X is Y Hz wide because of formula Z."

Cheers,

Andy K0SM/2

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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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