Subject: Re: [Elecraft] RTTY filter for K3

[Art]

Al
Not at all, I'm sure you've helped more than just me! Thanks for the info.

73 Art

Alan Bloom wrote:
> "Nyquist" in this context does not refer to the Nyquist criterion (i.e.
> that the sample rate must be more than twice the highest signal
> frequency).  It just means a filter that has no inter-symbol
> interference at a certain symbol rate (baud rate).
> 
> It turns out that if you start with completely unfiltered symbols (a
> series of infinitely-narrow impulses, one per symbol) a Nyquist filter
> has the property that the frequency response is anti-symmetrical around
> 1/2 the symbol rate.  One example of a Nyquist filter is a raised-cosine
> filter.  The frequency response (plotted on a linear, not dB scale) is
> 1.0 from zero Hz up to (1-alpha) * symbol_rate/2 and is zero for all
> frequencies above (1+alpha) * symbol_rate/2.  Between 1-alpha and
> 1+alpha, the frequency response has the shape of a raised cosine,
> smoothly going from 1.0 to 0.0.  If you plot that on graph paper, you
> can see that the frequency response is anti-symmetrical about
> symbol_rate/2.  (Alpha is a design parameter - the smaller the alpha the
> sharper the cutoff.)
> 
> It is much easier to make a Nyquist filter digitally than with analog
> components, but you could approximate one with coils and capacitors. 
> Actually, even the digital filter is an approximation since it
> theoretically has to have an impulse response that extends from
> -infinity to +infinity.
> 
> That's probably more than you wanted to know.  :=)
> 
> Al N1AL
> 
> 
> On Mon, 2008-05-19 at 18:35, Art wrote:
>> I must be confused by this filter discussion. Is not a Nyquist filter by 
>> its very definition a digital filter? Not directly comparable to the 
>> crystal filters in the K3 or any other rig.
>>
>> I would have supposed a Nyquist filter is one that samples at least 
>> twice per cycle at the highest observed frequency.
>>
>> As to what comes out of crystal filters, i.e. rtty tones, different 
>> discussion. If I have the processing chain figured correctly crystal 
>> filter output is then sampled by the audio card.
>>
>> I'm not a filter designer. Just asking based on previous experiences.
>>
>> 73 Art
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> 
>