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Re: [Discuss-gnuradio] Short pulses and ringing

From: Lamar Owen
Subject: Re: [Discuss-gnuradio] Short pulses and ringing
Date: Thu, 9 Mar 2006 11:07:37 -0500
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On Thursday 09 March 2006 08:45, Marcus Leech wrote:
> Pulsars produce narrow pulses with quite fast rise times, with pulse
> repetition rates of less than 1Hz
>   up to tens or hundreds of Hz.  The pulses tend to have fairly-fast
> rise times.

> How to FIR filters react to fast-rise-time pulses?  Do they ring like
> analog filters?

The only filter type I know off that has the step response you want is the 
Bessel, which is doable with an FIR filter with the right coefficients.  All 
the others (Butterworth, etc) ring to one extent or another.  The tradeoff is 
the Bessel's incredibly slow rolloff and absence of flatness in the passband.

That said, sound cards with inputs that go below 20Hz are rare as hen's teeth, 
and the rolloff that low is likely to be severe, with significant group delay 
and ringing of very low frequency pulses.  This is why I previously mentioned 
not trying to use the audio output; sound cards' audio output and input 
linearity in that frequency range stinks.  A USRP with an LFRX would be a 
better solution, but even then if there is a series cap anywhere the lows are 
going to be horrible.  But Matt says DC-30MHz, so maybe there are no series 
Lamar Owen
Director of Information Technology
Pisgah Astronomical Research Institute
1 PARI Drive
Rosman, NC  28772

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