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Re: [Discuss-gnuradio] Short pulses and ringing

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] Short pulses and ringing
Date: Fri, 10 Mar 2006 00:56:04 +0100
User-agent: Debian Thunderbird 1.0.2 (X11/20051002)

Lamar Owen wrote:
> 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 
> caps.
There are no series caps in the LFRX.
You do need to turn off the automatic DC removal in the usrp fpga. (can be done 
from python)

You also have to take care that DC is really DC.
(I added a big series cap (2200 uF) for my application (25 Hz - 40 MHz) because 
of the DC-offset from my signal source)


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