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Subject: Re: Building a simple video low pass filter
Date: Sun, 12 Jan 2003 16:36:18 +0100
Organization: Wanadoo, l'internet avec France Telecom
NNTP-Posting-Date: 12 Jan 2003 15:39:32 GMT
On 1/11/03 7:03 AM, in article NsOT9.firstname.lastname@example.org, "Ban"
> email@example.com wrote:
>> nonophil wrote:
>>> I am trying to build a simple video low pass filter. My goal is to
>>> reduce the output bandwidth of a digibox because my TV can't handle
>>> sharp signal transitions without ringing artefacts.
>>> The filter should cut frequencies > 5Mhz without creating an
>>> impedance mismatch (digibox, cable and TV are 75 Ohms).
>>> I used an interactive web-based L-C filter designer to mock-up a 3
>>> poles 75 Ohms low pass filter
>>> Would that work or am I missing something ? Any other ideas ?
>> Unless youve got miles of cable between the digibox and the
>> TV, I'd try something simpler.
>> A capacitor on the end of the cable will act as a lowpass filter.
>> Yes, it will screw up the impedance, but for a couple of meters of
>> cable it's not an issue.
> Actually the proposed filter will work well and will also "screw up" the
> impedance (200R at 2.7MHz,0R at 6+MHz), but since both sides are terminated
> (hopefully) the reflections will not have too much of an effect.
> I found 0.82uH 470pF and 5.6uH to give a very good pulse response. But try
> the 1capacitor solution first.
> ciao Ban
> electronic hardware designer
The 1 cap solution works with a slight ghosting. I don't know if it is a
reflection. My cable is quite short (1m). BTW putting a 1 series inductance
gives almost the same result as 1 parallel cap (including the ghosting).
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