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Old 21st June 2002, 10:18 AM   #1
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Question DIR1701 loop filter question

I'm a little bit puzzled regarding the values of the DIR1701's loop filter parts. Data sheet (page 16) gives 6K8, 0.022uf (22nF) and 0.0022uF (2.2nF), while the Demo board data sheet (page 3) replaces the 2.2nF with a 220pF. Which one is the right one (I think it's 2n2, but...) ?

By the way, has anybody tweaked those values, as it has been done for the CS8412/14 here ? Any equations ?

Thanks
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Old 21st June 2002, 03:51 PM   #2
tiroth is offline tiroth  United States
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Francios,

I built a 1701 reciever with 22nf and 2.2nf, worked fine. Did not try 220pF, but that sounds low. Have you looked at the 1703 datasheet? (68nF and 8.2nF I believe) Different values in the loop filter, and the transfer function corner starts at a much higher frequency. Since starting rejection at a lower frequency seems great, I didn't give much more thought to the issue, other than to wonder if the 1701 loop values could be used on a 1703.

Tweaking is definitely an interesting notion--I'd like to hear responses on that.
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Old 21st June 2002, 04:48 PM   #3
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Thanks Tiroth,

Yes, I was prepared to go with 2.2nF, so let's go. An almost recurrent problem with TI's datasheet for digital products is that they often provide only "surface" informations. Well, one can contact TI... I've done that five or six times, never receiving a reply , thus I've given up... AD and Crystal are more DIY-friendly

And yes, I've also wondered if the 1703's values were usable with the 1701, and vice-versa.

But let's hear about tweaks...
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Old 21st June 2002, 04:58 PM   #4
tiroth is offline tiroth  United States
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I actually got a response from TI re the 1703 datasheet, which I think is amazingly confusing unless you cross-reference the 1701 datasheet and make some assumptions to boot.

Luckily swapping SMD capacitors for loop tweaking is pretty painless
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