[hpsdr] LTC2208 input network design

[Dave Larsen]

Jeff

You can find the schematics here:

http://openhpsdr.org/support.php

Dave, KV0S
On Sep 26, 2012 10:22 PM, "jeff millar" <wa1hco at wa1hco.net> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> Mark...
>
> I've done quite a bit of work using LTC family of ADC's for cellular
> applications and understand the issues in driving the ADC properly.  But I
> can't find schematic for Hermes or Mercury.  Can you point me to them?
>
> In general, the ADC has sensitivity to signals and noise up to 1 GHz or
> more.  The ADC buffer/driver puts out wide band noise and all that noise
> could get folded into the output digital stream. So, any LC network between
> buffer and ADC is designed to roll off most of the noise above the intended
> operating frequencies for the ADC to prevent the wide band noise from
> making ADC noise figure and dynamic range worse.
>
> It's also important to keep a low output impedance of the network because
> the ADC has sample and hold transients that could kick voltage noise back
> into the input if the impedance was too high at some frequency.
>
> An LC simply rolls off faster than an RC and so does a bit better at
> limiting buffer output noise outside the band of interest.
>
> jeff, wa1hco
>
> On 09/26/2012 01:37 PM, Mark Haun wrote:
>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> Is there a Mercury or Hermes guru who could explain the selection of the
>> ADC
>> input network (L13, L17, and C87 in the Hermes schematic, between the
>> differential amp and the ADC) and the reasoning behind it?  I'm trying to
>> understand the design trade-offs, why LC instead of RC, and so on.  I
>> tried
>> searching the list archives but didn't find anything.
>>
>> Thanks,
>>
>> Mark  KJ6PC
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