[hpsdr] [OZY] Schematic update May 16, 2006

[Ray Anderson]

-----Original Message-----

>PDF Schematic at:

><http://www.philcovington.com/HPSDR/OZY/REVXA.3/>

>Added Panasonic RC filter networks to the local IO on OZY.

>Good news is that I will probably be able to add the 20 pin logic
>analyzer/debug port back into the design because of the space savings
>from using resistor networks and moving some of the passive components
>to the backside of the PCB.

>73 de Phil N8VB


Phil-

While reviewing your latest schematic changes I noticed the LC filters
(L2/C21 and L4/C22) feeding +1.2VA1 and +1.2VA2. Quick question for you:
What is the anticipated current draw on those outputs (and what sort of
things will they power))? The reason I ask is that I've seen a good
number of designs have issues with noise peaking when utilizing simple
LC filters like that, especially if they power sensitive circuitry like
PLLs or other noise sensitive loads. You can easily have more noise on
the output than you have on the input in the couple hundred kHz range.
This is a function of a low input impedance from the regulator and a
moderate load impedance. Large ratios of Zout/Zin can cause problem if
you don't manage the filter design correctly.

Lots of app notes from IC vendors specify these kinds of filters (and
whoever wrote the app notes usually wasn't aware of the issues
involved....)


-Ray	WB6TPU



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