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

[Ray Anderson]

>Note that the inductors are 0805 SMT so a 0805 resistor can be
>substituted if the inductors are a problem.  Each PLL analog supply
>looks like it pulls < 7 mA worst case...

>73 de Phil N8VB


Phil-

Do you a particular part in mind for the inductors? All the schematic
presently says is 'IND_0805'. If you can identify a candidate part I can
run a quick simulation to determine if there may be a problem lurking.
If so, the solution on the simple side may entail the addition of a
single resistor per filter and on the complex side, the addition of an
extra resistor & capacitor per filter. Replacing the ferrite bead with a
resistor can indeed provide a RC filter, but the size of the resistor
required to provide the desired rolloff may induce intolerable DC drop
in the supply. When you only have 1.2 volts to start with, the IR drop
can be significant percentagewise.

In the 12 years I was at Sun Microsystems consulting with various
hardware design teams I witnessed  numerous  simple  LC filter designs
that caused up to 20dB increase in noise in the 50kHz to 200kHz range on
the PLL power pins of various ASICS. In all cases we were able to tame
them with just a few additional parts.

-Ray   WB6TPU


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