Monday, April 23, 2012

More 30m Low Pass Filters

Today was again, Low Pass Filter Day.

I built three 30m Low Pass Filters to use with the Propeller as the Keyer and RF exciter. Previous filter builds included a DC blocking Cap to avoid DC loading the Prop, but a Low Pass Filter should not include elements that prevent Low Frequency signals to pass even if they are not wanted. DC blocking should come before the LP Filter on the Prop.

My previous built filter were a little flimsy, they were built on .031mil PCB material, components in line with the axes tended to crack and stop working. For these board, I used .064mil PCB material.

The circuit I am using was calculated via the CalculatorEdge online service (there may be other online calculators that would work as well, but this is the one that came up first in Google search).

The Calculator suggested the following, which became (=>) the available parts used.
  • Two Inductors 594.4nH => 680nH as hand wound (I could have been more precise here, but, . . .  :-)
  • Two End Caps  1086.5pF => 990pF (3 - 330pF)
  • Middle Cap 1416.1pF => 1320pF (4 - 330pF)
30m Low Pass Filter
I updated the circuit on my CircuitLabs Workspace, and ran some plots with the slightly different component values that I have available. The second harmonic rejection is reported as down -45db.

Down -45db at 20MHz
The results:
30m Low Pass Filters
I am going to use SMA connectors for all of my projects now, thanks to Jeff - KO7M I had enough connectors to complete two filers. More SMA Connectors and Jumpers Cables are on order.

One of the above Filters will be sent to a friend.


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