Sunday, December 10, 2017

commercial filter question?

I was asked a couple of questions that I hope this will answer.

This is the filter in a kit sold by BG Micro. Dave White's "beginner's and Experimenters" it uses 10 MHz
This is a partial schematic for a now obsolete commercial radio. It used the same basic circuit for the different bands and The,8, 11, and 14.xxxxx filters. The chart is for band selection.
I found this one on E-bay for $30 US. I think we can make our own for less. If the junk box has no crystals in it you can buy some for less than a dollar each.
When you get your feet under the bench it would be worth a little time to lash one up. A simple circuit with the generator feeding the crystal and a 50 Ohm resistor in series and measuring across the resistor. If you don't have a meter to read the signal with you can do as I do and put a diode feeding an AF amp and use a modulated signal to drive the loop. It is quite touchy as the crystal has a sharp resonant response. When you try 1 Xtal add another and see how it responds. If your generator has any back lash in the adjustment you may find the adjustment is very touchy by the time you have 4 Xtals in the circuit.

Edit: I saw some 16 Mhz Xtals on E-bay 5 cents each. (20 for 99 cents)

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