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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: one more questions Chris. posted by lovetube on November 7, 2007 at 03:13:35:
The basic R-C filter as used here presents a resistive load to the cable above what is called the "corner frequency" of the pair. Below the corner frequency, the impedance is high and decreasing as frequency increases. The impedance is very high at audio frequencies, and infinite at DC.
The corner frequency is given by this formula:
Fc = 1/(2*Pi*R*C)
where Pi is 3.14159..., R is the resistor value in ohms, and C is the capacitor value in farads. A 10,000 picofarad capacitor has a capacitance value of 0.00000001 farads.
For 10 ohms and 10,000 picofarads, the corner frequency is 1.6 MHz. If you use a larger resistor, say 20 ohms, the corner frequency will be reduced by the proportion of the resistor increase. 10 ohms is a good compromise if you don't know the cable impedance, or don't want to make the filter specific to a particular cable. However, for damping the cable resonances, the best resistor value to use is the cable characteristic impedance (Z0 = SQRT[L/C]).
If you do choose to use a larger resistor value, you can compensate by making the capacitor smaller.
The purpose of using multiple R-C pairs is to cover more of the radio frequency range. I found through measurements (with some expensive HP equipment) that the 10,000 pF capacitors have internal resonances in the 15 to 21 MHz range. This means they do not load cable resonance modes above their own resonances. Smaller caps have higher resonances. Adding a 10-ohm and 1,000 pF pair to the 10-ohm and 10,000 pF pair extends the total resistive loading range to the primary resonance of the smaller cap, whereas the 10,000 pF cap pair would stop working at about 15 MHz. Adding a pair with a 100 pF cap extends the range further. The cascade used by Chris should work well up to 100 MHz and beyond, and I'm not sure it adds value to go any higher.
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Follow Ups
- Your combination should work fine. - Al Sekela 11:54:36 11/07/07 (0)
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