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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: Re: Damping cables... posted by RadioWonder on August 16, 2006 at 04:40:28:
The series R-C filter presents a large impedance at low frequencies (infinite at DC), which decreases linearly with frequency to the so-called 'corner' frequency. Above the corner frequency, the impedance is constant and equal to the resistance. It is this constant impedance (assuming ideal behavior of the resistor and capacitor) which loads the cable at frequencies where ringing can occur.The corner frequency-to-R-C network relationship is determined by the following calculation:
First, the frequency in cycles per second, Hertz, is converted to radians per second by multiplying by 2 time Pi (6.28...).
Second, the radian frequency is inverted to give the time constant in seconds.
Third, the resistance or capacitance is determined from the relationship that the time constant is equal to the resistance in ohms times the capacitance in farads.
Thus, if you know the resistance you want to use, you can calculate the value of the capacitor that will give the appropriate corner frequency, or vice versa.
I'd keep the corner frequency well above 40 KHz. The Walker Audio High Definition Links use 10 ohms and 0.01 microfarads for the R and C values. The product of these is 1X10^-7 (one-tenth of a microsecond). The inverse of this is 10 million radians per second, or 1.6 MHz. This is good for reasonable cable lengths: a typical one-meter cable has its fundamental ringing frequency about 50 MHz, and this is proportional to the inverse of length. A four-meter version of the same cable would ring at 12.5 MHz.
If you know or can measure the characteristic impedance of your cable, setting R to be closer to this value gives improved matching. However, the capacitor then needs to be adjusted to compensate if you want to keep the corner frequency the same.
The reality of resistors and capacitors is that they all have some parasitic inductance. Capacitors resonate at a frequency where the parasitic inductance and the capacitance have the same impedance magnitude. This results in a very low impedance through the capacitor, but the problem is that the impedance then rises with frequency above that point. The filter thus stops working for higher frequencies, which may well include significant RF noise sources or higher cable resonance modes. The Walker capacitor resonates at 21 MHz.
As long as you are using good-quality resistors (with low self-inductance), it is worthwhile to bypass the main capacitor with smaller ones. The smaller a capacitor of a given type, the less parasitic inductance it will have and the higher its resonant frequency will be. You can get the filter to work up to beyond 100 MHz by using suitably small bypass capacitors.
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Follow Ups
- Not sure which one to use... - Al Sekela 14:45:03 08/16/06 (3)
- could you clarify this interesting point ? - tonemaniac 20:27:56 08/16/06 (2)
- Some answers... - Al Sekela 14:56:32 08/17/06 (1)
- excellent... thanks - tonemaniac 17:18:46 08/17/06 (0)
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