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Message ID: 4402     Entry time: Thu Mar 10 17:03:48 2011
Author: Larisa Thorne 
Type: Configuration 
Category: Electronics 
Subject: calculations for passive low pass filter on X arm 

[Kiwamu, Larisa] 


We want to increase gain in the lower frequencies, so a circuit must be designed (a passive low pass filter). 


First, measurements were taken at the X arm for impedance and capacitance, which were 104.5kOhms and 84.7pF respectively. Kiwamu decided to make the circuit resemble a voltage divider for ease of calculation, such that Vout/Vin would be a ratio of some values of the equivalent circuit reactance values. After a few algebra mistakes, this Vout/Vin value was simplified in terms of the R, C measured and the R', C' that would be needed to complete the circuit. 

Since the measured C was very small and the measure R was fairly high, the simplified form allowed us to pick values of R' and C' that would make the critical frequency occur at 0.1Hz: set the R' resistance to 1MOhm and C' capacitance to 10uF, which would yield a gain ~1.

With these values a circuit we can start actually making the circuit.

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