TI-H: TI-86 Slowdown?


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TI-H: TI-86 Slowdown?



Ok, apparently no one on the list is both proficent in analog electronics
and willing to help out with this problem, so here is what my next attempt
will be.
 
What I'm guessing I need is a low-pass filter that will cut the noise I
am detecting (50mV total amplitude, 10kHz).  Since I haven't found
anyone who could tell me how to design a decent filter, I am drawing
on another hobby, audio systems.
 
Since the noise is in the audio frequency range, I'll attempt to use
the low pass filter design I've used for woofer crossovers.  This is
a third order Butterworth crossover, -3dB at the crossover frequency,
-18dB/octave.
 
       L1     L2
Vcc---oooo-+-oooo--+
           |       |
        C1 =      Load
           |       |
Gnd--------+-------+
 
In order to attenuate the noise as much as possable without having
to use excessivly large value components, I intend to set the filter
at 2500Hz, 2 octaves below the target frequency, which should give
about -39dB of attenuation ((2 octaves*18db/octave)+3db at crossover
frequency), dropping the noise to below 0.01mV (each -3db represents
a halving of the power of the signal).
 
The equations I have for this filter are as follows:
 
             Rw
L1 = ------------------- * 10e3
       1 1/3 * Pi * Fc
 
             Rw
L2 = ------------------- * 10e3
           4 * Pi * Fc
 
                 1
C1 = ------------------------ * 10e6
       1 1/2 * Pi * Fc * Rw
 
Where Rw = impedance of the load, Fc = Crossover frequency.
(The x 10e? parts simply change the units to mH and uF)
 
I have no way to test the impedance of the inverters inputs,
but the DC resistance measures as 5000 ohms.  If this value
is off, I hope it is low, otherwise the crossover frequency
will be shifted up.
 
With Fc = 2500 Hz and Rw = 5000 ohms, I get:
L1 = 477mH
L2 = 160mH
C1 = 0.017uF
 
I don't know yet if I can find inductors of that value small
enough to fit into the calculator, or if the filter will even
work, given that the input impedance of the inverter could be
much higher than 5000 ohms.  I'll find out answers to both of
those questions tomorrow.
 
Now, does anyone have any comments or suggestions on how
I might be able to acomplish this better, or do none of
you have any experiance with analog electronics?
 
Thanks,
DK

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