Non Standard Low Pass Filter Options; Setting The Anti-Aliasing Filter - GE Bently Nevada 3300/25 Operation Manual

3300/25 Dual Accelerometer Input Monitor Operation Manual
19.11

Non Standard Low Pass Filter Options

Use this procedure only if the desired low pass corner frequency is not listed in the table
in the previous section. The procedure consists of these two parts:
• Set the anti-aliasing filter (
• Set the low-pass corner frequency (
Since the low-pass filter used on the filter board is a switched capacitor type, an anti-
aliasing filter is required to band-limit the input signal. Configure the anti-aliasing filter to
have a corner frequency,
f
frequency, .
LP
19.11.1

Setting the Anti-Aliasing Filter

1. Select the low-pass corner frequency you require,
kHz.
NOTE: the -3dB frequency is 1.06 x
2. Calculate the necessary anti-aliasing corner frequency,
required low-pass corner frequency,
f
≤ 68.57 kHZ, go to step 3.
If
AA
f
If > 68.57 kHZ, go to step 5 and use a value of M=255 and K=268.9. This will set the
AA
anti-alias corner frequency to its highest value.
3. Select K according to this table:
Anti-Aliasing Corner Frequency, f
Range
120 Hz to 4.81 kHz
>4.81 kHz to 68.57 kHz
4. Calculate the multiplier value, M, by using this formula:
f
where is the anti-aliasing corner frequency (in Hz) from step 2
AA
K is the constant selected in step 3
M is the multiplier value. (M should be in the range 6 to 255)
70
f
).
AA
f
).
LP
f
, five times that of the selected low-pass filter corner
AA
f
.
LP
f
LP
f = 5
AA
,
AA
f
M =
f
in the range 24 Hz to 22.372
LP,
f
AA
.
f ×
LP
Use this value for K
19.188
268.9
AA
K
, based on your