FIR (Fc @
-
3 dB) Filter (Digital Anti
-
Alias)
Passband
Stopband
δp : Passband ripple
δs : Stopband attenuation
ωp
ωc
ωs
: Passband frequency
: Corner frequency
: Stopband frequency
-3 dB
Frequency [kHz]
Magnitude [dB]
+ δp
- δp
δs
ωp
ωs
ωc
Figure L.34:
Digital FIR (Fc @
-
3 dB) filter
When FIR (Fc @
-
3 dB) filter is selected, this is always a combination of an analog Butterworth anti
-
alias filter and a digital FIR (Fc @
-
3 dB)
filter. The FIR (Fc @
-
3 dB) is an adapted FIR filter. Its ωp is reduced by a factor of ≈1.4 compared to the FIR (Fc @
-
0.1 dB) filter. It is supported
by Perception V6.40 and higher.
Analog anti
-
aliasing filter bandwidth
370 kHz
±
20 kHz (
-
3 dB)
Analog anti
-
aliasing filter characteristic
7
-
pole Butterworth, extended passband response
FIR (Fc @
-
3 dB) filter characteristic
12
-
pole FIR; FIR is a purely digital characteristic. Its closest analog resemblance is to an
Elliptic filter. However, FIR has both overshoot and pre
-
shoot on the step response. This
means that the ringing on the signal starts before the step input is started and that the ringing
continues after the step input is complete.
FIR (Fc @
-
3 dB) filter user selection
Auto tracking for sample rate divided by: 4, 10, 20, 40
Divided by 40 not available for 1 MS/s & 500 kS/s sample rate
The user selects a division factor from the current sample rate; software then adjusts the
filter when the sample rate is changed.
FIR (Fc @
-
3 dB) filter bandwidth (ωc)
Auto tracking the sample rate with the selected FIR (Fc @
-
3 dB) filter, user selection from
0.031 Hz to 250 kHz
FIR (Fc @
-
3 dB) filter passband flatness (ωp)
(1)
0.1 dB; DC to ≈ωc/1.4 (Adapted FIR filter behavior)
0.1 dB; DC to 125 kHz; FIR (Fc @
-
3 dB) filter bandwidth selection of ωc = 250 kHz, limited
by the 370 kHz analog anti
-
alias filter amplitude response. All other bandwidth selections
are not affected.
FIR (Fc @
-
3 dB) filter stopband attenuation (δs)
60 dB
With the FIR (Fc @
-
3 dB) filter bandwidth selection of ωc = 250 kHz, a peak of
-
35 dB
occurs between 500 kHz and 1 MHz due to limited analog anti
-
alias filter amplitude
reduction. At lower bandwidth selections, the digital filter reduces this peak to
-
60 dB.
FIR (Fc @
-
3 dB) filter roll
-
off
72 dB/Octave
FIR (Fc @ -3 dB) 250 kHz Overview
10
0
-10
-20
-30
-40
-50
-60
-70
-80
Frequency [kHz]
Ma
gni
tu
de
[dB
]
Passband flatness
Frequency [kHz]
Ma
gni
tu
de
[dB
]
0.2
0.1
0.05
0.15
-0.1
-0.15
-0.05
-0.2
0
10000
1000
100
10
1
0.1
1000
100
10
1
0.1
Figure L.35:
Representative FIR examples
(1)
Measured using a Fluke 5700 calibrator, DC normalized
GEN3i
1074
I3763-3.1 en HBM: public
