Chapter 5
Signal Generation Fundamentals
NI PXIe-5450 User Manual
5-14
ni.com
Sample clock, f
s
, is twice that of the signal bandwidth, f
o
, images are
introduced in the output signal at |f
o
± nf
s
|, as shown in the following figure.
The images shown in the previous figure degrade the spectral purity of the
signal, creating the need to filter these images out of the signal.
To create quality signals, all NI signal generators can lowpass filter the
generated signal. A lowpass filter can smooth the raw DAC output. The
filter removes high-frequency aliased components that are introduced
through the digital generation of the signal. You can implement the lowpass
filter through both analog and digital filters.
Designing an analog filter that rejects the images and yet gets maximum
output bandwidth (0 to 0.43f
s
) is difficult and is represented by the curve
Analog Filter 1 in the following figure. Analog Filter 2 represents a more
practical filter. This filter is not as aggressive as Analog Filter 1. Analog
Filter 2 does not filter out the images near f
s
, but it does reject all the others.
Analog filters have trade-offs between the roll-off of the attenuation after
the 3 dB point and the flatness of the attenuation before the 3 dB point.
Another aspect of the analog filter is group delay—the amount of time
needed for a signal having finite time duration, such as a pulse, to pass
through the analog filter. Ideally, in an analog filter with linear group delay,
all frequencies present in the signal should have the same time delay so that
the signal is not distorted.
f
s
0
2f
s
3
f
s
0.5f
s
Images
f
0
Signal
Power
4f
s
