Digital Signal Processing Reference
In-Depth Information
spurious signals
outside IF-band
non-linearities
self-mixing
components
LO-based downconversion
+ feedthrough
Mixer switch
non-linearities
self-mixing
feedthrough
DC
f
LO
2f
LO
3f
LO
4f
LO
5f
LO
frequency
weak baseband
spurious signals
near baseband
causes clipping
further in chain
Matched-filter receiver
signal
T
sym
b
1/ T
sy
m
b
self-mixing
frequency
Figure 5.6.
The template signal causes spurious components near and inside the
signal band in a matched filter based pulse detection system. This effect
desensitizes the receiver.
uniformly in time,
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removing the unwanted spurs from the signal spectrum
requires an extremely sharp filter since the nearest component is located at the
Nyquist frequency of the symbol sampling frequency. As a consequence, the
if-amplifier of a matched filter receiver must be operated at some back-off
point from the optimum gain value so that the receiver is desensitized. In fact,
the mechanism is the same as for an externally injected narrowband interferer.
Replacing the template signal by a single
lo-frequency
A general rule-of-thumb is that switching or repetitive signal compo-
nents should be avoided inside the analog signal path of a receiver, or
at least for frequencies that could corrupt the spectrum of the signal-
of-interest. Although a matched filter may be theoretically the most
optimal solution, the adverse consequences at transistor-level make it
perform much worse than would be expected at system-level. The most
convenient solution to the problem of self-mixing is to replace the tem-
plate waveform by a continuous running single-frequency lo-signal.
Doing so also avoids that the template signal must be aligned to the
unknown and varying arrival time of the pulses.
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This results in spurious components that are located at discrete multiples of the symbol repetition
frequency.
