Digital Signal Processing Reference
In-Depth Information
Summarizing, the first countermeasure against clock leakage into the signal
path is to use frequency planning and make the clock spurs an out-of-band is-
sue. The second precaution to protect the weak rf input signal is to use a fully
differential signal path. Using a well-considered transistor-level design of the
window block, one can make sure that the injected noise becomes a common-
mode component, orthogonal to the signal dimension of the rf pulses. The in-
jected signal is then suppressed by the common-mode rejection ratio (cmrr)
of the subsequent building block, in this particular case the downconversion
mixer. On the other hand, it is not worth the trouble to put too much effort in
the suppression of clock feedthrough. From the moment the overhead of the
injected spurs has been brought down to approximately the same power level
as this of the signal-of-interest, the remaining portion of switching noise can
be removed by the signal processor in the back-end of the receiver. The repet-
itive nature of the receive slot causes that interferer energy stays concentrated
around some discrete locations in the baseband spectrum. Mixing products be-
tween the clock signal and the local oscillator can be easily predicted while
and affected frequency bands are subsequently repaired by the issr routine.
Remark that the problem of clock injection also works the other way around.
In order to prevent that switching noise leaks back into the receive antenna, a
direct path between the windowing circuit and the antenna terminals should be
avoided. The same argument applies even more strongly to the lo-signal of the
mixer, as this signal is within the frequency band of the antenna. Usually, in
a narrowband receiver, reverse isolation is the responsibility of the low-noise
amplifier (lna). However, it may be more appropriate to use the word
antenna
buffer
: for a wideband receiver with a considerable risk of high power in-band
blockers, the linearity of the input buffer is at least as important as the noise
figure of the input stage. Also, the noise figure requirements of a wideband
system are more relaxed than this of a narrowband receiver, due to the larger
share of thermal channel noise in the link budget.
6.2
Multiphase clock generator
The heart of the pulse-based receive unit is formed by a multiphase clock gen-
erator, used to orchestrate the interactions between the different subcircuits.
All internal clocking is derived from an on-chip prescaler running at twice the
lo frequency of the downconversion mixer. For practical reasons, the high-
speed prescaler is injection-locked on the third-order harmonic of an exter-
nally supplied clock. The prescaler has a wideband pull-in range between 0
.
2
and 5
.
3 GHz, corresponding to an internal operating speed starting from 0
.
6
up to 16 GHz. The prescaler produces four 90
◦
shifted lo signals (carrier fre-
quency 0
.
3-8
.
0 GHz) which are applied to the inputs of the double-balanced
quadrature mixer (see Section 6.3) and two divider blocks.
