Digital Signal Processing Reference
In-Depth Information
Then, the rf signal also has a repetitive nature, caused by the generator looping
over a limited block length of the internal data register. As a result of this repe-
tition, the power of this rf signal is not evenly spread over the frequency band,
but is clustered around discrete frequencies. The spectral lines are separated by
a distance 1/T
repetition
, but also the peak psd is scaled with the distance between
two consecutive lines (T
repetition
), possibly violating the spectral mask if the to-
tal transmission power is kept constant. In a regular pulse-based transmission,
this problem is avoided by introducing sufficient randomness in the baseband
qpsk symbol stream. In most cases, this condition is easily met when the en-
coded bit stream is scrambled with a pseudo-random noise (prn) vector.
However, the issue above is not relevant for the outcome of further measure-
ment results. The i/q quadrature outputs of the receiver are connected to a
digitizing oscilloscope in x-y mode. The external trigger input of the oscillo-
scope is connected to one of the general purpose drivers of the receiver, and
internally linked to the trigger channel of the multiphase clock generator (see
Section 6.2). Triggering on the baseband signal itself is avoided because this
results in unstable triggering points when no signal is present in the baseband
output of the receiver. The synchronous trigger signal of the receiver ensures
that the oscilloscope always starts to acquire data at a fixed phase angle, even
if no signal is present. This situation typically occurs at the startup of the mea-
surement setup. At that moment, chances are quite slim that the receiver is able
to immediately pick up a signal from the data generator. The reason for this is
that the pulses from the data generator are not necessarily synchronized with
the single receive slot of the prototype pulse-based receiver. Unfortunately, the
prototype chip also lacks any form of synchronization mechanism, as this is
the responsibility of the digital back-end processor (see Section 5.2). There-
fore, after the startup of the receiver, the rf signal from the data generator
needs to be aligned manually until the transmitted pulses come within sight of
the activated receive slot. Because a reallocation of the receive slot itself is not
currently supported by the prototype receiver, this is done by delaying the rf
stream from the data generator with respect to the oscillator clock. The multi-
phase clock generator (which controls the position of the receive window) is
synchronized with the on-chip high-speed prescaler, while the prescaler itself
is (injection-) locked on the clock of the data generator. The rf pulses will thus
stay within scope of the receiver for as long as the receiver remains locked on
the data generator.
The result of all this effort is shown in Figure 6.9. It shows the i/q constel-
lation plot of the received rf pulse stream after pulse-to-baseband conver-
sion to a regular qpsk signal by the pulse-based radio receiver. Remark that
the constellation points are not located exactly on top of each other, but form
small
clouds
around the ideal constellation points. The deviation from the ideal
