Digital Signal Processing Reference
In-Depth Information
of the delay blocks can be programmed in 16 discrete steps, with a temporal
resolution better than 200 ps. Because the varying output load of the differen-
tial pair affects the slope of the clock signal, the analog delay cell is followed
by a regenerative latch. The result of all this tremendous effort is a four-way
multiphase clock generator, with independently triggerable edges.
The first two clock phases are used to mark the opening and closing times of
the receive slot: the rising edge of the first phase will open the receive window,
while the rising edge of the second phase disables the window. This process
repeats at the beginning of every master clock cycle. The third edge of the
multiphase clock generator triggers the differential offset compensation cir-
cuit of the open-loop variable gain amplifier.
3
Finally, the fourth edge of the
multiphase clock is used as a synchronous trigger signal for the (off-chip) ad-
converters of the i/q baseband section. This allows to dynamically adjust the
optimum sampling point of the converter.
Measuring the aperture of the receive window
Connecting the pulse-based receiver to the outside world is done using four
buffers. They are capable of directly driving the 50
load impedance of
external measurement equipment. Two of these unity-gain analog buffers are
exclusively dedicated to the i/q quadrature baseband outputs of the variable
gain amplifier. The remaining two other analog output channels are used as
general purpose buffers.
4
A number of interesting internal circuit nodes can be
routed to these buffers for testing purposes. Their main application is to verify
the correct functioning of the receiver.
Apart from some voltage measurements on internal nodes, the general pur-
pose buffers can also be used to monitor the outputs of the multiphase clock
generator. By connecting two different phases of the multiphase generator to
the output, it becomes possible to perform some direct measurements of the
phase delay between the respective clock lines. The accuracy of comparative
measurements between two channels, however, is very susceptible to small dif-
ferences in the connection to the external equipment. At this level, even small
deviations of the length of the propagation path affects the practical accuracy.
For example, bending the coaxial cables resulted in a relative time delay of
about 10 ps. The same effects were also noticed at the rf inputs of the re-
ceiver, where slight bending of the cables resulted in a rotation of the baseband
constellation of up to 20
◦
.
5
Direct access to the clock lines of the multiphase
3
Internally, the offset compensation circuit is based on the switch-cap topology, because it allows to realize
a low-frequency pole without the need for an excessive amount of chip area.
4
The general purpose buffers can be put to sleep to reduce switching noise on the supply voltage.
5
Measured with f
lo
=
6
.
85 GHz. This only shows the importance of the phase-tracking mechanism of the
signal processor, especially for the wireless channel: the differences in the propagation path length here are
much larger and faster varying than in the artificial environment of a measurement setup.
