Digital Signal Processing Reference
In-Depth Information
implementation which is described in the next section uses analog differential
current drivers with a reduced voltage swing for this purpose, a method that is
closely related to lvds.
18
A last design detail worth mentioning is that the internal clock distribution
lines are oriented perpendicular with respect to the rf-signal carrying path,
in order to minimize inductive coupling between both. The distance between
the clock buffers driving these lines and the input stages of the receiver must
be as large as possible and a common supply voltages should be avoided as
much as possible. At the same time, low-impedance ground/supply reference
planes are indispensable in many cases, since the clock lines are not terminated
differentially but directly drive (charge/discharge) the gate of cmos transistors.
The return path of the clock signal is thus formed by the closest ac-ground
terminal (i.e. the ground- or power plane) instead of the inverse clock line in a
differentially clocked system.
5.5
Comprehensive overview of the pulse-based
radio system
This section provides a more general overview of the pulse-based radio sys-
tem and briefly explains the coming about of its underlying principles. The
whole story starts with the fcc deciding to open up 7,500 MHz of spectrum
in the 3
.
1to10
.
6 GHz frequency band for use by unlicensed radio commu-
nication systems [Com02]. The considerable amount of bandwidth involved
here offers the potential for new possibilities in terms of wideband commu-
nication networks and immediately draws the attention of both industry and
academics. Ultra-Wideband (uwb) communication systems as they are called,
became soon marketed as being the wireless cable replacement technology of
the future. What is often not well recognized is that the maximum power spec-
tral density (psd) which these wideband communication devices are allowed
to emit is very low: less than
41
.
3 dBm/MHz eirp.
19
In fact, exactly the
same limits were already into force for a long time for unintentional radiators
(computers, switching power supplies). The main difference that there is at this
moment no further discrimination based on the origins of the transmission (i.e.
intended or not). It follows that there are some inevitable drawbacks that need
our attention.
−
18
lvds: low voltage differential signaling [Ass01].
19
eirp: Effective Isotropic Radiated Power.
