Digital Signal Processing Reference
In-Depth Information
Receive window is
Pulse-to-baseband
Multistage wideband open-
Output buffers employ
embedded in mixer.
conversion filter.
loop variable gain amplifier.
differential signaling.
I
2
2
2
buf 1
Q
2
2
2
buf 2
reset cmfb
close window
trigger out
open window
buf 3
1:1
1:2
1:4
LO
I
Q
0
1:64
90
buf 4
13.7 GHz
6.85 GHz
107/54/27 MHz
high-speed I/Q divider
lock-in: 0.6-16 GHz
Programmable divider.
1:32 / 1:64 / 1:128
Multiphase clock generator.
(Discussed in the next section)
Analog current-mode
output drivers. Z
out
=100Ω
Figure 6.1.
Schematic overview of the pulse-based receive unit. Note that in the
actual design of the prototype receiver chip, the receive window was
erroneously placed after the downconversion mixer, causing problems
with in-band spurious noise.
of which is essential for the operation of the pulse-based system. Only during
the receive slot assigned to a particular receive unit, the window is briefly acti-
vated in order to allow the receiver to capture some rf power from the channel.
During the off-state of the receive window, unwanted noise and in-band inter-
ferer power is prevented from entering the signal chain, which significantly
reduces the linearity requirements of the ensuing signal processing blocks. A
time-domain filter approach is in fact the only viable option for a wideband
receiver, because it guarantees a minimum of interferer suppression under all
possible conditions, without the need for a (strictly controlled) bank of tunable
analog notch filters. Based on the above described setup of the receiver, it is
already possible to derive some specifications for the windowing circuit.
6.1
Receive window specifications
Because the windowing subcircuit itself is built around a non-ideal network
of cmos switches, it can only offer a limited performance in terms of inser-
tion loss in the on-state and forward isolation during its off-state. Of course,
one can make the ratio between the former two parameters very large, but it
must not be forgotten that this structure is located within the very sensitive
