Digital Signal Processing Reference
In-Depth Information
nVdd
Vcmfb
mix_outn
mix_outp
315fF
VGA
Pulse-to-baseband conver-
sion capacitors.
315fF
LO_
p
LO_n
L
O
_
p
Receive window switching
noise is upconverted to RF.
Cascode transistors shield
clock spurs from antenna
and provide a low RF input
impedance.
receive_window
Vref
V
re
f
RF_inp
Antenna input signal is
injected directly into the
mixer lines and integrated
on the output capacitors.
RF_inn
Vbias
Vbias
nGND
Figure 6.5.
The input stage and downconversion mixer of the prototype chip
are based on a generic Gilbert mixing stage. Remark that the rf
input signal is injected directly into the mixer lines, for improved
linearity performance (iip
3
better than
−
1dBm).
For linearity reasons, the prototype receiver does not feature an active low-
noise amplifier at its input, so the rf input signals are directly delivered to
the mixing stage. The result is that there is no active gain involved in the
pulse-to-baseband conversion process. It is important to realize that, during
the pass-through state of the receive window, the injected rf current is routed
through the mixer switches and is then integrated on the capacitor between the
output terminals. The only parameter that matters at this point is the charge that
the signal-of-interest induces on this capacitor. The the
rmal n
oise floor mea-
sured across this capacitor is determined by
v
noise, rms
=
√
kT /C
[Sar93], but its
capacitance is not relevant for the snr of the output signal. This is because the
capacitor samples a current that is being injected in the rf input, rather than a
voltage value from a low-impedance voltage source. The pulse energy stored
on the capacitor is independent from its value, because the total charge that is
extracted from a single time-limited rf pulse applied to the input is limited
