UWB for Around-the-Body Data Streaming - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 60

Fig. 17 a LNA schematic. b Simulated notch effects of the LNA
The LNA employs a dual reactive loop structure as in Fig. 17 , which is a differential
version from the structure proposed in [ 14 ]. The transformer, consisting of Lp1 and
Ls1, is used for negative feedback, increasing the bandwidth at the expense of the
closed loop gain. The transformer, consisting of Lp2 and Ls2, boosts the gain in the
middle of the band. In addition, it provides an opportunity for notch filtering out-
of-band blockers and, thus, protecting the following stages from saturation. Here,
capacitor C1 is put in series with Ls1 to create a notch, for example, at the 1800 MHz
as shown in Fig. 17 b, which reduces the need for an external band filter. The above
features result in a power-efficient, low-cost, and robust implementation, complying
with worldwide regulations and prepared for upcoming standards.
At the mixer stage, an active differential mixer is chosen over the passive mixer.
Passive mixers shows typically a higher linearity at the expenses of no gain. Moreover,
the down-converted 500 MHz baseband signal is a challenge for the passive mixer.
The differential structure is selected for two reasons. First, the receiver is highly
differential; i.e., the antenna, LNA, and the quadrature DCO are implemented, fully
differential. Second, a double-balanced mixer eliminates the LO-to-IF and RF-to-IF
feed through.
The conventional double-balanced switching mixer (shown in Fig. 18 a) provides
high gain, reduce the noise contributed by subsequent stages and are widely used
in RF systems. The transistors in the switching stage (M3, M4, M5, and M6) are
stacked on top of the transistors that comprise the transconductor (M1 and M2). Also
the load resistor (R) is placed on top of the switching stage. This way of connecting
the transconductor, the switching stage and the load resistors is conflicting with
operation at low supply voltages. Therefore, at a low voltage supply, the voltages
drops across the load resistors, the switching transistors and the transistor in the
transconductor become critical. In terms of operation at low supply voltages, several
techniques have been proposed to overcome this limitation such as current bleeding,
folded mixer and switch-gm mixer. Hence, a folded-switching mixer in Fig. 18 b
is implemented based on a Gilbert-cell mixer combined with a folded approach.