29-03-2012, 11:41 AM
RF MEMs PHASE SHIFTERS
RF MEMS.pdf (Size: 9.63 MB / Downloads: 166)
Microwave and millimeter-wave phase shifters are essential
components in phased-array antennas for telecommunications
and radar applications and are currently based on ferrite materials,
p-i-n diodes, or field-effect transistor (FET) switches.
Solid-state phase shifters provide a good planar solution at microwave
frequencies and have been extensively used in modern phased-array systems.
A moderate amount of dc power (3-10 mW per diode) is consumed by p-i-n
diode phase shifters, which results in low-loss designs, especially at X-band frequencies.
FET-based phase shifters consume virtually no dc power and can be integrated
with low-noise or medium-power amplifiers on the same chip, thereby
reducing the assembly cost of phased-array systems. However, they introduce a lot
of loss in the front end, around 4-6 dB at 12-18 GHz [1], [2] and 8-9 dB at 35 GHz for
4-bit (b) designs [3], [4].
The Case for RF MEMS Phase Shifters
MEMS switches result in lower loss phase shifters at
any frequency, especially from 8-120 GHz. The average
loss of state-of-the-art 3-b MEMS phase shifters is
−0.9 dB at 10-14 GHz, which is a 3-4 dB improvement
over comparable on-wafer designs using FET
switches. This translates to a 6-8 dB improvement in a
radar or two-way telecommunication system. The improvement
is quite high for Ka-band (35 GHz), V-band
(60 GHz), or W-band (77 GHz, 94 GHz) systems (see
Table 1). This means that one can eliminate an amplifier
stage (or two) in the transmit/receive (T/R) chain,
thereby resulting in a dc power reduction of 20-100
mW per element at X- to
V-band frequencies.
RF MEMS in Reflect Arrays
RF MEMS phase shifters render the idea of a passive
reflect array possible at X- to K-band frequencies. Reflect
arrays are much less expensive than standard phased
arrays and provide ample space for drivers and receiver
electronics. They are space fed; therefore, they do not suffer
from the high transmission-line (corporate feed) loss
as the array size increases. The accompanying figure presents
the performance of a passive reflect array using RF
MEMS phase shifters. Note that one can obtain a one-way
system loss of −2.4 to −3.2 dB at X- to K-band frequencies.