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(Ed.)
Acoustic devices have played a major role in
telecommunications for decades as the leading technology
for filtering in RF and microwave frequencies. While filter
requirements for insertion loss and bandwidth become more
stringent, more functionality is desired for many applications
to improve overall system level performance. For instance, a
filter with non-reciprocal transmission can minimize losses
due to mismatch and protect the source from reflections
while also performing its filtering duties. A device such as
this one was originally researched by scientists decades ago.
These devices were based on the acoustoelectric effect where
surface acoustic waves (SAW) traveling in the same direction
are as drift carriers in a nearby semiconductor are amplified.
While several experiments were successfully demonstrated
in [1], [2], [3]. these devices suffered from extremely high
operating electric fields and noise figure [4], [5].
In the past few years, new techniques have been
developed for implementing non-reciprocal devices such
as isolators and circulators without utilizing magnetic
materials [6], [7], [8], [9]. The most popular technique has
been spatio-temporal modulation (STM) where commutated
clock signals synchronized with delay elements result in
non-reciprocal transmission through the network. STM has
also been adapted by researchers to create non-reciprocal
filters. The work in [10] utilizes 4 clocks signals to obtain
a non-reciprocal filter with an insertion loss of -6.6 dB an
isolation of 25.4 dB. Another filter demonstrated in [11]
utilizes 6 synchronized clock signals to obtain a non-reciprocal
filter with an insertion loss of -5.6 dB and an Isolation of 20
dB.
In this work, a novel non-reciprocal topology is explored
with the use of only one modulation signal. The design is
based on asymmetrical SAW delay lines with a parametric
amplifier. The device can operate in two different modes:
phase coherent mode and phase incoherent mode. In phase
coherent mode, the device is capable of over +12 dB of gain
and 20.2 dB of isolation. A unique feature of this mode is
that the phase of the pump signal can be utilized to tune the
frequency response of the filter. Under the phase-incoherent
mode, the pump frequency remains constant and the device
behaves as a normal filter with non-reciprocal transmission
exhibiting over +7 dB of gain and 17.33 dB of isolation. While
the tuning capability is lost in this mode, phase-coherence is no
longer necessary so the device can be utilized in most filtering
applications.
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