RF & MICROWAVE THE MECHANICS OF
VIRAL TRANSFER
and measurement equipment.
“Increased power enables a very
short pulse that provides increased
spectral coverage of the sample,”
added Dunnam. “However, at half-awatt
we can’t afford to give up anything
through insertion loss, hence we
require an extremely efficient isolator.”
Faraday rotation isolators
– ‘isolators’ for short – allow
electromagnetic signals to pass in
the forward direction with minimal
attenuation, but with maximum
absorption in the opposite direction.
However, standard isolators fall short
at extreme GHz frequencies. Above
the WR-5.1 band (140-220 GHz) loss
can climb to more than 5 dB, making
obtaining the lowest possible insertion
loss a very formidable engineering
challenge.
Expanding the limits of MMW R&D
For Cornell’s ACERT application,
reducing forward attenuation as much
as possible was particularly crucial
because at 500 milliwatts, while still
high for a coherent solid-state source,
any loss can significantly diminish the
spectrometer effectiveness.
“Virginia Diodes referred me to
Micro Harmonics,” recalled Dunnam.
“I was surprised to find there was
a company producing a component
that not only operates at such a high
frequency, but was an ideal fit for the
proposed ACERT instrumentation. They
had an isolator with the single most
important parameter I needed, low
insertion loss. They were ultimately
able to select one with just 1.2 dB
losses at 240 GHz, which is pretty
phenomenal.”
Micro Harmonics designs solutions
for components used in MMW
products. Under a two-phased NASA
contract, the company successfully
developed an advanced line of
isolators for WR-15 through WR-3.4
(50 GHz to 330 GHz) applications.
“They fully characterise each
individual isolator, which is really
important since each one is handassembled,
meaning there are slight
variations in performance,” explained
Dunnam. “This allowed selection of the
device that displayed good performance
in the narrow frequency range I was
interested in, 239-241 GHz.”
Minimising insertion loss to as
low as l.2 dB is achieved by reducing
the length of an internal ferrite rod
to the shortest possible length. The
design developed for NASA saturates
the ferrite with an unusually strong
magnetic bias field, which enables the
incident signal to rotate the required
45 degrees in the shortened ferrite
rod.
Dunnam went on to point out
other engineering challenges faced at
such high frequencies. The relatively
increased power enables a very short
pulse that provides increased spectral
coverage of the protein sample. But
after the last transmitted pulse in this
application, full receiver sensitivity
to the reflected decaying electron
spin signal is required within 10
nanoseconds.
“Any kind of ringing in the system
that is due to reflections can increase
‘dead time’ and obscure results, so
all the impedance mismatches and
spurious signals in the system must
be less than 0.1%,” said Dunnam.
“Additionally, the solid-state source
multiplier conversion efficiency and
stability can be adversely impacted by
out-of-phase signals being reflected
back. Low port reflection in the isolator
helps keep all that in check.”
“The solid-state source could
be destroyed by too much reflected
energy,” explained Dunnam. “As the
source was expensive to develop
and fabricate, I want to protect that
investment.”
Lastly, the issue of heat absorption
needed to be addressed even at subwatt
transmitter power levels. Power
in the reverse signal is absorbed
within the isolator, resulting in heat.
Historically, high heat was not an
issue as there was very little power
available from solid-state sources at
MMW frequencies, but as Dunnam
pointed out, 500 milliwatts at 240 GHz
is, relatively, a lot.
In his research centre’s application
of studying viral proteins, wavelengths
are short and components are
necessarily small and fragile.
To overcome the problem of high
heat loads, the Micro Harmonics
isolators selected by Dunnam
incorporate diamond heat sinks into
their design. Diamond is the ultimate
thermal conductor, approaching 2200
W/m•K (watts per meter-Kelvin), more
than five times higher than copper.
Diamond effectively channels heat
from the resistive layer in the isolator
to the metal waveguide block, and
thus lowers operating temperatures for
improved reliability.
Engineering a healthier future
By pushing the limits of mmWave
technology, engineers are laying
the groundwork for the next wave of
medical advancement that will help
fight against future outbreaks and
pandemics.
“What’s exciting about our work is
we’re providing insight into how all this
machinery works, which is essential
for the development of new antivirals,”
summed up Daniel.
Above: Spectrometer
set up at Cornell
www.newelectronics.co.uk 9 February 2021 17
/www.newelectronics.co.uk