Feature: MMW Insertion loss comparison
Isolator from Micro Harmonics
3. Low port reflection A good isolator must also have low port reflections. Voltage Standing Wave Ratio (VSWR) is a measure of reflections at the input and output ports. A good range at MMW frequencies is 1.5:1 or less, whereas 1:1 means no reflection. The importance of low port reflection is often overlooked.
An isolator with high port reflections creates an alternate set of standing waves. The adjacent components are still adversely impacted by out-of-phase signals reflected back into their ports. High isolation and low insertion loss are of little value if the port reflections are large.
4. High power rating Power in the reverse travelling signal is absorbed in the isolator, resulting in heat. Te more heat it can handle, the higher the power rating. Historically, high heat was not an issue as there was very little power available at MMW frequencies. However, as higher power sources become available, the importance of power ratings increases. To handle high heat loads, some newer isolators are already
incorporating diamond heat sinks into their designs. Diamond is the ultimate thermal conductor, approaching 2200W/mK, more than five times higher than copper. Diamond effectively channels heat from the resistive layer in the isolator to the metal waveguide block, thus lowering operating temperatures for improved reliability.
Connecting measurement instruments
5. Small footprint Minimising the size and weight of MMW components is especially important in today’s wireless applications. “A standard traditional-style isolator in the WR10 band is
about three inches long, with a cylindrical section in the centre that’s about 1.3 inches in diameter,” said Porterfield. “But the newest design shapes are rectangular and can be as small as 0.75 inches per side and 0.45 inches thick.” Te same technology used to reduce insertion loss – using
the shortest possible length of ferrite – also helps reduce the footprint.
Other properties In addition to the five critical characteristics, other properties of modern isolators improve their utility at MMW frequencies, such as wide bandwidth, for example. Standard waveguide bands typically extend to 40% on either side of the centre frequency. Newer, high-performing isolators operate over extended bandwidths exceeding 50% from centre frequency, giving designers greater freedom to build more bandwidth into their systems. Additional advances include isolators that operate in
cryogenic conditions, which is critical because a traditional isolator designed for room-temperature operation will perform poorly when cooled.
Isolator incorporating a diamond heat sink
22 September/October 2020
www.electronicsworld.co.uk
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