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Filters selectively attenuate certain frequency components of a microwave signal.
Low-pass filters, for example, remove frequencies above a certain cutoff.
Components like I/Q mixers frequently generate higher harmonics (e.g. they turn
a 5 GHz signal into mostly 5 GHz with some 10 GHz, 15 GHz, etc.). These undesired
harmonics can be filtered out easily with off-the-shelf low-pass filters.
I/Q mixers split an incoming microwave signal into two halves (I and Q) of
equal amplitude. One of the halves (Q) is then phase-shifted by 90 ◦ . Each half is
passed through a mixer, which multiplies its amplitude by another input signal.
Finally, the two halves are summed together to create the output. I/Q mixers can
be used to generate signals of arbitrary phase and amplitude. This can be seen
from simple trigonometrics. The input signal, sin ωt, is split into a sin ωt part
(I) and a cos ωt part (Q). These are then multiplied by an I and Q input signal
and summed, yielding: I sin ωt + Q cos ωt. This can be rewritten as A sin (ωt + δ)
where A 2 = I 2 + Q 2 and tan δ = I Q . 121