Page 20 - EngineerIT July 2022
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ADVANCED FILTER TECHNOLOGY
Figure 2: (a) Sub-octave pre-selection mitigates IMD2 issues; (b) filter bands become wider as frequency increases.
side and at 24 GHz on the high side. These differences mean many unwanted signals (see Figure 4) need to be rejected before
more filters are needed to cover lower frequencies bands than they reach the mixer to avoid generating unwanted MxN spur
high frequency bands. A frequency spectrum example of the pre- products. If the LO signal is at a single frequency, then a fixed
selector filtering is shown in Figure 2. band-pass filter is sufficient and can be optimised to pass only
The image/IF rejection filtering is typically further down the the desired signal. In wideband signal chains, a tunable LO signal
signal chain, between the LNA and the mixer. It is used to reject is usually implemented and therefore requires either a set of
the image frequencies and unwanted IF frequencies. The image switched filters or a tunable filter.
is a frequency band that, when present at the mixer input, will
generate signals equal in amplitude to the desired signals at the
mixer output. Image mitigation can be achieved from several
components in the signal chain, such as pre-selector filters,
dedicated image reject filters, and image rejection from single-
sideband (SSB) mixers. IF signal rejection is required to knock
down spectrum at IF frequencies before the mixer, to prevent
them from leaking directly across the mixer and showing up as
unwanted spurs. A frequency spectrum example of the unwanted
image and IF bands is shown in Figure 3.
Depending on the circuitry used to generate the LO, filtering
requirements may vary at this point in the signal chain. The
desired signal feeding the LO port of the mixer is a clean sine
wave or a square wave. Often, the LO circuitry creates sub-
harmonics and harmonics of the desired LO signal. These Figure 4: LO harmonic filtering.
Figure 3: (a) An image band and (b) an IF band must be rejected before the mixer.
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