This document contains simplified graphical examples that illustrate different types of cross-allocation interference that can occur in radio systems, including adjacent channel interference, overload interference, and intermodulation interference. It shows how front-end band filters and IF channel filters work together to mitigate these interference effects by suppressing unwanted signals near and far from the desired channel. The examples demonstrate the roles of automatic gain control, mixing, and filtering in managing interference across multiple channels in the radio frequency spectrum.

1. Cross-allocation Interference
Simplified graphical examples
Pierre de Vries
2 Dec, 2009

2. Filter Operation: Generic Case
Front-end (band) filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal at antenna Radio Frequency
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after front-end
filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after AGC (B3
suppressed due to
strong B5)
Signal after mixing IF (channel) filter
down to IF, tuned to
B3 Intermediate Frequency
IF When tuning to a signal in
the middle of the band, both
band and channel selectivity
help
Signal after IF filter

3. Adjacent Channel (all at IF)
Sidebands spill into
Good IF (channel) filter, i.e.
victim channel
cuts off at channel edge
1. Transmitter Sideband Case
Intermediate Frequency
Signal after IF filter
Bad IF (channel) filter, i.e.
Exceeds channel edge
Sidebands do not
spill into victim
2. Poor IF filtering Case channel
Intermediate Frequency
Signal after IF filter

4. Overload (in-band)
Front-end (band) filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal at antenna Radio Frequency
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after front-end
filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after AGC (B3
suppressed due to
strong B5)
Signal after mixing IF (channel) filter
down to IF, tuned to
B3 Intermediate Frequency
IF
Very weak resulting B3 signal
due to overload from signal
in channel B5
Signal after IF filter

5. Overload (OOB) near band edge
Front-end (band) filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal at antenna Radio Frequency
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after front-end
filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after AGC (no
change)
Signal after mixing IF (channel) filter
down to IF, tuned to
B1 Intermediate Frequency
Have to depend on IF
selectivity to remove strong IF
unwanted signal
Signal after IF filter

6. Overload (OOB) far from band edge
Front-end (band) filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal at antenna Radio Frequency
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after front-end
filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after AGC (no
change)
Signal after mixing IF (channel) filter
down to IF, tuned to
B3 Intermediate Frequency
Both front-end and IF IF
selectivity provide
protection
Signal after IF filter

7. Intermodulation
Front-end (band) filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal at antenna Radio Frequency
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after front-end
filter
A15 A16 A17 A18 A19 A20 B1 B2 B3 B4 B5 C1 C2 C3
Signal after RF
amplification
Signal after mixing IF (channel) filter
down to IF; with
intermod of A19 & Intermediate Frequency
A20
IF
Intermodulation
product interference
Signal after IF filter