Cross-allocation InterferenceSimplified graphical examples

Pierre de Vries

2 Dec, 2009

C2B2A17A15 C3B3 B4 B5B1A16

Filter Operation: Generic Case

C1

Signal at antenna

A19 A20A18

Front-end (band) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1

Signal after front-end filter

A19 A20A18

Signal after mixing down to IF, tuned to B3

IF

Radio Frequency

Intermediate Frequency

IF (channel) filter

Signal after IF filter

C2B2A17A15 C3B3 B4 B5B1A16 C1

Signal after AGC (B3 suppressed due to strong B5)

A19 A20A18

When tuning to a signal in the middle of the band, both band and channel selectivity

help

Adjacent Channel (all at IF)

1. Transmitter Sideband Case

Intermediate Frequency

Bad IF (channel) filter, i.e.Exceeds channel edge

Signal after IF filter

2. Poor IF filtering Case

Signal after IF filter

Sidebands spill into victim channel

Intermediate Frequency

Good IF (channel) filter, i.e.cuts off at channel edge

Sidebands do not spill into victim

channel

C2B2A17A15 C3B3 B4 B5B1A16

Overload (in-band)

C1A19 A20A18

Front-end (band) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1A19 A20A18

IF

Radio Frequency

Intermediate Frequency

IF (channel) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1A19 A20A18

Signal at antenna

Signal after front-end filter

Signal after AGC (B3suppressed due to strong B5)

Very weak resulting B3 signal due to overload from signal

in channel B5

Signal after mixing down to IF, tuned to B3

Signal after IF filter

C2B2A17A15 C3B3 B4 B5B1A16

Overload (OOB) near band edge

C1A19 A20A18

Front-end (band) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1A19 A20A18

IF

Radio Frequency

Intermediate Frequency

IF (channel) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1A19 A20A18

Signal at antenna

Signal after front-end filter

Signal after AGC (no change)

Have to depend on IF selectivity to remove strong

unwanted signal

Signal after mixing down to IF, tuned to B1

Signal after IF filter

C2B2A17A15 C3B3 B4 B5B1A16

Overload (OOB) far from band edge

C1A19 A20A18

Front-end (band) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1A19 A20A18

IF

Radio Frequency

Intermediate Frequency

IF (channel) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1A19 A20A18

Signal at antenna

Signal after front-end filter

Signal after AGC (no change)

Both front-end and IF selectivity provide

protection

A19

Signal after mixing down to IF, tuned to B3

Signal after IF filter

C2B2A17A15 C3B3 B4 B5B1A16

Intermodulation

C1

Signal at antenna

A19 A20A18

Front-end (band) filter

C2B2A17A15 C3B3 B4 B5B1A16 C1

Signal after front-end filter

A19 A20A18

Signal after mixing down to IF; with intermod of A19 & A20

IF

Radio Frequency

Intermediate Frequency

IF (channel) filter

Signal after IF filter

C2B2A17A15 C3B3 B4 B5B1A16 C1

Signal after RF amplification

A19 A20A18

Intermodulation product interference