AN1993

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Application note

Rev. 3 — 13 August 2014

4 of 23

NXP Semiconductors

AN1993

High sensitivity applications of low-power RF/IF integrated circuits

If the single conversion architecture of Figure 1 were implemented with a 455 kHz IF, any

interfering image would be received almost as well as the desired frequency. For this

reason, dual conversion, as shown in Figure 3, has been popular.

In the application of Figure 3, the first IF must be high enough to permit the pre-selector to

reject the images of the first mixer and must have a narrow enough bandwidth that the

second mixer images and the intermod products due to the first mixer can be attenuated.

There is more to it than that, but those are the basics. The multiple conversion heterodyne

works well, but — as Figure 3 suggests — compared to Figure 2 it is more complicated.

Why then, don’t we use the approach of Figure 2?

a. Attenuation of low-side image with 10.7 MHz

b. Attenuation of low-side 455 kHz image

Fig 2.

Effects of pre-selection on images

∆ pre-selector freq. (MHz)

−40

40

10

−10

002aah474

−30

−10

10

−60

−20

0

20

−40

−20

0

desired

frequency

pre-selector

filter

LO

image

10.7 MHz IF

∆ pre-selector freq. (MHz)

−40

40

10

−10

002aah475

−30

−10

10

−60

−20

0

20

−40

−20

0

desired

frequency

pre-selector

filter

LO

image

455 kHz IF

Fig 3.

Dual conversion

002aah476

first

mixer

first

LO

pre-select

filter

first

IF filter

IF amp

demod

audio

and/or

data

second

mixer

second

LO

second

IF filter