AN1993_15 Datasheet, PDF (4/23 Pages) NXP Semiconductors – High sensitivity applications of low-power RF/IF integrated circuits

NXP Semiconductors
AN1993
High sensitivity applications of low-power RF/IF integrated circuits
10
0
pre-selector
filter
−10
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10.7 MHz IF
−20
−30
−40
−60
−40
LO
image
desired
frequency
−20 −10
0
10
20
40
∆ pre-selector freq. (MHz)
a. Attenuation of low-side image with 10.7 MHz
Fig 2. Effects of pre-selection on images
10
0
image
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455 kHz IF
−10
pre-selector
−20
filter
−30
LO
−40
−60
−40 −20 −10
desired
frequency
0
10
20
40
∆ pre-selector freq. (MHz)
b. Attenuation of low-side 455 kHz image
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?
first
first
mixer IF filter
pre-select
filter
first
LO
second second
mixer IF filter
second
LO
IF amp
demod
audio
and/or
data
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Fig 3. Dual conversion
AN1993
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 3 — 13 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
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