PR

EL

IM

INA

RY

EVB71122

300 to 930MHz Receiver

Evaluation Board Description

39012 71122 01

Page 7 of 32

EVB Description

Rev. 001

Sept/06

2

Functional Description

2.1

Frequency Planning

Because of the double conversion architecture that employs two mixers and two IF signals, there are four

different combinations for injecting the LO1 and LO2 signals:

•

LO1 high side and LO2 high side:

•

LO1 high side and LO2 low side:

•

LO1 low side and LO2 high side:

•

LO1 low side and LO2 low side:

As a result, four different radio frequencies (RFs) could yield one and the same second IF (IF2). Fig. 2

bandpass shape can be achieved either with a SAW filter (featuring just a couple of MHz bandwidth), or by

the tank circuits at the LNA input and output (this typically yields 30 to 60MHz bandwidth). In any case, the

mixer MIX2. This mixer features image rejection with so-called single-sideband (SSB) selection. This means

either the upper or lower sideband of IF1 can be selected. In the example of Fig. 2, LO2 high-side injection

Fig. 2:

The four receiving frequencies in a double conversion superhet receiver

It can be seen from the block diagram of Fig. 1 that there is a fixed relationship between the LO1 signal fre-

LO2

LO1

LO2

f

f

N

LO2DIV

=

=

(1)

means of an integer-N PLL synthesizer. The PLL consists of a dual-modulus prescaler (P/P+1), a program

counter N and a swallow counter A.

tot

PFD

PFD

RO

LO1

N

f

A)

P

(N

f

A)

P

(N

R

f

f

⋅

=

+

⋅

=

+

⋅

=

(2)

f

LO2

f

LO2

f

LO1

f

RF

f

RF

f

RF

f

RF