EVB71122_15 Datasheet, PDF (8/31 Pages) Melexis Microelectronic Systems – 300 to 930MHz Receiver Evaluation Board Description

EVB71122
300 to 930MHz Receiver
Evaluation Board Description
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:
receiving at f_RF(high-high)
receiving at f_RF(high-low)
receiving at f_RF(low-high)
receiving at f_RF(low-low)
As a result, four different radio frequencies (RFs) could yield one and the same second IF (IF2). Fig. 2 shows
this for the case of receiving at f_RF(high-high). In the example of Fig. 2, the image signals at f_RF(low-high) and
f_RF(low-low) are suppressed by the bandpass characteristic provided by the RF front-end. The 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 high value
of the first IF (IF1) helps to suppress the image signals at f_RF(low-high) and f_RF(low-low).
The two remaining signals at IF1 resulting from f_RF(high-high) and f_RF(high-low) are entering the second 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 has
been chosen to select the IF2 signal resulting from f_RF(high-high).
f _LO2
f _LO2
f _RF
f _RF
f _LO1
f _RF
f _RF
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-
quency f_LO1 and the LO2 signal frequency f_LO2.
LO2DIV 
N _LO2

f _LO1
f _LO2
(1)
The LO1 signal frequency f_LO1 is directly synthesized from the crystal reference oscillator frequency f_RO by
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.
f _LO1

f _RO
R
(N  P

A)

f_PFD (N  P

A)

f _PFD

N _tot
(2)
39012 71122 01
Rev. 005
Page 8 of 31
EVB Description
Nov/15