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 fRF(high-high)
receiving at fRF(high-low)
receiving at fRF(low-high)
receiving at fRF(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 fRF(high-high). In the example of Fig. 2, the image signals at fRF(low-
high) and fRF(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 fRF(low-high) and fRF(low-low).
The two remaining signals at IF1 resulting from fRF(high-high) and fRF(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 fRF(high-high).
fLO2
fLO2
fRF
fRF
fRF
fLO1
fRF
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 fLO1 and the LO2 signal frequency fLO2
.
fLO1
LO2DIV = NLO2
=
(1)
fLO2
The LO1 signal frequency fLO1 is directly synthesized from the crystal reference oscillator frequency fRO 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.
fRO
fLO1
=
(N ⋅ P + A) = fPFD (N ⋅ P + A) = fPFD ⋅ Ntot
(2)
R
39012 71122 01
Rev. 001
Page 7 of 32
EVB Description
Sept/06
MELEXIS [ Melexis Microelectronic Systems ]
