TH71101_13 Datasheet, PDF (5/20 Pages) Melexis Microelectronic Systems – FSK/ASK Receiver

TH71101
315/433MHz
FSK/ASK Receiver
By using the internal PLL synthesizer of the TH71101 with the fixed feedback divider ratio of N = 16
(DIV_16), two types of down-conversion are possible: low-side injection of LO and high-side injection of LO.
The following table summarizes some equations that are useful to calculate the crystal reference frequency
(REF) and the LO frequency, for a given RF and IF.
Injection type
REF
LO
IF
RF image
low
(RF – IF)/16
16 • REF
RF – LO
RF – 2IF
high
(RF + IF)/16
16 • REF
LO – RF
RF + 2IF
1.6.1 Selected Frequency Plans
The following table depicts crystal, LO and image signals considering the examples of 315 MHz and
433.92 MHz RF reception at IF = 10.7 MHz.
Signal type
Injection type
REF / MHz
LO / MHz
RF image / MHz
RF = 315 MHz
low
19.01875
304.3
293.6
RF = 315 MHz
high
20.35625
325.7
336.4
RF = 433.92 MHz
low
26.45125
423.22
412.52
RF = 433.92 MHz
high
27.78875
444.62
455.32
The selection of the reference crystal frequency is based on some assumptions. As for example: the image
frequency should not be in a radio band where strong interfering signals might occur (because they could
represent parasitic receiving signals), the LO signal should be in the range of 300 MHz to 450 MHz (because
this is the optimum frequency range of the VCO1). Furthermore the IF should be as high as possible to
achieve highest RF image rejection. The columns in bold depict the selected frequency plans to receive at
315 MHz and 433.92 MHz, respectively.
1.6.2 Maximum Frequency Coverage
Parameter
Injection type
RF / MHz
REF / MHz
LO / MHz
IF/ MHz
f_min
high
289.3
18.75
300
10.7
f_max
low
460.7
28.125
450
10.7
39010 71101
Rev. 018
Page 5 of 21
Data Sheet
Jun/13