TH71101
315/433MHz
FSK/ASK Receiver
39010 71101 Page 5 of 21 Data Sheet
Rev. 018 Jun/13
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 low high
REF (RF – IF)/16 (RF + IF)/16
LO
16 • REF 16 • REF
IF RF – LO LO – RF
RF image RF – 2IF 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 RF = 315 MHz RF = 315 MHz RF = 433.92 MHz RF = 433.92 MHz
Injection type low high low high
REF / MHz 19.01875 20.35625 26.45125 27.78875
LO / MHz 304.3 325.7 423.22 444.62
RF image / MHz 293.6 336.4 412.52 455.32
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 f
min
f
max
Injection type high low
RF / MHz 289.3 460.7
REF / MHz 18.75 28.125
LO / MHz 300 450
IF/ MHz 10.7 10.7
