TH71101 Datasheet, PDF (4/20 Pages) List of Unclassifed Manufacturers – FSK/FM/ASK Receiver

TH71101
315/433MHz
FSK/FM/ASK Receiver
Frequency Planning
Frequency planning is straightforward for single-conversion applications because there is only one IF that
might be chosen, and then the only possible choice is low-side or high-side injection of the LO signal (which
is now the one and only LO signal in the receiver).
The receiver’s single-conversion architecture requires careful frequency planning. Besides the desired RF
input signal, there are a number of spurious signals that may cause an undesired response at the output.
Among them is the image of the RF signal that must be suppressed by the RF front-end filter.
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 fol-
lowing table summarizes some equations that are useful to calculate the crystal reference frequency (REF),
the IF and the LO frequency respectively, for a given RF.
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
The following table depicts crystal, LO and image signals considering the examples of 315 MHz and
433.6 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 = 433.6 MHz
low
26.43125
422.9
412.2
RF = 433.6 MHz
high
27.76875
444.3
455.0
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 430 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 433.6 MHz, respectively.
TH71101 Data Sheet
3901071101
Page 4 of 20
Nov. 2001
Rev. 005