TH71102 Datasheet, PDF (4/20 Pages) List of Unclassifed Manufacturers – FSK/FM/ASK Receiver

TH71102
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 LO1 signal (which
is now the one and only LO signal in the receiver).
The receiver’s double-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 are the image of the RF signal (that must be suppressed by the RF front-end filter), spurious
signals injected to the first IF (IF1) and their images which could be mixed down to the same second IF (IF2)
as the desired RF signal (they must be suppressed by the LC filter at IF1 and/or by low-crosstalk design).
By configuring the TH71102 for double conversion and using its internal PLL synthesizer with fixed feedback
divider ratios of N1 = 8 (DIV_8) and N2 = 2 (DIV_2), four types of down-conversion are possible: low-side
injection of LO1 and LO2 (low-low), LO1 low-side and LO2 high-side (low-high), LO1 high-side and LO2
low-side (high-low) or LO1 and LO2 high-side (high-high). The following table summarizes some equations
that are useful to calculate the crystal reference frequency (REF), the first IF (IF1) and the VCO1 or first LO
frequency (LO1), respectively, for a given RF and second IF (IF2).
Injection type
REF
LO1
IF1
LO2
IF2
high-high
(RF – IF2)/14
16•REF
LO1 – RF
2•REF
LO2 – IF1
low-low
(RF – IF2)/18
16•REF
RF – LO1
2•REF
IF1 – LO2
high-low
(RF + IF2)/14
16•REF
LO1 – RF
2•REF
IF1 – LO2
low-high
(RF + IF2)/18
16•REF
RF – LO1
2•REF
LO2 – IF1
The following table depicts generated, desired, possible images and some undesired signals considering the
examples of 315 MHz and 433.6 MHz RF reception at IF2 = 10.7 MHz.
Signal type RF = 315 RF = 315 RF = 315 RF = 315 RF =
RF =
RF =
RF =
MHz
MHz
MHz
MHz 433.6 MHz 433.6 MHz 433.6 MHz 433.6 MHz
Injection type high-high low-low high-low low-high high-high low-low high-low low-high
REF / MHz
21.73571 16.90556 23.26429 18.09444 30.20714 23.49444 31.73571 24.68333
LO1 / MHz
347.77143 270.48889 372.22857 289.51111 483.31429 375.91111 507.77143 394.93333
IF1 / MHz
LO2 / MHz
32.77143 44.51111 57.22857 25.48889 49.71429 57.68889 74.17143 38.66667
43.47143 33.81111 46.52857 36.18889 60.41429 46.98889 63.47143 49.36667
RF image/MHz 380.54286 225.97778 429.45714 264.02222 533.02857 318.22222 581.94286 356.26667
IF1 image/MHz 54.17143 23.11111 35.82857 46.88889 71.11429 36.28889 52.77143 60.06667
The selection of the reference crystal frequency is based on some assumptions. As for example: the first IF
and the image frequencies should not be in a radio band where strong interfering signals might occur
(because they could represent parasitic receiving signals), the LO1 signal should be in the range of 300 MHz
to 430 MHz (because this is the optimum frequency range of the VCO1). Furthermore the first 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.
TH71102 Data Sheet
3901071102
Page 4 of 20
Nov. 2001
Rev. 005