TH7111 Datasheet, PDF (4/20 Pages) List of Unclassifed Manufacturers – FSK /FM/ASK Receiver

TH7111
868/915MHz
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 TH7111 for double conversion and using its internal PLL synthesizer with fixed feedback
divider ratios of N1 = 16 (DIV_16) 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)/30
32•REF
LO1 – RF
2•REF
LO2 – IF1
low-low
(RF – IF2)/34
32•REF
RF – LO1
2•REF
IF1 – LO2
high-low
(RF + IF2)/30
32•REF
LO1 – RF
2•REF
IF1 – LO2
low-high
(RF + IF2)/34
32•REF
RF – LO1
2•REF
LO2 – IF1
The following table depicts generated, desired, possible images and some undesired signals considering the
examples of 868.3 MHz and 915 MHz RF reception at IF2 = 10.7 MHz.
Signal type
RF =
RF =
RF =
RF =
RF = 915 RF = 915 RF = 915 RF = 915
868.3 MHz 868.3 MHz 868.3 MHz 868.3 MHz MHz
MHz
MHz
MHz
Injection type high-high low-low
REF / MHz
28.58667 25.22353
LO1 / MHz
914.77333 807.15294
IF1 / MHz
46.47333 61.14706
LO2 / MHz
57.17333 50.44706
RF image/MHz 961.24667 746.00588
IF1 image/MHz 67.87333 39.74706
high-low
29.3
937.6
69.3
58.6
1006.9
47.9
low-high high-high low-low high-low low-high
25.85294 30.14333 26.59706 30.85667 27.22647
827.29412 964.58667 851.10588 987.41333 871.24706
41.00588 49.58667 63.89412 72.41333 43.75294
51.70588 60.28667 53.19412 61.71333 54.45294
786.28824 1014.17 787.21176 1059.83 827.49412
62.40588 70.98667 42.49412 51.01333 65.15294
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 800 MHz
to 915 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 868.3 MHz and 915 MHz, respectively.
TH7111 Data Sheet
3901007111
Page 4 of 20
Nov. 2001
Rev. 005