EVB71111

868/915MHz Receiver

Evaluation Board Description

39012 71111 01

Page 5 of 14

EVB Description

Rev. 011

June/07

By using the internal PLL synthesizer of the TH71111 with the fixed feedback divider ratio of N = 16

(DIV_32), 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)/32

(RF + IF)/32

LO

32

• REF

32

• 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 868.3 MHz and

915 MHz RF reception at IF = 10.7 MHz.

Signal type

RF = 868.3 MHz

RF = 868.3 MHz

RF = 915 MHz

RF = 915 MHz

Injection type

low

high

low

high

REF / MHz

26.80000

27.46875

28.25938

28.92813

LO / MHz

857.6

879.0

904.3

925.7

RF image / MHz

846.9

889.7

893.6

936.4

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 800 MHz to 930 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

868.3 MHz and 915 MHz, respectively.

1.6.2

Maximum Frequency Coverage

Parameter

Injection type

high

low

RF / MHz

789.3

940.7

REF / MHz

25.0

29.0625

LO / MHz

800

930

IF/ MHz

10.7

10.7