EVB71102

315/433MHz Receiver

Evaluation Board Description

39012 71102 01

Page 5 of 14

EVB Description

Rev. 011

June/07

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

high-high

low-low

high-low

low-high

REF

(RF – IF2)/14

(RF – IF2)/18

(RF + IF2)/14

(RF + IF2)/18

LO1

16

•REF

16

•REF

16

•REF

16

•REF

IF1

LO1 – RF

RF – LO1

LO1 – RF

RF – LO1

LO2

2

•REF

2

•REF

2

•REF

2

•REF

IF2

LO2 – IF1

IF1 – LO2

IF1 – LO2

LO2 – IF1

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 IF2 = 10.7 MHz. The columns in bold depict the selected frequency plans to

receive at 315 MHz and 433.92 MHz, respectively.

Signal type

RF =

315

MHz

RF =

315

MHz

RF =

315

MHz

RF =

315

MHz

RF =

433.92

MHz

RF =

433.92

MHz

RF =

433.92

MHz

RF =

433.92

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

30.23000

23.51222

31.75857

24.70111

LO1 / MHz

347.77143

270.48889

372.22857

289.51111

483.68000

376.19556

508.13714

395.21778

IF1 / MHz

32.77143

44.51111

57.22857

25.48889

49.76000

57.72444

74.21714

38.70222

LO2 / MHz

43.47143

33.81111

46.52857

36.18889

60.46000

47.02444

63.51714

49.40222

RF image/MHz

380.54286

225.97778

429.45714

264.02222

533.44000

318.47112

582.35428

356.51556

IF1 image/MHz

54.17143

23.11111

35.82857

46.88889

71.16000

36.32444

52.81717

60.10222

1.6.2

Maximum Frequency Coverage

Parameter

Injection type

high-low

low-low

RF / MHz

251.8

516.95

REF / MHz

18.75

28.125

LO1 / MHz

300

450

IF1 / MHz

48.2

66.95

LO2 / MHz

37.5

56.25

IF2/ MHz

10.7

10.7

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 450 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.