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LND-TRM34

LOW POWER 300 TO 500 MHz FSK TRANSMITTER

Linear Dimensions Semiconductor 

LND TRM34

LOW POWER 300 TO 500 MHz FSK TRANSMITTER

Matching

network and

antenna options

(continued)

Figure 2 shows a typical application diagram.

In order to minimise second harmonic distortion,

a differential output configuration is recom-

Table 5: Output Power Settings

R

(kOhms)

Output power

(dBm)

Current cons.

(mA)

mended. The LND TRM34 needs the following:

2.8

2

25

1) a dc bias reference to VCC (preferrably through

3.8

0

22

direct connection of a loop antenna connected

5.2

-2

20

to VCC at its centre, or use of printed bias

6.7

-4

19

inductors with a connection to VCC). Linear’s

8.5

-6

18

TRM3x series of evaluation boards provide

9.7

-8

17

large and small loop antenna solutions for cus-

10.3

-10

16

tomers to copy. These boards require a minimum

10.7

-12

16

number of external components, and importantly

no discrete inductors are required. For driving an 3) the PLL synthesizer is designed to operate with

external single-ended antenna, a high efficiency

an external second order loop as shown in

printed balun is also available.

figure 2, with component values as shown in

For optimum operation, the following points are

highlighted:

1) The IC can provide 7.5mA of current into the load

Table 7. These values determine the loop

bandwidth and dynamics and operation of the

loop is not guaranteed for any other values

2) for maximum output power, it is recommended

that the antenna load be transformed to an

impedance of 200Ohms (RPS is set at

3.9kOhms)

4) The bias current for the power amplifier directly

controls the output current and hence the O/P

power. This bias current is set by an external

resistor connected between PS and ground.

Output power versus RI values is shown in

Table 5, for a differential load impedance of 200 Ω.

Frequency

deviation

External

components

The amount of frequency deviation depends

upon the type of external crystal used. An on-

chip capacitor of 3pF is switched in parallel

with the crystal to shift its frequency of operation.

For instance, on the TRM3x series of evalua-

tion boards, the peak-to-peak frequency

deviation possible with the supplied crystal is

over 15kHz in SOP packaged parts and 25 kHz

for SSOP packaged parts. The crystal param-

eters are specified as shown aside:

For smaller frequency deviations, then a less

pullable crystal can be specified (eg 20 pF or

30 pF load crystals are less pullable). If peak-to-

peak frequency deviations of much greater than

15kHz are required, then please consult Linear

Dimensions for information on how to achieve this.

Table 6 - Crystal specifications

Supplier/part number

Frequency

Package

Operating mode

Calibration tolerance

Temperature stability

Operating temp range

Circuit loading

Maximum ESR, R1

Static capacitance (typical), Co

Motional capacitance (typical), C1

Pullability

Euroquartz: B537

Tel.:

0044146076477

6.78 MHz

HC49/U

AT

+/-10 ppm

+/-10 ppm

-10 to + 60°C

10 pF

50 Ohms

5 pF

20 fF

40 ppm/pF

Table 7: Recommended External Components

Component

Function

C1

Crystal Oscillator

C2

Crystal Oscillator

C5

Supply Decoupling

C6

Regulator Decoupling

C7

PLL Loop Filter

C8

PLL Loop Filter

R1

O/P power set

R2

PLL Loop Filter

X1

Crystal

Value

22 pF

22 pF

100

100

1

47

3.9

3.3

6.78

Tolerance

± 5%

± 5%

± 20%

± 20%

± 5%

± 5%

± 5%

± 5%

< 20ppm

Units

pF

pF

nF

nF

nF

pF

kOhms

kOhms

MHz

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