AN4147

Transmitter parameter

Doc ID 023516 Rev 1

9/14

3

Transmitter parameter

All of the measurements reported here are measured using the following parameters:

Tc = 25

° C, Vdd = 3.0 V, f = 315 MHz, unless otherwise specified.

Regarding the output power and emission, the standard specifies the maximum field

strength (

µV/m). Microvolts per meter (µV/m) are the units used to describe the strength of

an electric field created by the operation of a transmitter. A particular transmitter that

generates a constant level of power (watts) can produce electric fields of different strengths

depending on the type of antenna connected to it. Because it is the electric field that causes

interference to authorized radio communications, and since particular electric field strength

does not directly correspond to a particular level of transmitter power, the emission limit of

short range devices are specified in field strength.

In order to simplify the testing environment, a conversion from the field strength to output

power is done, since the latter is easier to measure. Although the precise relation between

power and field strength can depend on a number of additional factors, a commonly-used

equation to approximate the field strength (V/m) and the power (W) is:

Equation 1

where:

–

P = transmitter power (EIRP) in watts

–

G = gain of the transmitter antenna relative to an isotropic source

–

d = distance of the measuring point from the electrical center of the antenna in

meters

–

E = field strength in volts/meter

–

4 * d2 = surface area of the sphere centered at the radiating source whose surface

is d meters from the radiating source

–

120 *

Π = characteristic impedance of free space in ohms

Assuming a unity gain antenna (G = 1) and considering the measurement distance of 3

meters (d = 3), a formula can be summarized with:

Equation 2

The field strength of the fundamental at 315 MHz is not given directly but has to be

calculated from the two values (260 to 470 MHz) defined in

Table 1. The calculated value of

the field strength of the fundamental at 315 MHz is 6043.5

µV/m which corresponds,

applying the formula above, to a value of - 19.6 dBm at 3 m distance. In a similar manner,

the field strength of the spurious emissions must be lower than 604.35

µV/m which

corresponds, applying the formula above, to a value of - 35 dBm at 3 m distance.

(P * G)/(4*d

P = 0.3 * E

2