DocID024318 Rev 1

3/20

AN4271

Application circuit

1

Application circuit

Figure 1 shows the SPIRIT1 with range extender application board photo. The application is

made up of 2 boards: a daughterboard and a motherboard. The daughterboard holds the

SPIRIT1 with the circuits necessary for its function. For correct operation, the

daughterboard must be plugged into the motherboard (see

Figure 2) by two header 5x2

connectors (J6 and J7).

The motherboard is equipped with an STM32L152VBT6 microcontroller to correctly

program the transceiver. The microcontroller is programmed with firmware developed for

the SPIRIT1 application. A graphical user interface (GUI) has been developed for

programming of the SPIRIT1.

The daughterboard includes a 50 MHz crystal to provide the correct oscillator to the

SPIRIT1.

The SPIRIT1 has an internal SMPS that drastically reduces power consumption, making the

SPIRIT1 the best-in-class for the application on this bandwidth. The SMPS is fed from the

battery (1.8 V to 3.6 V) and provides a programmable voltage (1.4 V typ) to the device. An

SMA connector is provided to connect the board to the antenna or to instrumentation in

order to verify correct functionality and confirm the ETSI standard required.

A few of passive devices (inductors and capacitors) are used as matching/filtering for the

power amplifier (PA) and balun network for the receiver.

A SAW filter is recommended in the TX path to attenuate the spurious emissions above the

carrier frequency, which would otherwise violate spurious emissions limits under ETSI 300

220 [

2]. The same SAW filter is provided in the RX path because, due the proximity of the

GSM band, the receiver can be saturated in an application environment. This SAW filter can

be bypassed using a short R1 resistor.

The SAW filter used is a Tai-Saw Technology CO., LTD. TA0801 868 MHz SAW filter [

5].

An external FEM (front-end module) is used to improve the output power at the +27 dBm

requested. The FEM utilized is a Skyworks SE2435L [

6]. The device includes a power

amplifier (PA) capable of about +27 dBm of transmitted output power. The receive channel

consists of an integrated LNA (low noise amplifier) with programmable bypass. An

integrated antenna switch, which provides an insertion loss of approximately 0.8 dB,

separates the TX and RX paths.