RFXCOM RFXtrx433 Especificaciones descargar pdf (Pagina 48)

Chapter 4. Implementation 41

Another drawback of using the internal USB hardware, is the high memory use of almost 2 kB

RAM and 10 kB ROM. The big ATXMega192A3U has enough memory to handle this, but it will

cost 30% of ROM and 50% RAM of the small ATXMega32A4U. Therefore, the MiniCtrl ﬁrmware

had to be optimized for memory, otherwise the code size was greater than the available memory.

Despite these disadvantages, the costs are lowered by approximately 3 € and the overall PCB size

is reduced (see ﬁgure 4.10 on page 46).

4.1.4. Ceramic antenna for WLAN

As already mentioned the performance of the PCB antenna was very poor and highly dependent

on the surroundings. Although the board can be retroﬁtted with a matching pi-ﬁlter, it would

only work for a certain environment and since as yet there is no housing for the WiﬁCtrl available,

major changes must be expected. To solve this problem, the PCB antenna was replaced with a

ceramic chip antenna (ﬁgure 4.6).

Figure 4.6.: The ceramic antenna on the left and the PCB antenna on the right. Both pi-ﬁlters

are bypassed. The yellow colour of the left PCB is due to the lack of solder resist.

Ceramic antennas are separate components that are soldered onto the PCB. Their body has a

high dielectric constant, which mainly determines the resonance frequency and the signal strength.

Therefore, they are robust against detuning, as changes in the environment aﬀect the resonance

frequency far less than with a PCB antenna. For PCB antennas, the reception quality is highly

dependent on the PCB thickness, that is why even the slightest variation of it can detune it and

make it necessary to rematch the ﬁlter network [32]. The ceramic antenna works very well; the

author had no more connection problems with the WiﬁCtrl, even with a bypassed pi-ﬁlter.

4.1.5. Reprogramming of the RFM22B radio module

The original ﬁrmware of the WiﬁControl 433 used the RFM22B just for transmitting data, but

not for receiving data. This is due to the fact that the functions for data reception could not be

completed within the time limitation of the students research project. To enable data reception

an extra receiver had to be connected to the expansion port, but this had been very cumbersome

and had wasted a lot of space. Therefore, one of the ﬁrst changes was the reprogramming of

the RFM22B driver. Before we go into details, here is a brief introduction of the RFM22B. The

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RFXCOM RFXtrx433 Especificaciones Pagina 48