What is the maximum allowed power for ZigBee applications under ETSI EN 300 328?
Although ETSI EN 300 328 allows 20dBm RF output power, there is a limitation for PSD (Power Spectral Density) for wide band modulations other than FHSS. This PSD limit restricts the maximum allowed power for ZigBee applications.
Section 188.8.131.52 in EN 300 328 V2.1.1 details the test method for PSD. The ZigBee modulated signal measured according to Option 2, Step 3 (with sweep time set to 10s):
CW signal with the same power level and spectrum analyzer settings:
The peak power difference between the ZigBee and CW signal measured with RMS detector is ~2.3dB. In order to meet the PSD limit of 10dBm per MHz, the maximum allowed power is 12.3dBm (EIRP).
Note: The 12.3dBm power limit is radiated (EIRP), which means that in case of an antenna with 0dBi, the maximum conducted power allowed is 12.3dBm. For antennas with lower gain than 0dBi, conducted power can be increased, while for antennas with higher gain than 0dBi, conducted power should be reduced.
Measurements were performed on an EFR32MG13 based reference board (BRD4158A Rev A01).
Ideally, it is assumed that the source (generator) and load has 50Ω impedance, thus to transmit a signal from source to load without any losses, the transmission medium also must have 50Ω impedance.
Practically, the source / load impedance solely cannot be guaranteed to be 50Ω, hence additional impedance matching network is required in the circuit. Matching the impedance of complete RF path will make sure that there is minimum reflection loss and thus the antenna will resonate all of the incoming energy at its resonant frequency.
The RF path of EFR32 Series 1 can be divided into 3 parts as follows:
a) EFR32 Matching Network:
After performing load pull experiments, the optimum termination impedance of power amplifier was found as:
Case A: [Tx power level < +10dBm] ----> 20+j10Ω
Case B: [+20dBm > Tx power level > +10dBm] ----> 23+j11.5Ω
This impedance at the power amplifier should be matched to 50Ω to achieve the maximum power transfer from EFR to antenna. A low pass filter is used to transform this impedance and reject unwanted signals.
For case A, 2 element ladder LC low pass filter is enough to transform the impedance from 20+j10Ω to 50Ω. As max Tx power is limited to +10dBm, additional filtering is not required.
For case B, At higher Tx power, 2nd and 3rd order harmonics go beyond the allowable limit by the regulatory bodies. Hence suppressing higher order harmonics becomes important. To suppress these harmonics, a 3 element Pi filter is combined with the 2 element LC match which then results in a 4 element LCLC ladder that acts as a matching network and a low pass filter.
(For component values, detailed analysis and other types of EFR matching networks, Please refer AN930 rev 0.4 or later).
Thus the impedance at the end of EFR matching network in either case has been successfully transformed to ~50Ω.
b) Pigtail connection (Optional):
Even though pigtail connection is optional, we always recommend everyone to keep a provision for a U.FL connector and a series zero Ω resistor in their prototype design, this will allow the engineer to perform some RF tests such as measuring reflection loss, perform conducted test and etc.
This section is divided into 2 subsections:
i) U.FL connector:
To perform RF conducted test, a provision for U.FL connector can be helpful in the circuit. For this the series zero Ω resistor (3 - Res) has to be removed and it has to be placed at branched path towards U.FL connector. This will help us to ensure that the signal sent by the EFR RFIC is equal to the received signal at U.FL connector. Conducted test helps the engineer to verify that there are no reflection losses in the RF path between EFR and pigtail connection.
ii) Pigtail connector:
To measure reflection loss between pigtail connection and antenna, a pigtail connector is used. This can be done by removing zero Ω resistor (3 – Res) and soldering a pigtail connector at the second pad of the removed component. Please make sure that the metal jacket of the pigtail connector is properly soldered to ground pour from the pad till the edge of the board. By measuring the reflection coefficient, the antenna can also be matched to 50 Ω impedance.
This pigtail connection does not include any kind of impedance transformation, thus the impedance at the end of this network and before antenna is ~50Ω.
c) Antenna Matching Network:
The impedance of the antenna also solely cannot be guaranteed to be 50Ω, hence an additional matching network is required. We usually recommend a 3 element pi structured filter irrespective of the antenna type. Using a pigtail connector would be the starting stage of the antenna matching exercise. More information on the antenna matching network can be found at this KBA.
Please note that detailed design procedure and antenna tuning information for inverted F PCB antenna is given in AN1088.