Question

At which pins should I apply DC feed for the sub-GHz and 2.4GHz PA of EFR32 Series 1 devices?

Answer

1. 2.4GHz PA DC feed

The EFR32 Series 1 2.4GHz PA has a dedicated power supply input pin (PAVDD), the 2.4GHz PA should be powered through this pin:

On the sub-GHz side, the PA does not have a dedicated power supply input pin, therefore the sub-GHz PA needs to be powered through the TX output pins: SUBGRF_OP and SUBGRF_ON. Depending on the matching network type and the sub-GHz frequency band, different power supply schemes are needed for the sub-GHz PA:

2. Discrete matching network + ceramic balun for high sub-GHz frequency bands (>500MHz)

For high frequency bands (>500MHz) the recommended ceramic balun type is 0900BL15C050 from Johanson Technology. This balun has a dedicated BIAS input pin, which is in connection with BAL1 and BAL2 pins of the balun, therefore the DC bias will be present on the TX output pins supplying the sub-GHz PA.

An additional VDD filtering capacitor (C10) is required at the BIAS pin of the balun with appropriate value (see the EFR32 Series 1 Reference designs for the proper component values).

3. Discrete matching network + ceramic balun for low sub-GHz frequency bands (<500MHz)

For low frequency bands (<500MHz) the recommended ceramic balun type is ATB2012-50011 from TDK Corporation. This balun has no dedicated BIAS input pin, which means that the PA power supply should be connected to the TX output pins on the other side of the ceramic balun. In order to avoid the PA supply circuit loading the PA matching network, the PA DC feed should be connected to the TX pins through choke inductors (L4 and L5) with appropriate values so that the inductors show high impedance at the frequency of operation.

An additional VDD filtering capacitor (C14) is required at the biasing point prior to the choke inductors (L4 and L5) with appropriate value (see the EFR32 Series 1 Reference designs for the proper component values).

4. Full discrete matching network for any sub-GHz frequency bands

In the full discrete match, the ceramic balun and the TX matching network is replaced by a 4-element discrete balun. The discrete balun contains a shunt inductor (L1), which can be separated into 2 halves, and the PA DC feed can be connected to the common point of these inductors (L1-1 and L1-2). Also, a capacitor for improved 2nd harmonic suppression (CH) is connected at this point.

In order to avoid the PA supply circuit loading the PA matching network, a choke inductor is necessary (LDC).

For further details on the full discrete matching solution, see AN1180: EFR32 Series 1 sub-GHz Discrete Matching Solutions.

5. IPD matching network for any sub-GHz frequency bands

EFR32 Series 1 IPDs are available for 434MHz and 868MHz frequency bands, see additional details in AN1081: Integrated Passive Devices for EFR32 Series 1 Sub-GHz RF Matching.

All 4 IPD types listed in AN1081 have dedicated VDD pins, which are in connection with the TXP and TXN pins of these IPDs, therefore the DC bias will be present on the TX output pins supplying the sub-GHz PA.

Note: When PAVDD (2.4GHz PA) = VBIAS (sub-GHz PA), limiting supply voltage of 2.4GHz PA to transmit at lower power will also limit the supply voltage at sub-GHz. Hence if different max TX power levels are expected from 2.4GHz and sub-GHz PA, then PAVDD and VBIAS net should be supplied separately with appropriate power supply voltages.

Question

At which pins should I apply DC feed for the sub-GHz and 2.4GHz PA of EFR32 Series 1 devices?

Answer

1. 2.4GHz PA DC feed

The EFR32 Series 1 2.4GHz PA has a dedicated power supply input pin (PAVDD), the 2.4GHz PA should be powered through this pin:

On the sub-GHz side, the PA does not have a dedicated power supply input pin, therefore the sub-GHz PA needs to be powered through the TX output pins: SUBGRF_OP and SUBGRF_ON. Depending on the matching network type and the sub-GHz frequency band, different power supply schemes are needed for the sub-GHz PA:

2. Discrete matching network + ceramic balun for high sub-GHz frequency bands (>500MHz)

For high frequency bands (>500MHz) the recommended ceramic balun type is 0900BL15C050 from Johanson Technology. This balun has a dedicated BIAS input pin, which is in connection with BAL1 and BAL2 pins of the balun, therefore the DC bias will be present on the TX output pins supplying the sub-GHz PA.

An additional VDD filtering capacitor (C10) is required at the BIAS pin of the balun with appropriate value (see the EFR32 Series 1 Reference designs for the proper component values).

3. Discrete matching network + ceramic balun for low sub-GHz frequency bands (<500MHz)

For low frequency bands (<500MHz) the recommended ceramic balun type is ATB2012-50011 from TDK Corporation. This balun has no dedicated BIAS input pin, which means that the PA power supply should be connected to the TX output pins on the other side of the ceramic balun. In order to avoid the PA supply circuit loading the PA matching network, the PA DC feed should be connected to the TX pins through choke inductors (L4 and L5) with appropriate values so that the inductors show high impedance at the frequency of operation.

An additional VDD filtering capacitor (C14) is required at the biasing point prior to the choke inductors (L4 and L5) with appropriate value (see the EFR32 Series 1 Reference designs for the proper component values).

4. Full discrete matching network for any sub-GHz frequency bands

In the full discrete match, the ceramic balun and the TX matching network is replaced by a 4-element discrete balun. The discrete balun contains a shunt inductor (L1), which can be separated into 2 halves, and the PA DC feed can be connected to the common point of these inductors (L1-1 and L1-2). Also, a capacitor for improved 2nd harmonic suppression (CH) is connected at this point.

In order to avoid the PA supply circuit loading the PA matching network, a choke inductor is necessary (LDC).

For further details on the full discrete matching solution, see AN1180: EFR32 Series 1 sub-GHz Discrete Matching Solutions.

5. IPD matching network for any sub-GHz frequency bands

EFR32 Series 1 IPDs are available for 434MHz and 868MHz frequency bands, see additional details in AN1081: Integrated Passive Devices for EFR32 Series 1 Sub-GHz RF Matching.

All 4 IPD types listed in AN1081 have dedicated VDD pins, which are in connection with the TXP and TXN pins of these IPDs, therefore the DC bias will be present on the TX output pins supplying the sub-GHz PA.

Note: When PAVDD (2.4GHz PA) = VBIAS (sub-GHz PA), limiting supply voltage of 2.4GHz PA to transmit at lower power will also limit the supply voltage at sub-GHz. Hence if different max TX power levels are expected from 2.4GHz and sub-GHz PA, then PAVDD and VBIAS net should be supplied separately with appropriate power supply voltages.

Question

What difference can be observed in RF performance when connecting the GND pin of TCXO to Pin 18 (GNDX) of Si446x or connecting it to the common GND?

Answer

In case of using TCXO, a few dB lower (2-3 dB) reference spurs (fxtal away on both sides of the carrier) can be observed when connecting the GND pin of TCXO to Pin 18 (GNDX). Connecting to either GNDX or to the common GND has no effect on any other RF parameters.

For the proper connection of GNDX pin, please refer to the following Knowledge Base Article:

https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2014/10/16/si446x_pin_18_gndx-ijHv

Question

Which data rate, deviation and modulation index should I use?

Answer

Based on https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2015/06/09/modulation_choice-oUw0

the recommended modulation is 2GFSK, so from now on, let's consider only this type of modulation.

The occupied bandwidth of a 2GFSK signal can be estimated based on the following formula:

The modulation index can be calculated in the following way:

According to the formula for occupied bandwidth, the OBW value is higher with higher data rate and deviation. If one is using lower OBW, the receiver BW can also be set lower, which reduces the noise level in the receiver, and this way improves sensitivity. Hence it seems a good idea to transmit with as low data rate as possible, but applications often require a minimum data rate to send the necessary information in the available time slot. The data rate and the crystal tolerance should be chosen to stay within the allowed channel under extreme conditions as well according to the related regulation standard. Beside this, depending on the crystal accuracy the possibility of frequency offset between the transmitter and the receiver can grow. The right choice for data rate and crystal type depends on several parameters: carrier frequency, carrier power, channel bandwidth, modulation index, and of course the data rate and the crystal type cannot be considered separately.  A higher frequency signal introduces higher absolute frequency error with the same type of crystal. A high power application can violate the modulation bandwidth regulation easier, especially for narrow band channels (e.g. 25 kHz). Also, the modulation index determines the occupied bandwidth: higher modulation index introduces higher bandwidth.

To investigate the modulation index, let’s consider 3 typical instances: H = 1, H = 0.5, H = 2. If we choose the desired data rate for our application, the deviation determines the used modulation index. Also, the deviation determines the distance between the transmitted symbols: higher modulation index introduces more distance between symbols. Simultaneously, a higher data rate introduces higher occupied bandwidth, which causes that wider bandwidth is necessary in the receiver, this way a higher thermal noise will appear and the sensitivity will be worse. Thus, by increasing the deviation, the distance between symbols will also grow, which improves sensitivity, but on the other side, the occupied bandwidth will be wider, which causes sensitivity loss. These two effects seems to compensate each other, but there is an optimal choice, at H = 1. This is the value where the receiver sensitivity is at the optimal level, thus this is the recommended modulation index.

To sum up, the recommended modulation index for 2GFSK modulated signals is H =1. The data rate and the crystal type should be chosen based on the requirements of the application to meet the standard regulations. If the data rate is chosen, the H =1 modulation index determines the necessary deviation.

Question

Which data rate, deviation and modulation index should I use?

Answer

Based on https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2015/06/09/modulation_choice-oUw0 the recommended modulation is 2GFSK, so from now on, let's consider only this type of modulation.

The occupied bandwidth of a 2GFSK signal can be estimated based on the following formula:

The modulation index can be calculated in the following way:

According to the formula for occupied bandwidth, the OBW value is higher with higher data rate and deviation. If one is using lower OBW, the receiver BW can also be set lower, which reduces the noise level in the receiver, and this way improves sensitivity. Hence it seems a good idea to transmit with as low data rate as possible, but applications often require a minimum data rate to send the necessary information in the available time slot. The data rate and the crystal tolerance should be chosen to stay within the allowed channel under extreme conditions as well according to the related regulation standard. Beside this, depending on the crystal accuracy the possibility of frequency offset between the transmitter and the receiver can grow. The right choice for data rate and crystal type depends on several parameters: carrier frequency, carrier power, channel bandwidth, modulation index, and of course the data rate and the crystal type cannot be considered separately.  A higher frequency signal introduces higher absolute frequency error with the same type of crystal. A high power application can violate the modulation bandwidth regulation easier, especially for narrow band channels (e.g. 25 kHz). Also, the modulation index determines the occupied bandwidth: higher modulation index introduces higher bandwidth.

To investigate the modulation index, let’s consider 3 typical instances: H = 1, H = 0.5, H = 2. If we choose the desired data rate for our application, the deviation determines the used modulation index. Also, the deviation determines the distance between the transmitted symbols: higher modulation index introduces more distance between symbols. Simultaneously, a higher data rate introduces higher occupied bandwidth, which causes that wider bandwidth is necessary in the receiver, this way a higher thermal noise will appear and the sensitivity will be worse. Thus, by increasing the deviation, the distance between symbols will also grow, which improves sensitivity, but on the other side, the occupied bandwidth will be wider, which causes sensitivity loss. These two effects seems to compensate each other, but there is an optimal choice, at H = 1. This is the value where the receiver sensitivity is at the optimal level, thus this is the recommended modulation index.

To sum up, the recommended modulation index for 2GFSK modulated signals is H =1. The data rate and the crystal type should be chosen based on the requirements of the application to meet the standard regulations. If the data rate is chosen, the H =1 modulation index determines the necessary deviation.

Question

Which data rate, deviation and modulation index should I use?

Answer

Based on https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2015/06/09/modulation_choice-oUw0, the recommended modulation is 2GFSK, so from now on, let's consider only this type of modulation.

The occupied bandwidth of a 2GFSK signal can be estimated based on the following formula:

The modulation index can be calculated in the following way:

According to the formula for occupied bandwidth, the OBW value is higher with higher data rate and deviation. If one is using lower OBW, the receiver BW can also be set lower, which reduces the noise level in the receiver, and this way improves sensitivity. Hence it seems a good idea to transmit with as low data rate as possible, but applications often require a minimum data rate to send the necessary information in the available time slot. The data rate and the crystal tolerance should be chosen to stay within the allowed channel under extreme conditions as well according to the related regulation standard. Beside this, depending on the crystal accuracy the possibility of frequency offset between the transmitter and the receiver can grow. The right choice for data rate and crystal type depends on several parameters: carrier frequency, carrier power, channel bandwidth, modulation index, and of course the data rate and the crystal type cannot be considered separately.  A higher frequency signal introduces higher absolute frequency error with the same type of crystal. A high power application can violate the modulation bandwidth regulation easier, especially for narrow band channels (e.g. 25 kHz). Also, the modulation index determines the occupied bandwidth: higher modulation index introduces higher bandwidth.

To investigate the modulation index, let’s consider 3 typical instances: H = 1, H = 0.5, H = 2. If we choose the desired data rate for our application, the deviation determines the used modulation index. Also, the deviation determines the distance between the transmitted symbols: higher modulation index introduces more distance between symbols. Simultaneously, a higher data rate introduces higher occupied bandwidth, which causes that wider bandwidth is necessary in the receiver, this way a higher thermal noise will appear and the sensitivity will be worse. Thus, by increasing the deviation, the distance between symbols will also grow, which improves sensitivity, but on the other side, the occupied bandwidth will be wider, which causes sensitivity loss. These two effects seems to compensate each other, but there is an optimal choice, at H = 1. This is the value where the receiver sensitivity is at the optimal level, thus this is the recommended modulation index.

To sum up, the recommended modulation index for 2GFSK modulated signals is H =1. The data rate and the crystal type should be chosen based on the requirements of the application to meet the standard regulations. If the data rate is chosen, the H =1 modulation index determines the necessary deviation.

Question

Which modulation (OOK, 2FSK, 2GFSK, 4FSK, 4GFSK) should I use?

Answer

Let’s compare the above listed modulation types in pairs.

1. OOK vs. FSK

OOK is a digital amplitude modulation type, which represents 2 different states: the presence or absence of the carrier based on the symbol to be transmitted. Contrarily, for FSK modulation the amplitude of the transmitted signal is fixed, but the frequency can be different based on the given symbol.

The consecutive on-off switching of the PA for OOK modulation causes that the bandwidth increases compared to FSK modulation using the same data rate, this way FSK is a more spectrally efficient modulation. The wider bandwidth makes it necessary to apply a wider filter in the receiver, which increases the noise level and this way the sensitivity will get worse. If the occupied bandwidth is the same for OOK and FSK modulation, the sensitivity is similar (but in this case OOK modulation has lower data rate to have the same bandwidth as FSK). Beside these, OOK modulation is more sensitive for fading, since the information is carried only by the amplitude. Along with this, FSK modulation is more sensitive for the frequency offsets between the transmitter and the receiver.

To sum up, FSK modulation is more spectrally efficient so has better sensitivity and is less sensitive for fading. OOK modulation is less sensitive for the frequency inaccuracy, and thus is commonly used in applications where the frequency accuracy can not be guaranteed.

The above figure shows the spectrum of an OOK modulated signal with 40 kbps data rate and a 2FSK modulated signal with 40 kbps data rate and 20 kHz deviation.

2. FSK vs. GFSK

FSK modulation creates high level spurious contents (at integer multiples of the symbol rate) as well as relatively high side lobes on the transmitter side, which can cause regulation standard violations. A Gaussian filter can be applied to the symbols before creating the frequency modulated signal to suppress these spurs and side lobes by smoothing the baseband signal. This way the bandwidth can be slightly reduced, but the distance between symbols will decrease which causes a slightly worse receiver sensitivity (~0.5 dB) for GFSK modulated signals. Still, GFSK is a generally used modulation type, since it reduces spurious contents on the transmitter side significantly and the loss in sensitivity is negligible.

The above figure shows the spectrum of a 2FSK and a 2GFSK modulated signal with 40 kbps data rate and 20 kHz deviation.

3. 2(G)FSK vs. 4(G)FSK

A typical application of 4(G)FSK modulation is transmitting with the same data rate but occupying only half the bandwidth of the 2(G)FSK signal. For example, 2GFSK, 40kbps data rate, 20kHz deviation (OBW = 80 kHz) and 4GFSK, 20ksps symbol rate (= 40 kbps data rate), 10 kHz outer deviation (= 10/3 kHz inner deviation) (OBW = 40 kHz). The smaller bandwidth introduces lower noise level in the receiver, this results sensitivity improvement. On the other hand, the deviation on inner symbols will be lower, which causes sensitivity loss. The two effects compensate each other, but the latter one has a stronger effect on sensitivity, so eventually in this case 4(G)FSK will have slightly (~2 dB) worse sensitivity than 2(G)FSK.

To have the same occupied bandwidth for 4(G)FSK as 2(G)FSK, we need to use higher data rate. For example, 2GFSK, 100kbps data rate, 50kHz deviation (OBW = 200 kHz) and 4GFSK, 100ksps symbol rate (= 200 kbps data rate), 50 kHz outer deviation (= 50/3 kHz inner deviation) (OBW = 200 kHz). This way, the occupied bandwidth will be the same, but the (inner) deviation will be less for 4(G)FSK, eventually this results sensitivity degradation (~5 dB).

For more details on sensitivity difference between 2(G)FSK and 4(G)FSK modulation, refer to https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2015/06/05/4gfsk_vs_2gfsk_sens-u6p6

To sum up, 4(G)FSK is generally used to transmit with the same data rate in half the bandwidth of 2(G)FSK, or to occupy the same bandwidth using a higher data rate. In both cases 4(G)FSK will have worse sensitivity.

The above figure shows the spectrum of a 2GFSK modulated signal with 40 kbps data rate and 20 kHz deviation and a 4GFSK modulated signal with 20 ksps symbol rate and 10 kHz outer deviation.

Based on the above explained reasons, our recommendation is 2GFSK modulation, since it is more spectrally efficient than OOK or 2FSK, the Gaussian filter suppresses the spurious contents and side lobes on the transmitter side significantly while the receiver sensitivity is just slightly worse than 2FSK and a few dB better than 4GFSK.

Question

Which modulation (OOK, 2FSK, 2GFSK, 4FSK, 4GFSK) should I use?

Answer

Let’s compare the above listed modulation types in pairs.

1. OOK vs. FSK

OOK is a digital amplitude modulation type, which represents 2 different states: the presence or absence of the carrier based on the symbol to be transmitted. Contrarily, for FSK modulation the amplitude of the transmitted signal is fixed, but the frequency can be different based on the given symbol.

The consecutive on-off switching of the PA for OOK modulation causes that the bandwidth increases compared to FSK modulation using the same data rate, this way FSK is a more spectrally efficient modulation. The wider bandwidth makes it necessary to apply a wider filter in the receiver, which increases the noise level and this way the sensitivity will get worse. If the occupied bandwidth is the same for OOK and FSK modulation, the sensitivity is similar (but in this case OOK modulation has lower data rate to have the same bandwidth as FSK). Beside these, OOK modulation is more sensitive for fading, since the information is carried only by the amplitude. Along with this, FSK modulation is more sensitive for the frequency offsets between the transmitter and the receiver.

To sum up, FSK modulation is more spectrally efficient so has better sensitivity and is less sensitive for fading. OOK modulation is less sensitive for the frequency inaccuracy, and thus is commonly used in applications where the frequency accuracy can not be guaranteed.

The above figure shows the spectrum of an OOK modulated signal with 40 kbps data rate and a 2FSK modulated signal with 40 kbps data rate and 20 kHz deviation.

2. FSK vs. GFSK

FSK modulation creates high level spurious contents (at integer multiples of the symbol rate) as well as relatively high side lobes on the transmitter side, which can cause regulation standard violations. A Gaussian filter can be applied to the symbols before creating the frequency modulated signal to suppress these spurs and side lobes by smoothing the baseband signal. This way the bandwidth can be slightly reduced, but the distance between symbols will decrease which causes a slightly worse receiver sensitivity (~0.5 dB) for GFSK modulated signals. Still, GFSK is a generally used modulation type, since it reduces spurious contents on the transmitter side significantly and the loss in sensitivity is negligible.

The above figure shows the spectrum of a 2FSK and a 2GFSK modulated signal with 40 kbps data rate and 20 kHz deviation.

3. 2(G)FSK vs. 4(G)FSK

A typical application of 4(G)FSK modulation is transmitting with the same data rate but occupying only half the bandwidth of the 2(G)FSK signal. For example, 2GFSK, 40kbps data rate, 20kHz deviation (OBW = 80 kHz) and 4GFSK, 20ksps symbol rate (= 40 kbps data rate), 10 kHz outer deviation (= 10/3 kHz inner deviation) (OBW = 40 kHz). The smaller bandwidth introduces lower noise level in the receiver, this results sensitivity improvement. On the other hand, the deviation on inner symbols will be lower, which causes sensitivity loss. The two effects compensate each other, but the latter one has a stronger effect on sensitivity, so eventually in this case 4(G)FSK will have slightly (~2 dB) worse sensitivity than 2(G)FSK.

To have the same occupied bandwidth for 4(G)FSK as 2(G)FSK, we need to use higher data rate. For example, 2GFSK, 100kbps data rate, 50kHz deviation (OBW = 200 kHz) and 4GFSK, 100ksps symbol rate (= 200 kbps data rate), 50 kHz outer deviation (= 50/3 kHz inner deviation) (OBW = 200 kHz). This way, the occupied bandwidth will be the same, but the (inner) deviation will be less for 4(G)FSK, eventually this results sensitivity degradation (~5 dB).

For more details on sensitivity difference between 2(G)FSK and 4(G)FSK modulation, refer to https://www.silabs.com/community/wireless/proprietary/knowledge-base.entry.html/2015/06/05/4gfsk_vs_2gfsk_sens-u6p6.

To sum up, 4(G)FSK is generally used to transmit with the same data rate in half the bandwidth of 2(G)FSK, or to occupy the same bandwidth using a higher data rate. In both cases 4(G)FSK will have worse sensitivity.

The above figure shows the spectrum of a 2GFSK modulated signal with 40 kbps data rate and 20 kHz deviation and a 4GFSK modulated signal with 20 ksps symbol rate and 10 kHz outer deviation.

Based on the above explained reasons, our recommendation is 2GFSK modulation, since it is more spectrally efficient than OOK or 2FSK, the Gaussian filter suppresses the spurious contents and side lobes on the transmitter side significantly while the receiver sensitivity is just slightly worse than 2FSK and a few dB better than 4GFSK.