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RF Front End Module for a dual-band radio/antenna design

Krishna Prasad Skrishprasads@gmail.com

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RFFE Module Description

• For the purpose of this report, the assumed design of Laird Sentrius module is considered

• Laird Sentrius is a module built by Laird Inc.• Module consists of LoRa radio + BLE radio• LoRa + BLE is a low cost recipe for enterprise IoT• Applications include:

– greenhouses, – grain storage, – cold chain monitoring, – cooling tanks– or any application that requires constant monitoring and

reporting of temperature fluctuations, humidity, and other critical metrics

http://www.summitdata.com/blog/new-lorawan-ble-sensor-platform-for-long-range-iot-applications/

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RFFE Module Block Diagram

SemtechSX1272

LoRa Radio

NordicnRF52xxx

BLE Radio

Balun

Diplexer

• Semtech’s LoRa radio operates at ~915MHz ISM band– Semtech radio outputs in differential (balanced) mode for

enhanced common mode noise immunity– Balun is designed to converted balanced to unbalanced (single

ended) trace to link the antenna• BLE radio works between 2.4 ~ 2.5GHz ISM band• Since both radios share the same antenna, a diplexer

is designed to filter out the respective signals• A dual band antenna design is also shown along with

it’s matching components

Antenna Matching

TxRx_P

TxRx_N

TxRx

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BALUN DESIGN

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Lumped Balun design

• A Balun is a component used for converting differential signals to single ended signals and vice-versa

• Thus, a balun is essentially a power divider that produces a phase shift of 180o between it’s outputs, producing a differential signal

• Each line produces a +90o and -90o through high-pass and low-pass filter circuitry

• The disadvantage of a lumped balun is that it’s bandwidth is very limited owing to the narrow Q factors of the L-C resonant circuits used

Balun

Rinner = 𝑅𝑖𝑛𝑅𝐿 = 100 . 50= 71 Ω

L = *+,,-./ .𝝅 .1

= 12.3 𝑛𝐻

C = 5/ .𝝅 .1. *+,,-.

= 2.45 𝑝𝐹

Where f = 915 MHz

https://analog.intgckts.com/impedance-matching/lumped-lc-balun/https://www.markimicrowave.com › assets › appnotes › balun_basics_primer

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Lumped Balun Performance

Like a divider, the balun shows 3dB insertion loss … and excellent return loss

across CM (4) and DM (5) ports

Outputs (2, 3) show a phase different of 180 degrees, as expected

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DIPLEXER DESIGN

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Lumped Diplexer design• A Diplexer is a pair of filters used for directing

the incoming signal to two different destinations based on it’s frequency

• The filter in this implementation is done using lumped elements in a ‘T’ configuration

• Diplexers provide low insertion loss in the passband and high isolation between each other, which is especially important to prevent intermods and harmonics

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ANTENNA DESIGN

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LoRa & BLE dual band PCB antenna design• An Inverted F antenna with dual arms

corresponding to dual resonance is shown• After the required matching network is installed,

the antenna shows excellent return loss and efficiency

• This antenna is then connected to the diplexer to direct the TX/RX signal to it’s respective destination

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Conclusion

• Thus the entire front end including a dual band antenna is designed, with the required balun and a diplexer