# A Low Cost BLE Transceiver with RX Matching Network Reusing PA Load Inductor for WSNs Applications
## 2 Proposed Front-End with RX Matching Network Reusing PA Load Inductor  
An integrated TRX-switch/matching network presents some design challenges. Firstly, reliability issues will be introduced by the large PA output power through stressing the low noise amplifier (LNA) input transistors. This requires an input voltage swing reduction of LNA at TX mode, and thus a large RX off-state impedance (ZRX-off). Secondly, the TX performance features, such as output power, error vector magnitude (EVM) should not be degraded by the ZRX-off. Similarly and finally, the off-state TX impedance (ZTX-off) should not deteriorate RX performance such as NF and linearity.  
BLE specification asks for only \(-70\) dBm sensitivity (1-MHz channel bandwidth) at GFSK modulation with a required SNR of 14 dB [4]. The NF can thus be obtained by:  
\[\text{NF}=-(-174\text{ dBm}/\text{Hz})-10\times\text{Log(BW)}-\text{SNR}_{ \text{out}}+\text{Sensitivity}, \tag{1}\]  
Figure 1: The conventional RF front-end configuration.  
where BW and SNR\({}_{\text{out}}\) represent channel bandwidth and required demodulator signal-to-noise ratio (SNR), respectively. The maximum allowable NF obtained using (1) is 30 dB. However, in order to increase the link budget, a NF below 7 dB was targeted in this work. This allows a reasonable matching network design trade-off between RX noise and power delivery.  
Figure 2a shows the simplified schematic of the proposed front-end with RX matching network reusing PA load inductor. The front-end integrated the TRX-switch and matching network. The PA load inductor L\({}_{1}\) is reused as the impedance matching component for RX. Capacitors C\({}_{1}\) and C\({}_{2}\) are employed to match the impedance in both RX and TX paths. Switches S\({}_{1}\)-S\({}_{9}\) are adopted to determine the mode of the front-end circuit.  
In TX mode, S\({}_{1}\), S\({}_{4}\), S\({}_{7}\), S\({}_{5}\) and S\({}_{9}\) are turned off while S\({}_{2}\), S\({}_{3}\), S\({}_{5}\), S\({}_{8}\) and S\({}_{9}\) are turned on. The inductor L\({}_{1}\) acts as the load of the PA which is typically supplied by DC-DC convertor through the PM\({}_{2}\). For PM\({}_{2}\) size, it is determined by the following three factors: (1) The on-resistance. Large on-resistance causes a large voltage drop, and thus reduces the efficiency of PA. (2) The parasitic resistance and capacitance when PM\({}_{2}\) is turned off. RX NF might thus be degraded. (3) The current density. The size of PM\({}_{2}\) should be large to accommodate large PA current. Therefore, the size of  
Figure 2: (a) Proposed front-end with RX matching network reusing PA load inductor; (b) Simplified LNA circuit in TX mode; (c) Simplified PA circuit in RX mode.  
PM\({}_{2}\) is chosen to be 0.11 \(\upmu\)m/200 \(\upmu\)m, where the width is 5 \(\upmu\)m and the finger number is 40. For LNA, the input transistors NM\({}_{1}\) and PM\({}_{1}\) are turned off and the simplified circuit is shown in Figure 2b. Consequently, the PA sees the impedance of the LNA path (ZRX-off) as a small capacitor shunted with a large resistor given as R\({}_{\rm RL}\|1/\rm j\omega C_{RL}\). Around 2.45 GHz, simulation shows the real and imaginary parts of ZRX-off are 2.6 k\(\Omega\) and 95 fF, respectively. Simulation shows about 4% PA efficiency loss due to ZRX-off and PM\({}_{2}\).  
In RX mode, S\({}_{1}\), S\({}_{4}\), S\({}_{7}\), S\({}_{5}\) and S\({}_{9}\) are turned on and S\({}_{2}\), S\({}_{3}\), S\({}_{5}\), S\({}_{8}\) and S\({}_{9}\) are turned off. For PA, the transistors (NM\({}_{2}\), NM\({}_{3}\) and PM\({}_{2}\)) are turned off and the simplified circuit is shown in Figure 2c. Similarly, LNA sees PA path as a large impedance (ZTX-off), consisting of a small capacitor shunted with a large resistor and can be given as ZTX-off = R\({}_{\rm TL}\|1/\rm j\omega C_{TL}\). Around 2.45 GHz, the simulated real and imaginary parts of ZTX-off are 16 k\(\Omega\) and 66 fF, respectively. This large impedance ZTX-off results in a 0.4 dB insertion loss, which is acceptable for the NF requirement of BLE.