# A Low Cost BLE Transceiver with RX Matching Network Reusing PA Load Inductor for WSNs Applications
## 3 Transceiver Circuit Implementation  
### Transceiver Architecture  
Recent designs for the 2.4 GHz ISM band receiver employed either the sliding-IF [19; 20; 21], direct conversion [22; 23] or low-IF architecture [9], which have been demonstrated to be the most feasible to meet BLE performance requirements under severe power and chip area constraints. The sliding-IF architecture shifts the RF signal into the analog baseband (ABB) signal with twice frequency conversions [24; 25]. This architecture facilitates LO generation and distribution at a favorably lower frequency, but causes a systematic, difficult-to-avoid susceptibility to out-of-band (around 1.45 GHz) image interference. Furthermore, GFSK modulation used in BLE contains significant energy at very low frequencies close to DC. Considering direct conversion architecture is susceptible to DC offset and flicker noise, low-IF architecture is adopted in this work. Figure 7 shows the simplified block diagram of the proposed BLE transceiver. It includes the aforementioned TRX-switch and matching network integrated front-end, a low IF RX, a two-point modulation based TX, a fractional-N synthesizer, digital modulator and demodulator, and several digital signal processing. The RF signal passes first through the integrated TRX-switch and matching network, in which the inductor is shared between RX and TX. The on-chip switch drives the single-end variable-gain LNA, which amplifies the RX signal before it is fed to a current bleeding quadrature down conversion  
Figure 6: (a) RX impedance movement in smith chart; (b) The final NF and S\({}_{11}\) of RX impedance from 2.4 GHz to 2.5 GHz. mixer. A 4 th order complex band pass filter (BPF) follows the mixer, which performs channel selection, image rejection and anti-aliasing filtering, and also serves as a programmable gain amplifier. The IF signal is then digitized by a 3 rd order delta-sigma continuous time (CT) ADC [26]. After that, the signals are further processed in the digital sections, such as additional channel select digital filtering, RSSI estimation, dc offset cancellation, automatic gain control (AGC), IQ imbalance calibration, and demodulation [27]. For BLE intermodulation characteristics, the required signal shall be measured at a power level of 6 dB over the reference sensitivity level [4]. The minimum BLE requirements for the input 3 rd intermodulation point (IIP\({}_{3}\)) and input 2 nd intermodulation point (IIP\({}_{2}\)) can be calculated as follows [28]. IIP\({}_{3}\) (min) = Pin + 1/2 \(\times\) (P\({}_{1}\) - P\({}_{3}\)). Where P\({}_{\text{in}}\) = \(-\)50 dBm, P\({}_{3}\) = \(-\)70 (sensitivity) + 6 (dB) \(-\)14 (SNR) = \(-\)78 dBm and P\({}_{1}\) = \(-\)50 dBm. Thus, IIP\({}_{3}\) (min) = \(-\)36 dBm. A \(-\)22 dBm IIP\({}_{2}\) (min) is calculated in a similar way. However, in order to increase the link margin, IIP\({}_{3}\) above \(-\)20 dBm and IIP\({}_{2}\) above 0 dBm was targeted in this work. BLE TXs usually adopt two architectures: conventionally mixer-based [29], and two-point PLL-based TX [22]. The mixer-based TXs can support universal modulations, but at the expense of high circuit complexity and power consumption. Additionally, the severe PA-to-VCO coupling in mixer-based TXs [30] gives rise to frequency pulling effect, which degrades the TX modulation accuracy and increases output spectral regrowth [31; 32]. In this work, the TX employed a two-point PLL-based architecture with a non-linear PA to eliminate the power-hungry RF mixers and quadrature local oscillator (LO) generators, as shown in Figure 7. High frequency (HF) and low frequency (LF) data paths are produced from digital domain to directly modulate a fractional-N synthesizer working at two times carrier frequency. LF acted as the first point modulation which can be modulated by a slowly-varying frequency modulation (FM) signal. On the other hand, HF extended the FM bandwidth to beyond the PLL bandwidth, which is the second point modulation. After dividing by 2, the modulated signal is directly fed to a nonlinear PA.