# Spintronic RF-Direct on-off Keying Modulation Using a Frequency Division Multiplex
## 4 Design of the Spin Demodulator  
As shown in Figure 4, we applied a broadband antenna to the receiver to accommodate all channels with the same type of antenna, and located a band pass filter (BPF) immediately after each antenna in order to separately select and detect the target signal on a channel assigned at each STNO. The antenna used is ANT1085-4R1-01A from TDK, and the bandwidth of the antenna used is sufficiently covered from 3.1 GHz to 5.2 GHz, with an antenna gain of 2 dBi. BPF applied [PERSON]'s 3600 BP for the 3.5 GHz band and [PERSON]'s 4224-to-4752 for the 4.2 GHz band. BPF has an insertion loss of 1.5 dB and 2.0 dB for 3.5 GHz and 4.2 GHz, respectively. In particular, isolation between channels is important for receiving a signal without interference in the condition of two channels on a single board. Isolation for BPF is above 35.5 dB at 4.2 GHz when transmitting a 3.5 GHz signal, and above 14.8 dB at 3.5 GHz when transmitting a 4.2 GHz signal. For good sensitivity of the receiver, a Low Noise Amplifier (LNA) was placed immediately after each BPF. The LNA used was applied to Avago's MGA86563. The LNA has a noise figure of 1.8 dB and 2.0 dB at 3.5 GHz and 4.2 GHz, respectively, and the gain is measured as 20.0 dB and 17.1 dB, respectively. In particular, isolation between channels is important for receiving a signal well without interference in a two channels condition on a single board. Figure 9 shows the isolation and gain characteristics obtained from measurement, together with the BPF and LNA. The measured results are a gain of 20.0 dB and 17.1 dB, and isolation of 14.8 dB and 35.5 dB at 3.5 GHz and 4.2 GHz, respectively, considering BPF loss of 1.5 dB and 2.0 dB. We additionally could situate the attenuator of \(-8\) dB after the LNA at the 3.5 GHz path to secure more isolation even in the wired condition between the transmitter and receiver. In this study, we targeted wireless communication under 10 mm distance between the transmitter and receiver, considering the used MTJ pillar has approximately a \(-71\) dBm output power level. The link-budget for short distance range communication is calculated with the bandwidth, taking into consideration the signal purity as well as the power level, antenna characteristics, path loss, and noise figure in the receiver. Table 1 shows the link-budget of the wireless communication for 10 mm distance when making the STNO operate at  
Figure 8: Transient results comparing simulation and measurement results for the modulation. 3.5 GHz and 4.2 GHz. The link budget is calculated with the air loss as 5 dB in a distance of 10 mm. The power level transmitted through the antenna is \(-72.0\) dBm and \(-72.5\) dBm at 3.5 GHz and 4.5 GHz, and the thermal noise level is \(-92.9\) dBm in the 130 MHz STNO linewidth. The SNR in the transmitter is thus 20.9 dB (\(=-72.0+92.9\)) and 20.4 dB (\(=-72.5+92.9\)), respectively. The transmission margin is 11.9 dB (\(=20.9-9.0\)) and 11.4 dB (\(=20.4-9.0\)), as the required SNR is 9.0 dB for the \(10^{-3}\) bit error rate (BER) in the OOK system. In the receiver we tested at a distance 10 mm, the system noise figure is 4.3 dB and 4.5 dB at 3.5 GHz and 4.2 GHz, respectively, and the minimum sensitivity of the receiver is optimally \(-79.6\) dBm and \(-79.4\) dBm at 3.5 GHz and 4.2 GHz, respectively, considering the system NF and the required SNR. The transmission margin is then 4.6 dB (\(=-75.0+79.6\)) and 3.9 dB (\(=-75.5+79.4\)) at 3.5 GHz and 4.2 GHz, respectively. The margin for 3.9 dB corresponds to supporting transmission up to 15.8 mm distance in a 4.2 GHz communication system. Figure 9: Measurement results of the BPF and LNA for the 3.5 GHz and 4.2 GHz signal paths. (**a**) Results of the gain and isolation at 3.5 GHz when transmitting a 4.2 GHz signal (20.0 dB for gain, 14.8 dB for isolation) (**b**) Results of the gain and isolation at 4.2 GHz when transmitting a 3.5 GHz signal (17.1 dB for gain, 35.5 dB for isolation).