Modulation rate study in spin torque oscillator based wireless communication system

Abstract

Wireless transmission requires a circuitry to generate GHz range signal . Spin transfer torque [1,2] devices are one of the promising candidates for generating stable signal in the GHz range through collective precession of the local spins in one of the ferromagnetic layer, known as free layer . Another ferromagnetic layer, fixed layer, acts as a medium to convert the incoming electric current into spin polarized current . This spin polarized current drives local spins of the free layer into constant precession . Factors like high linewidth, low output power and modulation rate are yet to be optimized to make these devices ready for use in practical wireless communication system . Recently, H . S . Choi et al. [4], has demonstrated the first ever complete wireless communication system using spin torque nano-oscillators (STNOs) through the amplitude shift keying (ASK) modulation . Here, we present a more detailed study of wireless communication as a function of the modulation frequency and distance between receiver and transmitting antennas (T x ) in order to determine modulation rate limits . The STNO device under study is the same as in ref [3] . The setup used for wireless communication is shown in Fig 1(a) . The setup uses a function generator to excite the device with a square wave voltage signal (V bias ) . The resulted output signal is then transmitted by an antenna with 5 dB gain after 35 dB amplification using a low noise amplifier (LNA) . On the receiver part, the signal is first amplified by another LNA (33 dB gain), placed next to the receiver antenna . The signal is finally detected by a 4 GHz oscilloscope .We use a numerical envelope detector to extract the envelope out of the received waveform . We performed a systematic study by varying the modulation frequency (f mod =1 kHz to 2 MHz) and distance T x = 25 cm to 150 cm . Here we choose an applied field H ext = 220 Oe, V bias = 260 mV (equivalent to 4 mA), and φ =192 (the angle between free and reference layer) to get a single mode oscillation with maximum power [3] . At this condition, the STNO frequency is 3 .6 GHz with peak power of -73 dBm (without amplification) and linewidth of 101 MHz . The Power of the signal is measured at points 1 and 2 by connecting a spectrum analyzer in separate measurements as shown in fig . 1(a) . The total loss during the wireless transmission can be calculated as: Loss =Power at point 1 + Transmitter antenna gain (5 dB) -Air path loss + Receiver antenna gain (5 dB) -Power at point 2 . , where, air path loss=20log 10 (4πT x /λ) . It can be seen from figures 2(a-b) that increasing the distance degrades the signal peak amplitude and increases linewidth . Figure 2(c) shows an increase of loss with distance . In fig . 2(d) we discuss modulation frequency dependence of modulated signal before transmission . We found a reduction of peak amplitude V ON with increasing f mod during ASK modulation on the transmitter side . This leads to an increase of total loss as the modulation frequency is increased . With tolerable losses...

Analog and digital phase modulation of spin torque nano-oscillators

“…Such applications require solutions which are wideband, compact, low cost and autonomous. Recent advances in the field of spintronics 3,4 led to active developments of communication systems [5][6][7] based on spin-torque nano-oscillators (STNO) using basic modulation techniques 5,8 . Spin-torque nano-oscillators (STNOs) are potential candidates to be used as rf sources within transmitter-receiver blocks due to their large frequency tunability, nanoscale size and and multifunctionality.…”

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Evaluation of radiated immunity tests using adjusted pulse modulated signals

“…EMC laboratories are usually provided with simple signal generators, only capable of synthetising some analog modulation schemes, albeit enough to follow the RI standards requirements. On the other hand, real‐life waveforms are usually synthetised through complex processes, involving digital modulation of the base‐band data and many other steps [14–17]. Creating such waveform in an EMC laboratory might demand signal generators with a more specific hardware (e.g .…”

Current Modulation of Nanoconstriction Spin-Hall Nano-Oscillators

“…It also holds great promise even for the realization of universal memory [5][6][7]. Extensive research by the spintronics community has aimed at achieving nanoscale spin-based microwave components, such as microwave generators [8][9][10][11], rectifiers [12], spin diodes [13], spin-wave generators [14,15], and modulators [16][17][18][19][20][21][22][23], which could be used as building blocks of communication systems.…”