Analog and Digital Phase Modulation and Signal Transmission with Spin-Torque Nano-Oscillators

Spin-torque nano-oscillators (STNOs) are nanoscale devices with wide-band-frequency tunability. Their multifunctional rf properties are well suited to define schemes for wireless communications that use basic protocols for data transmission, such as amplitude-, frequency-, and phase-shift keying (ASK, FSK, and PSK, respectively). ASK and FSK are demonstrated for STNOs, but the implementation of PSK is more challenging because the STNO phase suffers from large fluctuations. Here, we introduce a special PSK technique for STNOs by combining their modulation and injection-locking functionality. The concept is validated using magnetic-tunnel-junction-based vortex STNOs for injection locking at 2f and f/2, showing phase shifts up to $0.70\pi \mathrm{rad}$ and data-transmission rates up to 4 Mbit/s. Quadrature phase-shift keying and analog phase modulation are also implemented, where the latter is employed for voice transmission over a distance of 10 m. This demonstrates that the STNO’s phase noise and output power meet the requested performances for operations in existing communication schemes.

Figure 1

Schematic of the electrical setup for digital PSK and analog PM using STNOs.

Figure 2

2f injection locking of the vortex-state STNO using a rf source of constant frequency $f_{\mathrm{source}}=600\mathrm{MHz}$. (a) STNO spectrum and linewidth, Δf (blue line), vs I_dc at $P_{\mathrm{source}}=-3\mathrm{dBm}$. (b) Arnold tongue as a function of input power, $P_{\mathrm{source}}$, and dc. (c) Phase noise and (d) corresponding time evolution of the STNO phase, ${\mathrm{\Phi }}_{\mathrm{STNO}}$, for three different current values, I_dc, corresponding to three different states: (1) unlocked free-running STNO $(I_{\mathrm{dc}}=11\mathrm{mA})$; (2) partially locked STNO $(I_{\mathrm{dc}}=10\mathrm{mA)}$ with phase slips; (3) injection locked STNO $(I_{\mathrm{dc}}=9.75\mathrm{mA)}$. Inset in (d) shows an enlarged scale of STNO phase vs time for point (3).

Figure 3

Demonstration of BPSK at 2f and f/2 injection locking using P_source= −3 dBm for different parameters of the binary modulation signal V_mod and f_mod. Time traces of the instantaneous STNO phase ${\mathrm{\Phi }}_{\mathrm{STNO}}$ (red) and modulation signal V_mod (black) are shown. Values of Nf, f_mod, and V_mod are (a) 2f, 500 kHz, 6 mV; (b) 2f, 500 kHz, 12 mV; (c) f/2, 500 kHz, 8 mV; (d) 2f, 2 MHz, 10 mV. (e) Table for maximum phase shift, $\mathrm{\Delta }\mathrm{\Phi }$, achieved at different values of V_mod and f_mod.

Figure 4

Demonstration of (a) QPSK and (b) analog PM using a sawtooth signal for 2f injection locking with P_source= −3 dBm and parameters of the modulation signal of V_mod= 8 mV and f_mod= 10 kHz. Time traces of the instantaneous STNO phase, ${\mathrm{\Phi }}_{\mathrm{STNO}}$ (red), and modulation signal, V_mod, are shown (black).

Figure 5

Schematics of the benchtop system used for wireless voice transmission. BPF is a bandpass filter with a band of 50 MHz and a central frequency of 300 MHz. LPF is low-pass filter with a cutoff frequency of 20 kHz.

Figure 6

(a) Frequency-time spectrogram at the SDR receiver for transmission of the following message: “Demonstration of voice transmission using phase modulation of spin-torque nano-oscillators”. Parameters are described in the text. During voice transmission, sidebands are generated, as indicated. (b) Comparison of PS plots of the STNO output voltage after amplification of 20 dB, as measured on a spectrum analyzer within a $\mathrm{RBW}\mathrm{of}1\mathrm{kHz}$ for the free-running STNO (gray curve), binary FSK modulated signal (black curve), and binary PSK signal (red curve). BPSK data are taken for injection locking at 2f with $P_{\mathrm{source}}=-3\mathrm{dBm}$, and amplitude of the modulation signal for binary frequency shift keying (BFSK) and BPSK signal is $V_{\mathrm{mod}}=6\mathrm{mV}$. PS corresponds to the power within the resolution bandwidth set to RBW = 1 kHz.

Figure 7

Signal-to-noise ratio for different parameters for (a) analog PM and (b) digital BPSK phase modulation as a function of distance between emitter and receiver. Horizontal dashed lines are SNR values at transmitter output. Vertical dashed lines correspond to the maximum distance for which the contribution of the internal STNO noise is larger than that from the receiver noise and, therefore, determines the SNR. Diagonal dashed lines correspond to a reduction of SNR due to free space path loss (FSPL) [36, 37]. Evaluation of SNR for (a) PM signal using carrier frequency of 300 MHz, amplification of 0 dB, and bandwidth of 5 kHz and (b) for a BPSK signal for different parameters of modulation bandwidth, additional amplification, and carrier frequency; blue curve, carrier frequency 300 MHz, amplification 0 dB, bandwidth 2 MHz; green curve, carrier frequency 3 GHz, amplification 0 dB, bandwidth 2 MHz; red curve, carrier frequency 3 GHz, amplification 30 dB, bandwidth 2 MHz; magenta curve, carrier frequency 3 GHz, amplification 30 dB, bandwidth 20 MHz.

Figure 8

MDR (left scale) and BER (right scale) for BPSK signal as a function of distance. Horizontal dashed lines show levels of MDR and BER at the transmitter. Vertical dashed line shows the distance at which SNR at the receiver drops by 3 dB. Blue and green lines correspond to equations for MDR and BER at the receiver, respectively.