Full-Wave SPICE

One of the highlighted features of Wavenology EM is the ability to co-simulate both complex circuits and RF systems together. This is done by hybridizing the SPICE circuit solver and the transient EM solver. To give evidences of such capability, we demonstrate several antenna design examples.


 

Ambient Power Collector

 

The first example is an ambient power collector, a device used to collect EM energy from the ambience (due to mobile phone signals, for example) and convert it to electric power. The power (albeit only milliwatts) is sufficient to operate devices such as clocks, smoke alarms, Ni-Cd battery chargers, and so on. The geometry of this ambient power collector is shown in Fig. 1, where 4 SPICE circuits, 4 lumped inductors, and 5 lumped capacitors are used. It includes 3 parts: (1) two antennas to receive EM energy; (2) a full-wave rectifier using 4 diodes to convert the AC signal to DC signal; and (3) a 9th order low pass filter to remove the high frequency components. The load resistance is 125 Ohm.

 



Fig. 1.  The geometry of an ambient power collector, in which 4 SPICE circuits, 4 lumped inductors, and 5 lumped capacitors are used.

 

Four semiconductor diodes are used in the full-wave rectifier, as shown in Fig. 2. These diodes are modeled by the SPICE diode model as follows:

 
.model DMOD1 D(IS = 5.1E-9 RS=4 N=1.02 IK=4.5m CT=0.87p VJ=0.59V M=0.5 FC=0.5 BV=20 IBV=10u)

 

(a) (b)

 


(c)
Fig. 2. (a) The full-wave rectifier used in the ambient power collector. (b) The model of the full-wave rectifier. (c) Configuration of the circuit in Wavenology EM.

 

The 9th order low pass filter is implemented by 4 lumped inductors and 5 lumped capacitors, as shown in Fig. 3.

 



Fig. 3. Four lumped inductors and five lumped capacitors are used to build a 9th order low pass filter.

 

The working frequency of the antennas is 2.4 GHz. Thus a plane wave of 2.4 GHz is used to model the ambient electromagnetic waves, as shown in Fig. 4(a). The amplitude of the incident plane wave is 300 V/m.

 



(a)(b)
Fig. 4.  (a) An incident electromagnetic plane wave of 2.4 GHz is used to model the ambient electromagnetic wave; (b) The voltage recorded at the load of the ambient power collector in Fig. 1. An electric DC power is obtained.

 

The voltage in the load (125 Ω, modeled by a lumped resistor) is recorded. The result is shown in Fig. 4(b). We can clearly see that the incident electromagnetic wave is converted to an electric DC power in the steady state, with a small amount of AC component remaining


 

RF Signal Amplifier

 

The second example is a radio frequency signal amplifier, in which the amplification circuit is modeled by a SPICE circuit. The basic geometry is shown in Fig. 5, where a patch antenna is used to receive electromagnetic signal. And the signal is transmitted by a coaxial cable. An amplification circuit is used to connect the inner and outer conductor of the coaxial cable to amplify the transmitted signal, as shown in Fig. 6.

 



Fig. 5. The basic geometry of the radio frequency signal amplifier. It includes a patch antenna to receive an electromagnetic wave and a coaxial cable to transmit the received signals.
 

(a)(b)
Fig. 6. (a) An amplification magnifier is built to connect the inner and outer conductor of the transmitted coaxial cable. (b) The amplification circuit.

 

The amplification circuit is modeled by a SPICE circuit as follows:

 



 

A plane wave of frequency 2.4 GHz with the amplitude of 10 V/m is used to model the electromagnetic signal. The voltage received at the coax cable is recorded. Two results are compared with and without the using of the amplification circuit. Clear amplification effect can be observed, as shown in Fig. 7.

 



Fig. 7. Voltages received with and without the amplification circuit. The blue curve is the input signal to the amplification circuit, which is in fact the signal received without the use of the amplification circuit.

 

Wavenology EM has a convenient feature to allow the user to access the SPICE circuit results. The SPICE results of voltage in all the nodes and currents in all the branches can be accessed by clicking some corresponding tree items under the results item, as shown in Fig. 8. For example, the voltages at all the nodes in the SPICE circuit are plotted in Fig. 9.

 



Fig. 8. Tree items implemented in the software to access the SPICE circuit results.
 

Fig. 9. Voltages at all the nodes of the SPICE circuit in Fig. 8.


 

End-to-End Simulation of an Antenna System

 

In the third case, we demonstrate the capability of Wavenology EM package to perform an end-to-end simulation of a more complex communication system, with a direct modulation transmitting antenna and a receiving antenna with amplifier and demodulation circuitries. This shows a realistic application of this software tool in communications.

 



Fig. 10. Configuration of the AM communication system to demonstrate the end-to-end simulation capabilities of Wavenology EM.  The transmitting patch antenna has the direct modulation, while the receiving monopole antenna has amplifier and demodulation circuitries.

 

Fig. 10 shows the configuration of the AM communication system with one transmitting antenna and one receiving antenna, each with multiple circuitries. This communication system consists of a transmitter and a receiver. The transmitting part is the diode-modulated patch antenna. The receiving part includes a monopole antenna, an amplifier, the demodulator and the filter.
 
The transmitter uses new modulation scheme – the direct antenna modulation. It has a lumped port as the carrier source, working at 1.44 GHz carrier frequency. Two diode modulation circuits are connected to the two corners of patch antenna. Each circuit is composed of a diode in series with a sinusoid voltage source (100 MHz signal). Fig. 11 shows the structure of transmitter and its modulation circuit diagram.

 



(a)(b)(c)
Fig. 11. (a) Transmitting antenna (perspective view) using direct modulation.   (b) Transmitting antenna (side view) showing two diodes. (c) Modulation circuit diagram.

 

Fig. 12. shows voltage signal applied on the patch after the modulation. It consists of the low frequency modulation signal and the high frequency (1.44 GHz) carrier frequency.

 



Fig. 12 Voltage applied on the patch after modulation.

 

The receiver is a monopole antenna, working at the frequency of 1.44 GHz. The structure is shown in Fig. 13, which is a monopole antenna formed by the inner conductor of a coaxial cable with a ground plane. The inner conductor of the coaxial feed is connected to a three-terminal amplifier-demodulation circuitry.

 



(a)(b)(c)
Fig. 13. (a) Monopole receiving antenna (angle view). (b) Monopole antenna (front view). (c) Circuits and load connection in coaxial feed.

 

In the circuitry at the receiving antenna, there is a pre-amplifier cascaded with a demodulator to pick up the signal as shown in Fig. 13(c). Fig. 14 gives the diagram of the RF amplifier followed by a diode detector and filter capacitor.
 
The communication configuration of the transmit-receive system is shown in Fig. 10, where the receiving antenna radiation pattern has its maximum oriented toward the maximum radiation pattern of the transmitter. Fig. 15(a) shows the voltage signal received by the monopole antenna through the active circuitry.  The peak signal is around 1 mV. Fig. 15(b) is the signal obtained after the pre-amplification, showing a peak voltage around 0.9 V. Fig. 15(c) shows the demodulated signal. Fig. 16 shows the snapshot of the electric field distribution from the transmitter to the receiver.

 



Fig. 14. the circuit diagram for the amplifier-demodulator of AM signals with a pre-amplifier, and a demodulator (detector).  This circuit is used in Fig. 13(c) together with the RF receiving antenna structure.
 

(a)

(b)

(c)
Fig. 15. (a) The modulated signal received by the monopole antenna. (b) The amplified modulated signal. (c) The demodulated signal after the demodulator in Fig. 14.
 

Fig. 16. The snapshot of the electric field distribution.

 

In this example, we have shown the end-to-end simulation of a communication system with realistic circuitries for transmitter signal modulation, receiver signal pre-amplification, and demodulation. Our preliminary results show that the Wavenology EM can conveniently integrate complex lumped circuit components in arbitrary full-wave environments, so that the signals in a end-to-end communication system can be predicted.