Rectennas – Limitations and Influences (Part II)

Dr. Senad Bulja — Tue, 13 Jun 2023 01:35:50 +0000

With reference to Fig. 2 (a) in Part I, each parameter influencing the efficiency of a rectifier operating at 10 GHz is examined. As a reminder, its efficiency is examined as a function of a) influence of threshold and breakdown voltages; b) influence of loss; c) influence of diode’s parasitic resistance; d) influence of diode’s junction capacitance; e) influence of termination type and f) influence of package parasitics.

A. Influence of threshold and breakdown voltages on efficiency

Let us now assume that all components in the circuit of Fig. 2 (a) of Part I are ideal apart from the threshold and breakdown voltage, which assume finite values. Table I below lists the assumed values, efficiencies recorded and the input power level at which the efficiency was recorded. As can be seen, maximum efficiency is obtained for the case when the breakdown voltage is high and when the threshold voltage is low. This is intuitively understandable and points to one way of increasing efficiency in rectifying circuits.

Table I Influence of threshold and breakdown voltage

B. Influence of loss

In a similar way compared to A., the influence of loss is also examined. For this purpose, 4 cases are identified, as given in Table II. As can be seen, losses negatively affect the overall efficiency, with efficiency dropping from 81% for 0.4 dB of insertion losses to 61% for 1.6 dB losses. The marginal increase in the maximum poweris explained by the fact that losses reduce the maximum power reaching the active device (diode in this case).

Table II Influence of losses

C. Influence of diode’s parasitic resistance

As with the previous two cases, a compiled Table III shows the effect of increase of parasitic resistance. As can be seen, the increase in the diode’s parasitic resistance reduces the overall efficiency of the rectifier, however, the reduction in the efficiency is not as dramatic as it was for case B, investigated previously.

Table III Influence of diode’s parasitic resistance

D. Influence of diode’s junction capacitance

Here, the influence of diode’s junction capacitance is investigated for frequencies up to 10 GHz. For the purpose of this exercise, it was assumed that the diode’s series resistance is Rs = 10 Ω and storage time TT = 10 ps. The diode’s zero voltage junction capacitance is assumed to be Cj0 = 0.2 pF. Table IV shows the effect of the finite capacitance on the overall efficiency as function of frequency. As can be seen, the influence of the diode’s capacitance negatively affects both the efficiency, maximum power and it also limits the rectifier’s dynamic range.

Table IV influence of diode’s junction capacitance

E. Influence of termination type

The implementation of a low-pass filter in the circuit of a rectifier strongly influences overall efficiencies. Usually, a simple capacitor is used to block high order harmonics (2nd and 3rd being of highest significance), however, in certain application a class-F terminations can also be used, which, effectively, provide a short circuit at harmonic frequencies, Fig. 1 or Part I. In Table V below, we provide a comparison between an ideal class F circuit providing a short termination at the second and third harmonics against a standard capacitor termination.

Fig. 1 Rectifier with a capacitive termination

Fig. 1 Rectifier with class-F termination

Table V Influence of termination type

As can be seen from Table V, class F terminations have the potential to increase overall efficiency, even though marginally. However, in this case the exact values of efficiency will be determined by the losses of the capacitive and class-F termination implementation types.

F. Influence of package parasitics

For this purpose, the influence on the designed single diode rectifier was examined for the case when the package parasitics are included in the diode model. For this purpose, an ideal capacitive termination of C = 100 pF was used in both cases. The diode case was assumed to be SOT-23 package [1]. Table VI shows the obtained results. As can be seen, the package does not influence overall efficiency, however, it does influence the maximum power at which the highest efficiency is observed.

Table VI Influence of package parasitics

In conclusion, the choice of the active device for the implementation in the circuit of a rectifier is very important as it has a significant impact on overall efficiency, dynamic range, and maximum input powers. As such, choosing the correct diode needs to be taken with great care.

References:

[1] https://en.wikipedia.org/wiki/Small-outline_transistor

Rectennas – Limitations and Influences (Part I)

Dr. Senad Bulja — Mon, 12 Jun 2023 09:05:29 +0000

Functionally, a rectenna is a circuit comprising of an antenna and a rectifying circuit, that can be used to convert Electro-Magnetic (EM) energy into DC. The origins of a rectenna can be traced all the way back to [1], where a rectenna was used to power a helicopter model [2]. The interest in rectennas have soared in recent years, driven by the rise of new ways of energy harvesting. Of note here is the use of Space Solar Power Satellites (SSPS), which collect and convert solar energy into electrical energy, which is beamed down to a ground station on Earth [3]. Here, the efficiency of conversion from EM to DC is of high importance as inefficiencies result in power waste and increased heat. The principle of EM to dc conversion is very simple, as presented in Fig. 1.

Fig. 1 Simplified block diagram of EM to dc converter

The crux for the correct operation lies with the nonlinear device. The nonlinear device acts as a multiplier, thus creating signals at the output with frequencies at integer multiples of the frequency of the corresponding input signal. If we assume that the frequency of the signal at the input if f1 , the nonlinear device creates signals at frequencies 2f1, 3f1, … and also n*f1 +/- mf1 with n and m being integers. As such, as the output, in addition to the dc signal (f1 -f1) signals at frequencies of f1, 2f1, 3f1…. High frequencies are eliminated by using a Low-Pass Filter (LPF) and its simplest implementation is a capacitor, as shown in simplified, standard rectifying circuits, employing 1, 2 and 4 diodes are shown in Fig. 2. In this figure, the chosen nonlinear device is a diode. The most commonly used diode is a Schottky diode or any other low or zero-barrier diode, in order to increase the efficiency of rectification. The choice between the single, double or 4-diode rectifier is dependent on the maximum input power that the device is expected to handle. Usually, for low power applications a single diode circuit is used, while high power applications usually require rectifiers with a greater number of diodes. However, this has to be carefully evaluated, as diodes with higher breakdown voltage may prove to provide a greater efficiency in a single diode rectifier circuit as compared to rectifiers with a greater number of low breakdown voltage diodes.

Fig. 2. Simplified rectifying circuits; (a) single diode

Fig. 2. Simplified rectifying circuits; (b) 2-diode

Fig. 2. Simplified rectifying circuits; (c) 4-diode

With reference to the single-diode rectifier circuit, we will examine the influence of each diode parameter on the efficiency of the rectifier. For this purpose, we examine the efficiency of a rectifier operating at a frequency of 10 GHz, as a function of: a) influence of threshold and breakdown voltages; b) influence of loss; c) influence of diode’s parasitic resistance; d) influence of diode’s junction capacitance; e) influence of termination type and f) influence of package parasitics. This will be done in Part II, with reference to the circuit of Fig. 2 (a).

References:

[1] US 3434678 Microwave to DC Converter William C. Brown, et al, filed 5 May 1965, granted 25 March 1969

[2]https://en.wikipedia.org/wiki/Rectenna#:~:text=A%20rectenna%20(rectifying%20antenna)%20is,trans mit%20power%20by%20radio%20waves.

[3] N.C. Au, D. M. Nguyen, T. D. Nhu and C. Seo, “A 5.8 GHz rectifier using diode connected MESFET for space solar satellite system”, in IEEE Transactions on Microwave Theory and Techniques, vol. 70, no. 10, October 2022.

Dr. Senad Bulja, PhD, FIET, SMIEEE