Design An Integrated Oscillator/Antenna

This design tightly integrated an active oscillator with a microstrip patch antenna for miniaturization in WLAN, Bluetooth, and other 2.45-GHz systems.

R. A. Abd-Alhameed,
D. Zhou, C. H. See, P. S.
Excell, A. Ghorbani,
and N. J. Mcewan
Mobile and Satellite Communications Research Centre, Bradford University, Bradford, BD7 1DP, United Kingdom; e-mail: [email protected].

Miniaturization often requires integration of multiple components into common, compact structures. With that goal in mind, the authors pursued the design of an active antenna with an integrated oscillator for a nominal operating frequency of 2.45 GHz. Voltage series feedback was used to broaden the unstable region of the active device while also maximizing input and output reflections. The design involved a striplinefed patch antenna as the output terminal element for the unstable active device, with coupling effects between the antenna and active RF circuitry ignored. Output power at the input of the antenna was optimized with respect to constraints imposed by specified phase noise and harmonic levels. In order to evaluate the characteristics of the active antenna oscillation without interfering with the radiating performance, a calibrated sensor was placed to the radiating edge of the antenna having the maximum voltage. As will be shown, target design specifications were met after performing adjustments of oscillation frequency.

An oscillator-type active microstrip antenna integrates an active device with a microstrip antenna to generate steadystate oscillations. The oscillator converts DC power to RF power using the active device's negative-resistance characteristics. An integrated version of such an active antenna has been developed for sensor applications at lower power levels. Additional research has sought to overcome the power limitations of solid-state sources for such designs due to interest in the potential of spatial power-combining techniques. The oscillator consists of an active device in conjunction with a microstrip antenna that simultaneously serves both as a load determining the frequency of oscillation and as an element radiating the generated RF power into space. Proper selection of an operating point of the active device is important for the operational performance.

For an oscillator-type active microstrip antenna, the active elements can be two-terminal devices,_1-3 for example, IMPATT devices and Gunn diodes, or they can be three-terminal devices such as metal-epitaxial-semiconductor, field-effect-transistor (MESFET), high-electron-mobility-transistor (HEMT), and heterojunction-bipolartransistor (HBT) devices._4-9 In general, each type of solid-state source has advantages and disadvantages. Two-terminal devices are suitable for high-power applications at millimeter- wave frequencies, but suffer low DC-to-RF efficiency, requiring careful attention to dissipation of heat in the circuit and system design. Threeterminal devices, on the other hand, can provide high DC-to-RF efficiency and low noise figure, but at reduced power levels.

Microstrip antennas have advantages of modest size, low profile, and planar geometry, leading to low manufacturing cost. The planar structure also lends itself to integration with the associated electronic circuits, e.g., in the form of an active antenna. The present work reports on an experiment to develop an active transmitting antenna for wireless local area networks (WLANs) as well as for Bluetooth. The antenna is an oscillator- type microstrip active antenna working at around 2.45 GHz, linked to a two-port unstable active device. The active device is directly integrated with a rectangular patch antenna, except that a short microstrip line was introduced between the antenna input port and the active device for measurement purposes. Usually, with such a design process, the feeding line loss would be considered negligible, but it was included in the present work.

The design steps for both the patch antenna and the oscillator were carried out in parallel. The radiation effects of the antenna feed line were introduced on the antenna side and the input impedance variations of the antenna at the feed line were taken as input parameters for the design of the oscillator. Voltage series feedback was employed to maximize the dynamic range of the oscillator output and to insure that operation be maintained in the active device's unstable region, as required to satisfy oscillation conditions.

The antenna was considered with a single-port input (two or more input ports may also be considered) and all the results associated with it, over the frequency band of interest, were transferred to the RF circuit simulator. However, coupling effects between the antenna and the other RF circuit elements (such as the matching elements and the DC feeder lines) were ignored. The design was carried out first by linear simulation to predict the required oscillation frequency and then optimized. Following this, nonlinear simulation was performed to predict the oscillation conditions, phase noise, and power performance.

The characteristics of the antenna including the feed line and the oscillator circuit were simulated and analyzed using the Advanced Design System (ADS) design software tools from Agilent Technologies.10 It should be noted that the antenna was modeled using the Momentum software package included in ADS. The design goal (see the table) was met, with fine adjustment and control of the oscillation frequency by insertion of a capacitor at the drain pin of the GaAs MESFET active device. It was observed that the range of the oscillation frequency obtained was around 6.87-percent deviation from the 2.45-GHz center frequency, with low phase noise and acceptable output power.

The measured frequency and the forward power at the antenna input port were determined using a sensor calibration factor that had been evaluated when the antenna was disconnected from the oscillator circuit.¹¹ The sensor was created by a small patch of dimension 3 5 mm, placed at the antenna edge developing maximum voltage. The distance between the sensor and the antenna edge was optimized without affecting the input return loss of the antenna port, also satisfying the linearity conditions of the calibration factor required. It was found that a 2-mm space (empirically trimmed) was needed for coupling of about 22.6 dB between the sensor and the antenna near the resonant frequency of the patch. The sensor patch was also connected to ground via a 50-O load, to improve the output match of the sensing circuit. A second pin connected the sensing patch to a coaxial probe at the rear of the board, and this fed the sensor output to a spectrum analyzer. The inclusion of the 50-O resistor ensures that such a sensor will function correctly and also that the output connector of the sensor appears as a relatively wellmatched source. This will reduce errors that might be caused by connecting it to a poorly matched power meter or spectrum analyzer. The calibration factor was measured first when the antenna was disconnected from the active RF circuit: this was then reconnected to measure the output power of the oscillator.

A model ATF-10136 GaAs MESFET with noise figure of 0.5 dB at 4 GHz from Agilent Technologies (www.agilent.com) was chosen as the unstable two-port active device for the integrated antenna/oscillator. Voltage series feedback was represented by an open transmission line connected to the FET's source port. The linear circuit was optimized for maximum reflections at input and output ports at 2.45 GHz. Fig. 1. shows the response of these reflections. The peak values of S₁₁ and S₂₂ at 2.45 GHz were found to be 1.9 and 1.3, respectively; these values were considered acceptable in terms the input and output stability circles required for the integrated antenna/oscillator design.

The antenna and the RF circuit elements were mounted on Duroid circuit-board material from Rogers Corp. (www.rogerscorporation.com) with the following specifications. The relative permittivity, loss tangent, and height of the substrate were 2.55, 0.0018, and 1.524 mm, respectively. The total area of the finite ground considered for the active antenna oscillator circuit is around 8 5 cm. The antenna was considered as a stripline-fed rectangular microstrip patch. The dimensions of the patch were 36 mm in length by 46 mm in width and the dimensions of the feed line were 15 mm in length by 2 mm in width. The magnitude and the phase of the return loss at the input of the feed line at 2.45 GHz were 0.299 and 147 deg., respectively. The two-port S-parameters between the antenna feed line and the output sensor when the antenna was disconnected from the RF circuit are shown in Fig. 2. The corresponding calibrated factor S'₂₁ from the measured data when the sensor is placed 2 mm from the end of the radiating patch was computed using Eq. 1:

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Fig. 3 shows the response of the calibration factor from 1.8 to 3.0 GHz. However, the variation of the calibration factor was checked for different distances, varying from 2 to 4 mm, and these measurements show that the maximum rate of amplitude change at 2.45 GHz related to the reading at the 2-mm distance was about 0.25 dB. The effect of the sensor on the input return loss of the antenna was also checked and found to be less than 0.01 dB, dependent on the coaxial feed used.

The antenna input impedance data was transferred to the RF circuit simulator and the resonant condition at the input port of the active devices was observed. Then the input matching circuit was optimized for maximum input power at the antenna port using the nonlinear model of the active device. The nonlinear model and the prototype of the active antenna oscillator circuit, including the sensor, are shown in Fig. 4 and Fig. 5, respectively. As the table shows, all of the specified design goals were met, as evidenced by the measured results.

Fig. 6 shows a spectrum analyzer plot of the free-running oscillation, with the marker set at 2.4240 GHz and -13.33 dBm. The difference between the measured oscillation-frequency from the specified target one is around 1.23 percent: this represents the errors associated with the RF elements used. Fine adjustment and control of the oscillation frequency around the targeted output power was also achieved by changing the susceptibility of the input antenna admittance. This has been done by connecting the MESFET output with 5-pF variable capacitor. The oscillation frequency range achieved was approximately within 6.4 percent of the targeted value.

In summary, an integrated oscillator- type active antenna, using positive series feedback and working at a center frequency around 2.45 GHz, was presented and designed. The design steps for both the patch antenna and the oscillator were carried out in parallel. The voltage series feedback results in good dynamic range at the oscillator output. The measured frequency and the forward power at the antenna input port, using a calibrated output sensor, gave reliable results without affecting the radiation performance of the antenna and the oscillator circuit elements, with all nominal design goals having been met.

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