A Low Cost Analog Phase Shifter Product Family for Military, Commercial and Public Safety Applications
A new product family of low cost continuously variable phase shifters (PS) has been introduced. The phase shifters are optimized to reduce size, weight and power consumption (SWAP) as well as unit cost, while increasing switching speed and reliability. These products are perfectly suited for commercial and military manufacturers to significantly reduce the number of components in the current RF communications systems, and design low cost, high performance solutions for next-generation communications systems that demand new functionalities such as reconfigurable, smart, intelligent devices and systems.
The PS family covers seven frequency bands from 700 MHz to 7 GHz with a minimum phase shift of 90° and a minimum relative bandwidth of 30 percent. The primary applications for these phase shifters include broadband, wireless local area networks (WLAN), base stations, satellite communications, phased-array radar and smart antennas. Table 1 lists the current models with custom models readily available to meet customer’s needs.
The phase shifters are manufactured utilizing nGimat’s proprietary thin film ferroelectric barium strontium titanate (BST) on sapphire technology. BST is a high dielectric constant material (?r > 500), whose dielectric constant can be tuned down when a DC bias is applied. This makes BST a promising candidate for reconfigurable and tunable devices in numerous RF and microwave applications such as phase shifters, tunable filters and tunable matching networks. nGimat’s BST technology is superior to others in the following attributes: (1) the BST thin films are epitaxially grown on inexpensive high Q sapphire substrate using nGimat’s proprietary low cost Combustion Chemical Vapor Deposition (CCVD), resulting in higher dielectric constant and tunability, as well as much reduced loss and manufacture cost; (2) a low voltage, planar capacitor structure has been developed by nGimat and the Georgia Institute of Technology, which yields improved intermodulation distortion (IMD) performance. The planar structure also facilitates processing and yields lower loss since thicker and more conductive metals can be deposited than the platinum base electrodes used with BST parallel plate designs.
The phase shifters are fabricated using integrated circuit processing techniques, enabling mass production at low costs. Furthermore, the manufacture of a PS requires fewer processing steps than those of a transistor fabrication, further reducing the fabrication cost when compared to monolithic microwave integrated circuit (MMIC) technology.
The PS is tuned by a single DC bias of +20 or –20 V max, reducing signal routing complexity and allowing precise, continuous phase shift control. Utilizing nGimat’s proprietary low voltage capacitor structure, the PS exhibits a high third-order input intercept point (IIP3 > 30 dBm). Lumped element circuits are used for the PS, enabling a compact design and lower loss. A bottom side photo of a flip-chip mountable S-band phase shifter (model nPS1726) is shown next to the article title. The die size is only < 0.080" × < 0.080" × < 0.02". Solder ball terminations and flip-chip packaging are used to reduce device loss and facilitate compatibility to automated industrial manufacturing. The die is fully passivated for high reliability in extreme environments.
Since only passive components are used (compared to conventional PIN diode phase shifters), the phase shifters essentially draw zero static current, enabling long battery life for mobile applications. The drive power required for the BST capacitor is equal to the charge needed to drive the capacitor for one cycle, multiplied by the number of cycles per second, multiplied by the voltage. Assuming a capacitance of 5 pF, a drive voltage of 20 V and an operating frequency of 10 kHz, the BST capacitor requires 10 µW of power. This represents orders of magnitude improvement over the power required to drive a PIN diode.
Figures 1, 2 and 3 show the measured S-parameters and differential phase shift of an S-band PS (model nPS1726). It should be noted that the measurement was done with the phase shifter flip chip mounted onto a printed circuit board. A loss improvement of 0.3 dB may be added to account for fixture losses. Therefore, the de-embedded phase shifter, from 1.7 to 2.6 GHz, has an insertion loss of 0.5 to 1.8 dB and a return loss of 14 to 30 dB. Over 100° of differential phase shift is obtained over a 30 percent fractional bandwidth. All the other models exhibit similar electrical performance with the center frequency being the only variable. The typical third-order input intercept point (IIP3) is greater than 30 dBm and can be designed to be 50 dBm, and the switching time of the phase shifter is in the range of tens of nanoseconds. Custom designs can be fabricated to function with 10 V or less control bias and also be integrated to yield up to 360° of phase shift from a single part.
Electronically scanned array antennas offer tremendous benefits over traditional mechanically steered antennas for a variety of radar and communications applications, such as fast, reliable electronic beam steering, compact volume profile, multi-target capability and larger antenna area (gain) for a given system volume. High production costs, however, have hindered the ubiquitous use of these systems. As an essential active component in a phased array, the current MMIC phase shifters account for nearly 50 percent of the total cost. The introduction of low cost ferroelectric phase shifters will reduce the manufacturing cost of a phased array by at least 50 percent, with additional benefits in SWAP and performance, enabling a broad range of military and civilian applications. Examples of applications include vehicle “on-the-move” antennas, mobile communications, collision avoidance radars, WiFi antennas, conformal antennas and body-born antennas.
Thanks to the explosive growth of WLAN and availability of easy-to-use systems, mobile and wireless communication has become an important part of everyday life. However, the current wireless communications systems have limitations in capacity and performance such as limited spectrum, delayed spread, co-channel interference and multipath fading. Utilizing 2.4 GHz ferroelectric phase shifters, nGimat and the Georgia Institute of Technology have demonstrated a smart antenna system that showed improved data throughput rate and extended range to the access point.
Figure 4 shows a photograph of the two element beamforming network (BFN) fabricated using early prototype phase shifters packaged in conventional leadless chip carrier ceramic packages. Each phase shifter chip consists of a two-section phase shifter, with over 200° of phase shift from 2.4 to 2.45 GHz. The input is connected to an external antenna port on the WLAN card. The BFN is adaptively controlled using the parallel port of the laptop PC connected to the WLAN card. The adaptation is carried out so as to maximize the received bit-error-rate (BER), and hence the data rate of the WLAN. Antenna patterns of the two-element array are measured at 2.4 GHz using an outdoor measurement range. The results are shown in Figure 5. The data show that nulls of more than 20 to 25 dB may be obtained in certain directions, while broad beams are maintained to receive the incoming multipath signals.
A system level field test is performed to ascertain the benefits of the adaptive nulling system as compared to the conventional switched diversity system currently employed on most WLAN devices, as shown in Figure 6. Interference signals at various levels were directed line-of-sight at the laptop computer attached to the WLAN card as it was receiving data from an access point that was not line-of-sight to the phased-array antenna. It is seen that the adaptive nulling afforded by the BST phase shifters achieves a throughput of 350 to 375 kByte/s. This is approximately equal to the throughput of the WLAN card alone until the interference level is increased above –15 dBm (at the source of its transmission). Above this interference level, the throughput rapidly degrades for the conventional switched diversity WLAN antenna, and eventually causes a link failure. In contrast, the adaptive nulling network is able to improve the signal-to-noise ratio sufficiently to maintain full throughput at up to 0 dBm interference level.
The adaptive nulling system described herein could also be applied to anti-jamming systems for global positioning systems, and for co-site interference systems commonly employed in military communications networks. In addition, the BST phase shifters also find applications in RF signal processing, such as adaptive notch filtering and reconfigurable transmitters.
A new series of low cost analog phase shifters has been described that covers from L- to C-band with improved SWAP performance. These new phase shifters are perfectly suited for phased-array antennas, smart antennas, and other communications and radar applications.
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