Microwave power amplifier with switch. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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The specificity of the use of some foreign means of communication in Russian conditions is the need to supplement the functionality of these means with a variety of set-top boxes of domestic production. For example, wireless communication systems of the IEEE802.11 standard [1], made in the design of PCMCIA cards, are used abroad to connect portable computers to public and corporate networks in shops, universities, airports, warehouses, etc. Since they are designed to work with a portable computer, their circuitry is minimized in terms of energy consumption, which is associated with a low transmitter power (30-100 mW). Due to the relatively low price, such devices in Russia are used everywhere for any wireless communication applications. And often their low output power is the only thing that does not satisfy the end customer. In this case, domestic power amplifiers appear on the scene, designed to be used with IEEE802.11 miniature devices and allowing them to significantly increase their output power. The design and testing of one of these amplifiers will be discussed in this article.

Usually IEEE802.11 devices have one microwave connector used to connect the antenna, the transmit/receive switching takes place inside the device, as this is a time division system. Therefore, the first problem that had to be solved in the development of a power amplifier was the separation of the receiving and transmitting channels in order to amplify the latter and, if possible, attenuate the former as minimally as possible. For the described separation, you can use passive devices - circulators. The design of power amplifiers with a circulator was described in detail in [2], so this article will focus on another way - using an active switch.

Rice. 1. Block diagram of a power amplifier with a switch

A feature of using microwave switches is the need to generate a control signal for switching reception-transmission modes. Of course, such a signal can be taken from the IEEE802.11 device itself, but in this case, the convenience of using the amplifier is lost, since in addition to the microwave cables, one more control cable must be connected. In addition, the receive-transmit switching signal is not explicitly output to the PCMCIA connector. To obtain the switch control signal, the developed amplifier uses a microwave detector based on a Schottky diode of the HSMS-2850 type from Agilent. The Schottky diode HSMS-2850, designed for detection, modulation, mixing and frequency division in the range from 915 MHz to 5.8 GHz at a frequency of 2.45 GHz (the average operating frequency of the developed amplifier), has a sensitivity of 35 mV/μW. More details about the technical parameters of this component can be found in [3] or on the Internet at agilent.ru. To match the Schottky diode at a frequency of 2.45 GHz, a resonant circuit consisting of two strips was used. Its calculation is given in [4], in addition, for its calculation, you can use the AppCad microwave calculator distributed free of charge by Agilent.

Rice. 2. Appearance of the device

The block diagram of the developed amplifier is shown in fig. 1, appearance - in the photograph, fig. 2. The field-effect transistor SHF-0289 based on gallium arsenide from Stanford Microdevices was used as an active element of the amplifier. This future-proof component delivers at least 30 dBm output power at 2.45 GHz with 20 dBm input power. Some disadvantage of its use can be called the need for a supply voltage of 8 volts, but as experiments have shown, it works satisfactorily even at a voltage of 5 volts, if you do not require full output power from it. The switching circuit of the transistor, given in the application documentation [5], is rather complicated, while the values ​​of some components during tuning must be varied to obtain acceptable parameters, but such is the fate of all field-effect transistors.

From the output of the transistor amplifier, the amplified transmitter signal is fed to the SW-438 microwave switch by MA-COM. Housed in a SOT-363 plastic surface mount package, this low-cost GaAs switch provides low forward attenuation (0.7 dB or less at 2.4 GHz), high isolation (25 dB or more), and virtually no power consumption (less than 10 µA at 3 Volts). Typically, microwave field switches are controlled by negative voltage, so another of its advantages is the ability to control both negative and positive voltage - this is very convenient when designing your driver. Detailed technical documentation [6] for this component can be found on the manufacturer's website: macom.com.

Fig. 3. Schematic diagram

The microwave switch is controlled by a driver, which in this circuit is a high-speed analogue signal multiplexer from Analog Devices ADG774ABRQ. Its functions include simultaneous switching of 0 and +2.5 Volt signals at the control inputs of the SW-438 switch according to the power detection signal at the input of the microwave detector, transmitted through the actuating element - the KT-3130 transistor. Level + 2.5 Volts is formed by a resistive divider R7 / R8 - see the circuit diagram in fig. 3. The ADG774ABRQ has a low on-resistance of 2.2 ohms, can operate at either 5V or 3V, and is TTL/CMOS compliant with control inputs. The main advantage that distinguishes the ADG774ABRQ from its first implementation, the ADG774BRQ, is the double analog signal bandwidth - 400 MHz and the short switching time - 3 ns, which makes it possible to use such a multiplexer in any modern telecommunications, more about it in [7].

Rice. 4. Test results

The test results of the developed device are shown in the graph, Figure 4. The gain of the device in decibels and the output power in decibels per milliwatt are shown in the graph depending on the operating frequency. In conclusion, we would like to note that the use of foiled PTFE 1 mm thick instead of foiled fiberglass 1.5 mm thick as the amplifier substrate material would significantly improve the results obtained.

List of used literature

1. Shakhnovich I. "Wireless LANs. Anatomy of IEEE802.11 Standards". - Electronics: Science, Technology, Business. - No. 1, 2003. - S. 38-48.
2. Malygin I.V. "Development and research of power amplifiers of microwave radio modems of the IEEE802.11 standard" - Components and technologies. - No. 9, 2002. - S. 24 - 25.
3. Surface Mount Microwave Schottky Detector Diodes HSMS-2850 Series. technical data. Communications Components Designer's Catalog. Hewlett Packard. 5966-0895E (9/97).
4. 2.45 GHz Detector Demonstration Board. assembly and operating instructions. HSMS-2850. Semiconductor Products Designer's Catalog. Agilent Technologies. CD. 5968-7805E (11/99).
5. SHF-0289. DC - 3 GHz, 1.0 Watt, CaAs HFET. product description. Stanford Microdevices, stanfordmicro.com. EDS-101241 Rev A.
6. GaAs SPDT Switch DC-3.0 GHz SW-438. Ver. 2.00, MA-COM, Tyco/Electronics.
7. Low Voltage 400 VHz Quad 2:1 Mux with 3 ns Switching Time ADG774A. REV.0. Analog Devices Inc., 2001, analog.com.

Author: Malygin I.V.; Publication: library.espec.ws

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