- Buyers Guide
High Efficiency L- and S-band Transmitters
Emhiser Research Inc.-
A new line of miniature airborne telemetry transmitters has been introduced that uses a proprietary approach to amplifier design and features previously unattainable DC-to-RF efficiencies. The breakthrough design utilizes GaAs FETs in a configuration that eliminates the usual transmitter design trade-off of a linear regulator (very poor efficiency) vs. a switching regulator (electromagnetic/RF interference problems).
In many telemetry applications, the transmitter represents the major portion of the current draw and becomes the determining factor in flight time, range, battery size/weight, heat dissipation and other flight parameters. By dramatically lowering the current draw requirement, the new transmitter completely resets this trade-off equation.
Another common situation requiring design compromises is an existing system's increased data rate specification. For example, a requirement to double the data rate of an existing system requires the pre-detect bandwidth of the receiver to double. A 3 dB gain in received power is needed for the link to maintain the original range and bit error rate.
Since lowering the low noise amplifier (LNA) noise figure by 3 dB is probably impossible, the system designer is usually left with the choices of increasing the gain of the transmit and/or receive antenna(s) or increasing the transmitter power. Assuming the existing telemetry pack system represented an optimal design prior to the introduction of this series of transmitters, the designer's choices are narrowed even further to increasing the transmit antenna gain (difficult due to flight constraints) or increasing the receive antenna gain (usually very expensive).
The new line of ultra-high efficiency (UHE) transmitters effectively solves this system design dilemma. For example, doubling the transmit power from 5 W (conventional design) to 10 W (UHE) actually lowers the current draw requirement and heat dissipation. Other link elements, including the transmit antenna, receive antenna and LNA, as well as system elements, such as battery size and heat radiators, remain unchanged.
Yet another situation posing a design dilemma is one where higher RF power is not needed, but more current is required for additional processing circuitry and, of course, no additional supply current is available. In this case, changing from a conventional 5 W transmitter to a new UHE model frees up nearly 1 A of supply current.
Most conventional telemetry transmitters are configured similar to the block diagram shown in Figure 1. A phase-locked exciter generates the RF carrier and determines the modulation characteristics. This exciter drives several cascaded linear (class A) amplifier stages, which, in turn, drive the power stage(s) operating in class C. The power stage(s) is typically a silicon bipolar transistor.
In contrast to high power bipolar transistors, high power GaAs FETs are optimized for class A or AB rather than class C operation. While a class C stage is more efficient than a class A or AB stage in terms of DC-to-RF power conversion, the situation reverses when taking into account the much higher gain S21 of GaAs FETs.1 Typical L- and S-band GaAs FET gains are 16 to18 dB, as opposed to approximately 8 dB for silicon bipolar transistors. Therefore, the power-added efficiency (PAE) of GaAs FETs far exceeds the efficiency of a class C silicon bipolar device.
A block diagram of the new UHE transmitter is shown in Figure 2. The multiple class A bipolar driver stages (two stages minimally, three typically) and class C power stage are entirely replaced by the new GaAs FET amplifier. The high gain of the GaAs FETs permits the removal of several amplifier stages and their attendant DC power losses.
The elimination of amplifier stages, the increased GaAs FET RF power out/DC power in efficiency (up to 70 percent) over silicon bipolar transistors and the GaAs FET amplifier configuration significantly reduce the total current draw. In addition, the overall transmitter efficiency increases as the RF power output of the transmitter increases since the overhead current of the exciter represents a smaller percentage of the total current draw.
Performance comparisons between the UHE line of telemetry transmitters and conventional designs dramatically demonstrate the advantages of the new transmitters. Figures 3 and 4 show the decreased current draw and power dissipation, respectively, while Figure 5 shows the increased transmitter efficiency. Using these figures, the following examples evaluate the replacement of conventional telemetry transmitters with the company's new UHE line.
A change from a 10 W conventional transmitter to a 10 W UHE series transmitter with the same data rate results in no reduction in range, a 60 percent reduction in current and 67 percent less power dissipation. Changing from a 5 W conventional transmitter to a 10 W UHE series transmitter with the same data rate provides 41 percent additional range with a 22 percent reduction in current draw and a 35 percent reduction in power dissipation. A similar change from a 5 W conventional transmitter to a 10 W UHE series transmitter with double the data rate produces no range change but a 22 percent reduction in current draw and 35 percent less power dissipation. A final example is a change from a 2 W conventional transmitter to a 5 W UHE series transmitter with the same data rate. This replacement results in a 58 percent increase in range, a 25 percent reduction in current draw and a 38 percent reduction in power dissipation.
With standard input voltage levels of +28 V ±4 V, the frequency-agile UHE 2 W design draws 400 mA (max), the 5 W draws 750 mA (max) and the 10 W draws 1.4 A (max). Conventional frequency-agile telemetry transmitters draw 1.0, 1.8 and 3.5 A for 2, 5 and 10 W, respectively.
A new line of UHE telemetry transmitters offers designers new choices for system optimization. The transmitters are available in 2, 5 or 10 W RF power levels in the standard nine- or 11-cubic-inch packages (2.5" x 3.5" x 1" or 2.5" x 3.5" x 1.3"), and in 2 and 5 W RF power levels in the five- or six-cubic-inch packages (2" x 3" x 0.8" or 2" x 3" x 1").
1. Tim Bambridge, "High Efficiency 5 W S-band Telemetry Transmitter Using Gallium Arsenide Field Effect Transistors," Proceedings of the 1995 International Telemetering Conference, Volume XXXI, Las Vegas, Nevada, 30 Oct. 1995 to 02 Nov. 1995.
Emhiser Research Inc.,
Verdi, NV (775) 345-2705.