By Dave Slack, Times Microwave Systems
System integrators and designers of aerospace receivers face a multitude of practical realities. One of the most pressing of these realities involves receiver location. Unfortunately, the ideal receiver location is not always practical, or even possible.
One ideal approach is to collocate the low-noise receiver front-end with its associated sensor or antenna, so that virtually every bit of signal that the antenna collects is available for detection and processing.
Quite often antennas are located at the extremes of an aircraft's physical geometry such as the tips of the wings or tail fins. These locations are often tiny, and quite harsh environments. The tips of wings and fins are usually high-vibration environments, and require anything placed in these locations to be extremely light and enormously durable.
For these reasons it is normal to compromise, and locate the receiver within the fuselage or some other benign location and route the received energy through transmission lines from the sensors to their receivers. Systems designers must add the associated insertion loss of this interconnecting path to the receiver noise figure. This reduces the receiver-system sensitivity and decreases the maximum signal-detection range.
Receiver sensitivity is the minimum power level that a receiver can receive and still perform its designed purpose; any signal the antenna receives that is weaker than the receiver's sensitivity will not register.
As power from an emitter decreases over distance, the signal power that equals a receiver's sensitivity suggests the receivers maximum range, which is specified by the manufacturer of that receiver.
For signals just strong enough to meet a receiver's sensitivity, any signal loss between the antenna and receiver can be critical. Interconnection losses between sensors and receivers will most assuredly degrade system performance and maximum range.
Receiver sensitivity is a function of thermal noise, receiver bandwidth, additive noise of the receiver (noise figure), and the required signal-to-noise ratio required for adequate system operation.
When designers insert a lossy transmission line between sensor and receiver, the receiver "system" sensitivity is directly reduced by the attenuation of the interconnection cable. It can be shown that a 6-decibel path loss between sensor and receiver will decrease maximum range by about half.
Collocating the antenna with its low-noise receiver front-end can eliminate this effect. Stated another way, integrating the receiver with the interconnection hardware will significantly increase maximum range. Removing a 6-decibel path loss between antenna and receiver can effectively double the system's maximum range.
Installing a low-noise amplifier at the antenna will provide significant system performance improvements. Amplifying small signals, those just at the threshold of detectability, before passing them through significant interconnection losses, can cause targets to be detected that may otherwise have been lost.
Designers should not discount the effects of noise, however. Adding a preamplifier introduces noise into the system. Designers can reduce the system noise figure by eliminating the cable loss. The designer also can increase the system noise with the amplifier.
The aggregate effect of adding the amplifier is a function of the amplifier noise figure (NF), the amplifier gain and the cable loss that is between the amplifier and the other receiving system components. The system noise floor should be kept as low as possible to maintain optimum system performance.
The important issue of note is the desirability of a low-noise, high-gain device early in the received signal path. Having a negative gain (or loss) immediately after the receive antenna is one of the most destructive design decisions that can be made regarding receiver system sensitivity and overall system performance.
Times Microwave Systems in Wallingford, Conn., has introduced a durable, lightweight, low-noise amplifier (LNA) integrated into existing passive interconnection hardware, which collocates the low-noise receiver front-end with its associated antenna, thus providing the optimum receiver-antenna relationship required for maximum receiver sensitivity and overall system performance.
These amplifiers, which are for harsh airborne and naval environments are incorporated into cable types that are MIL-T-81490 qualified and that are hermetically sealed to ensure long life and reliable performance.
These devices can easily be custom tailored for unique applications, which provides an unprecedented opportunity for system manufacturers to design systems with standard performance characteristics regardless of the platform on which the system will ultimately be deployed.
Adjusting the gain-frequency slope within the amplifier module can normalize differences in the platform topography and its associated interconnection cabling. This provides much greater system transportability across a wide range of platform architectures.
Because these LNA devices employ a modular design they can be easily removed and replaced without removing any part of the interconnection cabling.
These active cable assemblies are available in compact, rugged, and lightweight packages; are integrated into the MilTech series of aerospace grade microwave cable assemblies; and cover the operating range of radar warning receivers operating from 500 MHz to 18 GHz. The noise figure is about 5.5 dB with an available minimum gain of 18 dB.
Typical power output, measured at the 1-decibel compression point, is a minimum of 10 dBm (decibels referenced to milliwatts). These devices are available with a power-limiting option that will prevent the device from being damaged when subjected to input power levels of up 1 watt of average power.
Because of the lightning-sensitive location in which these amplifiers would be installed they are available with an integrated lightning-protection device. The gain and gain equalization are fully temperature-compensated from –55 to +85 degrees Celsius.
Power consumption is less than 2.5 watts. These devices can be operated from primary aircraft power ranging from 15- to 24-volts DC. They can be either powered through an external power pin or through the center conductor of the microwave coaxial cable structure.
The physical package has been specifically designed for use in severe aerospace environments. The package is quite low in mass at about 48 grams such that vibration-induced force loads are minimized.
The case is integrated into the cable and threads on using standard microwave connectors. The physical envelope, of the discrete package is 2.5 inches in length and less than 0.65 inches in diameter. The associated cable assemblies are fully qualified per MIL-STD-81490 and hermetically sealed for use in rugged aerospace environments. They are hermetically sealed and capable of operating at altitudes in excess of 40,000 feet.
These devices can be provided with a flat gain vs. frequency slope, or a custom-tuned gain profile. This option allows the possibility of the system avionics to be identical across a variety of platforms.
By changing the gain/loss vs. frequency profile of the interconnecting cable the system parameters will be identical regardless of platform topography or cable lengths, which offers an opportunity for system designers to provide greater commonality of parts and significantly reduce costs.