RADAR systems may be complex but their operation relies on a range of relatively simple resistor components. Resistors are used in pulse forming, sensing, switching, current limiting and RF termination applications, among others.
In the late 1800’s Heinrick Hertz proved that radio waves travel in a straight line and are reflected by a metal object. In RADAR equipment a pulse of high frequency radiation is emitted, based on the time and direction of the returning pulse the distance and direction of a metallic object may be calculated.
High Voltage Resistor
Pulse forming networks generate a pulse of high frequency radiation whose shape and duration are dependant on the electrical characteristics of the pulse forming network. High voltage resistors are a key element of these networks.
A high voltage resistor is required that is able to withstand voltages in the tens of Kilovolts range. They must deliver a high level of long term stability and durability, high average power per unit size, and low inductance.
A current sense resistor may be used to translate the current flow from the receiving element of the RADAR device so that it may be measured and interpreted. The current through the resistor may be represented by the voltage drop across that resistor. A precision sense amplifier may then be used to measure the voltage drop.
To control power loss and heat generation the resistance of the sense resistor must be minimised but the resistor must retain a high current carrying capability and be able to withstand high current surges. A low resistance value also limits self heating of the resistor element and therefore the impact of TCR. The impact of TCR errors may also be minimised by appropriate choice of materials.
Current Limiting Resistors
A compromise is required to ensure the resistor device may dissipate the pulse energy and avoid resistor element burn out but all within an acceptable device footprint.
System level application issues include the impact of short term temperature rise on adjacent components, the board area required to accommodate the current limiting resistor and potential solder joint failure caused by repeated temperature cycling.
RF Termination Resistors
Terminating resistors match impedances in the circuit and reduce reflections that would otherwise cause compromise system performance. Terminating an RF system is a complex issue with a number of conflicting factors to consider.
Terminating resistors have only one lead with the other lead is connected to a flange which is, in turn, bolted to a heatsink. They are available as 50 or 100 Ohm variants with typical power ratings between 30W and 800W. The issue for the system designer (and resistor manufacturer) is to maximise performance while minimising losses through the resistor.
The correct choice of a resistor in RADAR applications, therefore, presents a number of challenges. Electrical performance, component size, thermal issues, dimensions and system constraints all require careful consideration.
The choice of resistor technology is also a key issue, thin film, thick film, wirewound and carbon film all have their relative advantages and disadvantages. However, thick film technology tends to be a good all round choice due to its high power density, low inductance, high power handling capability, long term stability and robust construction.
When presented with a number of competing priorities in the design of sophisticated electronic systems compromising on performance to accommodate a standard resistor device from one of the major manufacturers may not be an option. A specialist manufacturer able to produce a custom device, at an acceptable cost, in low to medium volume may be the only solution.