Electronics Engineering

It depends on the amount of current available to charge or discharge. On the surface, I would say that you can discharge a capacitor faster than you can charge it, because the charge rate is limited by the current available in the power supply, while the discharge current could be quite high, because you could just apply a short circuit conductor around the capacitor. Of course, this could damage the capacitor. In truth, the equation of a capacitor is...

dv/dt = i/C, which means that, given the same charge or discharge current, the rate of change of voltage would be the same.

The time taken to fully charge, or to fully discharge, a capacitor is given by the equation: time = 5 CR, where C represents its capacitance, in farads, and R represents the resistance of the circuit supplying the capacitor, in ohms. By 'fully charge', we mean bring the potential-difference across the capacitor's plates to the same value as the applied potential difference.

If the external voltage source is replaced with a short circuit then, providing the resistance of the circuit hasn't changed, the discharge time will be exactly the same as the charging time. If the resistance is changed, then the same equation applies, but you need to insert the new value of resistance.

Radar Technology - The Main Components

All radars are composed of the items listed below. Their operation is organised in a processing chain and hence it is the weakest part that defines the systems capabilities.

The Transmitter

Transmitters are built around semiconductors (often contained in MMICs, millimetre-wave integrated circuits) or powerful vacuum tubes. The latter are rather complicated and sophisticated devices and often carry weird names ending in '-tron', such as Amplitron, Magnetron, Carcinotron, Stabilitron or Klystron.

Owners of a microwave oven are also owners of a magnetron. The fact that microwaves can heat up food was discovered by serendipity, when during the 1940s a radar researcher was astonished to see that a chocolate bar was melting in his trouser pocket while he was performing experiments with an unshielded magnetron1.

The Antenna

The radar antenna serves as the coupling element between the wiring in the radar hardware and free space. Radar antennae can be as small as a thumbtack or as big as a 30-storey building, depending on their operating frequency and beamwidth.

The Receiver

The receiver's task is to pick up the echo that was bounced off a target, filter out unwanted parts outside the radar's bandwidth, amplify the rest and feed it into the signal processor for further analysis. A good receiver is a radar's best defence against noise, its toughest enemy.

A receiver must be very sensitive in order to pick up weak echoes from far away. But usually it is located near the transmitter which can easily 'blind' or even destroy it by 'spillover' leaking into the receiver's input circuitry. In a pulsed radar, damage can be avoided by using a Transmit/Receive-Switch or T/R switch that disconnects the receiver's input from its antenna while the transmitter is operating. In a Continuous Wave Radar, the transmitter operates all the time and receiver protection is only feasible by blocking the frequency that is currently used. Both measures do fulfil their purpose but at the same time they introduce some problems of their own: they produce blind ranges and blind speeds.

Until not long ago, travelling wave tubes (TWT) were the mainstay of receiver construction. Like the -tron devices mentioned above, their inner workings are rather complicated as they are built around some chamber or structure where strong magnetic fields or electron beams interact with low power, high frequency signals. During the 1980s, TWTs were gradually replaced by semiconductors.

The Signal

The signal is what the radar transmits into space. A wide variety of types is available, and perhaps more than all the hardware components, the signal is what determines the quality and capabilities of a radar. The most powerful radar, equipped with an ultra-low sidelobe antenna of incredibly high gain can be blind at the most important range or target speed if the signal was chosen wrong. Some signals are likely to produce 'angels' and 'ghosts' on a screen - things that really make a radar operator's life interesting. More often than not, a single type of signal will not meet all the requirements, and fierce discussion about necessary expenditures ensues between manufacturer and customer.

The Signal Processor

The Signal Processor is the central element of a radar. It has to decide whether an echo really is an echo and whether or not it is worth being reported and displayed. This is not a simple task, as there is much natural noise around, and in the case of military applications there is man-made interference too.

The System

A collection of sophisticated components is a precondition, but not a guarantee, for a good radar. The first step during the design phase is to determine which part of the electromagnetic spectrum is to be used, followed by the selection of the signal that is most appropriate for the purpose in question. All this needs to be composed into a system that is more than the sum of its parts. There are only a few things that a radar cannot do, and the easiest way to find these is to look into the requirement specifications written by the customer.