Exciter (radar)
In modern radar devices , the assembly group that generates the finished pulse shape and transmission frequency for the transmitter or the active antenna is called an exciter .
In a waveform generator , the pulse shape is generated at the intermediate frequency level, i.e. at a frequency in the range from 30 to 75 MHz. This basic pulse is used to modulate a high frequency that is usually only a fraction of the final transmission frequency. The exciter contains a mother generator , which is quartz-stabilized or whose base frequency is even derived from a time standard of the GPS satellites. This basic frequency is multiplied several times, resulting in several frequencies, which, however, are still far below the required transmission frequency.
One of these frequencies is now modulated with the waveform from the waveform generator and now contains the pulse shape to be sent. The final transmission frequency in its complete pulse form is generated from the multiplied frequencies in one or more up- converters. As an alternative to a frequency multiplier, a VFO or a DCO can also generate a variable frequency regulated by a voltage or a data word and thus make the transmission frequency variable over a wide range.
The exciter is usually already in the antenna platform. This has the advantage that only a low-frequency signal has to be passed through the slipring unit . The high frequency generated by the exciter, which is still low in power, is pre-amplified and sent to the antenna. There the transmission power is provided by one or more high-performance amplifiers or all elements of the active antenna are fed in phase.
At the same time, one or two reference frequencies are generated with the frequencies, which are used in the receivers to feed the down mixers . The downward mixing on the receiving path also takes place in the antenna platform so that the echo signals can also pass through the slipring unit on the relatively uncritical intermediate frequency level.