Radar and Phased Array Antenna
Fiberoptic delay lines incorporate wideband lasers and photodiodes designed for analog modulation. Fiberoptics provide high bandwidth and sensitivity to facilitate long delays, and wide dynamic range for operation over a variety of loss budgets. The basic advantage of a fiberoptic delay line is that long lengths of optical fiber are extremely low loss, broadband, and can be coiled into small packages. Delays up to 2000 microseconds, phase linearity better than 0.01%, and RF bandwidths to 18 GHz can be achieved.
Basic applications include the following:
- Radar Systems BIT (built-in-test) - Fiberoptic delay lines are used as BIT equipment for radar systems. Radar systems generally have some dead time between last echo received and next transmitted pulse. Some self testing is accomplished during this time (noise performance, dc tests, etc.). In addition, the system may periodically break its operational cycle to perform self testing with a simulated echo. The same kind of testing is also performed during manufacturing and as part of regular ground-based system testing.
- Antenna Systems Variable Delays - Varying delays at multiple antennas may be used to store and time multiplex signals from a number of antennas for processing at a single receiver.
- Synthetic Aperture & Phased Arrays - Varying delays may also be used to direct multi-antenna beam patterns in synthetic aperture or phased array applications.
- Moving Target Indication and Clutter Cancellation - Each received echo pulse is subtracted from the previous echo, which has been stored in the delay line. Any component of the signal that has not changed will be subtracted from itself to give a zero output. This could be ground clutter or a stationary target. A moving target will generally have an amplitude change as well as a Doppler frequency shift. The difference between successive pulses in this case will result in a dc or low frequency output proportional to the frequency (phase) shift (speed information) and the change in amplitude.
- Communication Systems Delay - Delay lines are used to simulate the natural time delays in communicating networks such as cellular systems and geo-synchronous satellite round trips.
- Radar Warning Receivers - An echo is received at an IFM (instantaneous frequency measurement) preprocessor that identifies frequency and sets up a local oscillator (LO) to down convert the signal to the IF of a signal post processor. Delay lines hold the signal long enough to allow the IFM to tune the LO (~1 msec).
- Electronic Warfare (EW) Systems - Delay lines are used for jamming. Radar pulses may be received, processed, and retransmitted as false echoes containing false information on target size, speed, and direction.
- Phase Shift Discriminators – Delay lines are used in FM demodulators as well as elements in phase noise measurement systems. If the input is a CW signal, the output is proportional to the phase difference of the signal compared to the delay time.