Phase Receiver
In an EW system, the Angle of Arrival (AOA), also referred to as Direction of Arrival (DOA), of the signal is a very important parameter. This is often measured in DF systems which consist of a number of antennas, typically 4 to 8, covering all directions of interest. The antenna pointing directly towards the emitter will produce a stronger signal than the others. Thus, by comparing amplitudes of signals at all antennas the AOA can be established. This has been the traditional method which gives an accuracy of about 10 to 15 degrees for a 4-quadrant system.
Where greater accuracy is required an interferometric technique is employed in which the phase of the signals at the antennas are compared rather than the amplitudes. This method can give an accuracy of better than 1 degree. This technique requires the receiver to provide a high degree of phase tracking between antenna channels. The Sage phase receiver achieves this performance over a multi-octave bandwidth by using optimal filter design and constant phase limiting amplifiers.
The Sage Phase Receiver is a self-contained compact subsystem covering an input frequency of 0.85 -18 GHz giving a 10-bit ECL phase descriptor output. The subsystem consists of three major parts, an RF Front-End, Synthesized LO and Digitizer. The RF signal is first down converted to a convenient IF frequency to allow the use of better phased matched components. It is then amplified and limited as a constant drive to the phase discriminator. The synthesized LO is used to center the phase receiver on the band of interest. The IF bandwidth is sufficient to cover frequency agile signals.
The RF Front-End comprises two channels, each with RF down converters, IF filters and limiting amplifiers. Both channels then feed into a phase discriminator via a 90 degree hybrid to give the sin and cos of the phase angle as the outputs. The Phase Digitizer converts the sin and cos of the phase angle into a 10-bit ECL pipeline phase descriptor.
The phase receiver has a dedicated synthesizer which is used as the LO for down conversion, to provide a calibration signal for high DF accuracy and allow BIT. Combination of fast tuning with high accuracy is achieved by phase locking a number of high speed VCOs to an accurate reference. The synthesizer covers the output frequency of 1.325 to 17.525 GHz in 50 MHz steps with +13 dBm output power. The multi-octave frequency range is split into 6 bands consisting of 3 lowband VCOs covering 0.875 - 4.5 GHz and 3 highband VCOs covering 4.5 -17.975 GHz. The outputs of the VCOs are low pass filtered to remove harmonics. A pair of SP3T switches select the appropriate VCO in each band. A coupler is fitted at the output of the switch to drive the phase locking circuitry, the signal level is controlled with a pin attenuator and an amplifier is fitted to each path to provide the +13 dBm required to maintain output match over the full dynamic range. A SP2T switch selects the output from the required band.
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