Having seen the types of Feeds used to derive Tracking error information now we delve into the subject of overall Monopulse Tracking Systems consisting of Feed/Cables/and Tracking Receivers and also see how these subsystems are integrated with the antenna drive or servo system.
There are basically 3 parellel chains for 3 channels. The 3 outputs from Monopulse Comparator S, Dx and Dy are amplified in 3 LNAs. It follows that the components upto this stage have to have a very low loss because any loss therein causes a direct contribution to system noise and degrades the system G/T leading eventually to jitter while tracking.
The 3 channels which have been amplified are down converted using a common Local Oscillator ( L.O. ) because the input f relation amongst the channelshas to be maimtained upto Product Detector which generates the final dc voltage corresponding to f difference between S and Dx and S and Dy. These 3 IFs are finally processed in MONOPULSE PROCESSOR which is is described in detail below. For brevity we stick to a classic 3 Channel Monopulse Tracking Receiver:
The S chain ( Shown in dashed yellow box in the figure ) is basically conventional PLL receiver with a coherent AGC derived using a quadrature f detector.
Recall that in a PLL, the loop action maintains a 90o phaseshift between Input to f detector and the reference oscillator. Therefore a second f shifter ( indicated as ‘Amplitude Detector’ in the figure ), fed with an additional 90o phase shift in Ref Oscillator , detects amplitude changes and so becomes a Coherent Amplitude detector. The output voltage of this detector drives AGC loop ensuring a constant input to both the phase shifters. This voltage can also be used to drive the ‘Signal Strength’ Meter.
Crystal Oscillator can’t be pulled much from its centre frequency so generally a ( XN ) frequency multiplier is used to increase the operational frequency range of the receiver.
Since PLL receiver is not the the subject matter of this post we will not go in further details but would emphasize that the reader should go into the details of this very very interesting technique. We now concentrate on Monopulse Receiver.
Error channels viz., Dx and Dy, IF signals from down converter are fed to 2 parallel receiver channels which are identical to the S channel. These channels use LO and Ref Osc derived from S channel.
The AGC for these channels is also driven by the AGC voltage of S channel.
The SUM channel thus is a reference for both error chains and all the three channels behave identically.
It naturally follows that the three channels should have good dynamic performance of programmable gain control. While undergoing this gain control the inter-channel phase match should be maintained over full dynamic gain range else false error computations will result which demands a larger IF bandwith for AGC amplifiers than actually required.
At the end of these three channels now phase comparator ( technically named as Product Detectors ) derives the angular difference in antenna pointing using coherent detection method between SUM and ERROR channels. Output voltage of Product detector corresponds to the angular offset of antenna from target. The phase denotes direction and amplitude shows outness in pointing.
We have seen the basic configuration of Monopulse Tracking receiver, But a few additional components are required to be added to adopt the receiver to practical environment. Most importantly we need to add a error polarity reversibility ( if required ) and d.c. gain control is required in the final output to facilitate practical system integration with Power Drive electronics/Servo system.
Other important component is the Secant Correction in AZ channel as described in another post of this blog.
Coming to overall Tracking System, there should be no active component upto monopulse error deriving mechanism in the feed. It follows that feed and this passive electronics should be very low loss. Only after SUM and ERROR channels are derived the LNAs should be used.
Error channel LNAs could be a little cheap if economics is a priority. Again the LNAs should have good dynamic performance over frequency range and amplitude range. By 'good' , we mean that the phase and gain difference at input and output should not change much over the entire dynamic range.
Some phase control mechanism is a must prior to monopulse processor as well to keep required phase match between SUM and ERROR channels for best detection sensitivity.
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