Measuring the STC
Figure 1: Different STC curves (multiplied by different factors to set the maximum gain from 40 km)
Measuring the STC
The STC curvature is a time-dependent control voltage, which regulates the amplification in the high-frequency part of a radar receiver as a function of time. If this gain control does not work, this can result in target losses in the close range, because the fixed targets already saturate the receiver.
With a largely digital receiver, this is one of the few measurements that can still be made in analog form with an oscilloscope. A prerequisite is that this control voltage is also accessible via a test socket. Both external and internal synchronization can be used on the oscilloscope. On the oscilloscope, a voltage curve (depending on the radar unit) must be visible like a charge or discharge curve of a capacitor, depending on whether this regulation is to act as additional amplification or as damping. With some radar sets, the function of the STC curve can be changed by a switch, so that the form of the voltage curve changes. Theoretically, the form of the curve should approximately correspond to the function of a 4th root, which results from the basic radar range equation. However, depending on the influence of interfering fixed targets in the close range, the shape can lie between the function course of the 2nd root to the 7th root. Here the function of the square root is preferable for very strong targets. The 7th root would effect that only the direct coupling of the transmission impulse into the reception path should be attenuated.
Testing with a test signal
The presence of a control voltage with the STC curvature does not yet mean that this control will work. Ideally, the control element (adjustable amplifier or controlled attenuator with PIN diodes) should act immediately after the low-noise preamplifier.
To check this, a generated test signal can be fed into the receive path with a signal generator. This test signal should be adjustable in distance. It is first set to a distance of about 40 km and its level is adjusted to a level that forms a clearly visible but not too strong target signal. Then this test signal is pulled to a closer distance and should at some point no longer be visible when the STC-function is switched on.
These pairs of measured values (the distance of the disappearance of the test target and its power level set at the signal generator) can be plotted in a diagram and should produce an STC curvature similar to one of the curves in Figure 1 if several different pairs of measured values are used. If this diagram is successful, then the STC circuit works.