The invention relates generally to microwave circuits and more particularly to circuits that provide coordinated injection of test signals into a waveguide.
Modern radar systems include sub-systems built in for the automatic testing of various components to insure that maximum performance is achieved at all times.
As part of these testing systems it has been found useful to add appendage ports to various microwave devices whereby known signals may be injected and processed by the device. The output of the device is then analyzed and compared with the input signal and its operational effectiveness determined.
Typically, in modern radar systems there are two RF coupled ports for built-in testing. The first occurs at the receiver protector input port and is utilized for the injection of a coherent signal from a local stabilized oscillator. This signal is used to measure receiver gain and characteristics of the sensitivity time control.
The second port is coupled to the input port of a low noise field effect transistor amplifier. This injects a noise signal from which receiver noise is measured.
The disadvantage to this system is found in that the noise test is made at a point where a significant portion of the noise factor is omitted, thereby degrading the results of any measurements.
The receiver protector is typically a dynamic non linear device. Being a very lossy component, the receiver protector degrades the noise figure and changes receiver sensitivity as it changes its characteristics.
Properly, to obtain total noise the test signal should be injected into the input to the receiver, which is effectively the receiver protector. The difficulty with this approach in the past has been the injection of a coherent signal from the stabilized local oscillator at the same point. Until now, no way had been found which would allow both signals to be injected in a coordinated, effective manner. With the circuit to be described, it is now possible to obtain measurements of noise variations through the highly lossy, dynamic receiver protector as well as the remainder of the system.