Editing Continuous-wave radar (section)

===Leakage===
The transmit signal will leak into the receiver on practical systems. Significant leakage will come from nearby environmental reflections even if antenna components are perfect. As much as 120&nbsp;dB of leakage rejection is required to achieve acceptable performance.

Three approaches can be used to produce a practical system that will function correctly.
* Null
* Filter
* Interruption

Null and filter approaches must be used with bistatic radar, like [[semi-active radar homing]], for practical reasons because side-lobes from the illumination radar will illuminate the environment in addition to the main-lobe illumination on the target. Similar constraints apply to ground-based CW radar. This adds cost.

Interruption applies to cheap hand held mono-static radar systems (police radar and sporting goods). This is impractical for bistatic systems because of the cost and complexity associated with coordinating time with nanosecond precision in two different locations.

The design constraint that drives this requirement is the [[dynamic range]] limitation of practical receiver components that include band pass filters that take time to settle out.

====Null====
The null approach takes two signals:
* A sample of the transmit signal leaking into the receiver
* A sample of the actual transmit signal

The actual transmit signal is rotated 180 degrees, attenuated, and fed into the receiver. The phase shift and attenuation are set using feedback obtained from the receiver to cancel most of the leakage. Typical improvement is on the order of 30&nbsp;dB to 70&nbsp;dB.

====Filter====
The filter approach relies on using a very narrow band reject filter that will eliminate low velocity signals from nearby reflectors. The band reject area spans 10 mile per hour to 100 mile per hour depending upon the anticipated environment. Typical improvement is on the order of 30&nbsp;dB to 70&nbsp;dB.

====Interruption, FMICW====
While interrupted carrier systems are not considered to be CW systems, performance characteristics are sufficiently similar to group interrupted CW systems with pure CW radar because the pulse rate is high enough that range measurements cannot be done without frequency modulation (FM).

This technique turns the transmitter off for a period before receiver sampling begins. Receiver interference declines by about 8.7&nbsp;dB per time constant. Leakage reduction of 120&nbsp;dB requires 14 recover bandwidth time constants between when the transmitter is turned off and receiver sampling begins.

The interruption concept is widely used, especially in long-range radar applications where the receiver sensitivity is very important. It is commonly known as "frequency modulated interrupted continuous wave", or FMICW.