Careful Spec & Test of Spur Performance key to MTI and SAR radar performance

RF System Spurs frequently set the quality of imaging in synthetic aperture radars, and the false target limit in MTI radars requiring high linear processing gain.
 

8 Ways to Clarify Spurious Emissions | EE Times

Avoid confusion among engineers by defining terms such as carrier and bandwidth right up front.

Most RF devices have a requirement for allowable spurious content, based on
Mil Standards, FCC regulations, NTIA requirements, or simply from performance needs. Unfortunately, the requirement is often ambiguous. Vigorous debate may erupt during the verification process, regarding the interpretation of spurious requirements. You can minimize the debate by clarifying the terms related to spurious content in RF signals used in radar and communication systems. Here are some suggestions on how to reduce the confusion.Spurious signals are unintended signals that can result from harmonics,  intermodulation, frequency conversion, or EMI (electromagnetic interference). The DUT (device under test) will most likely have both noise requirements and spurious requirements. So, what differentiates a spur from a region of elevated noise? The noise

requirement will likely be specified in terms of dBc/Hz, as a density, while the spurious requirement will be specified in terms of dBc. This determines how far below the carrier, in dB, the spurious content must be. A good place to begin is in the definition of the term "carrier."

1. Define the carrier conditions

Figure shows the effect of pulsing a CW carrier.

Power of the CW carrier frequency (blue) dropped

by 20log10(D) (red), D is duty cycle =PWxPRF

when the PW (Pulse Width) is 1.75 µs

and PRF is 180 kHz. The red lines

depict the PRF lines.

 2. Define the term "spurious."

Spurious signals are usually measured at points
between subsystems within an RF receiver.

spurs are clearly visible in a spectrum analyzer display

3. Define the input power level.

4. Does the requirement apply to pre- or post-channel combining?

 

Spurious signals in a carrier are a result of channel combining

 5. Define where in the signal processing the spurious requirement applies.

A Typical LFM Pulse - spectrum changes
pre/post compression

 6. Define the bandwidth pertaining to the requirement.

CW Spectrum in which the spurious requirement applies

 7. Define the measurement bandwidth



 

 

Spur bandwidth increasing with the order of the spur

8. specify a maximum number of filters that can be desensitized by spurs

Conclusion


The system engineer has to perform allocations from the system requirements to the subsystem hardware designer. The allocation will determine how much of the allowable spurious requirement may be consumed by each subsystem. This assures that when all subsystems are integrated into the system, the entire system will meet its requirement. The requirements should be specified in a way that's relevant to the manner in which the system is intended to be used.