# Swept Noise (Radar)

Written By Massimo Annulli

Swept Spot Noise uses a saw-tooth, triangular or piecewise frequency modulation superimposed to a spot noise FM.
• Linear saw-tooth sweep
the sawtooth waveform is a non-sinusoidal waveform and can be considered as a sequence of ramp waveforms.
The various ramps that make up the sawtooth can be the same or different, both in amplitude and in duration, thus adding a further amplitude and/or frequency modulation.
Given an F1-F2 frequency range, the sawtooth wave rises from F1 to F2 with a certain slope and then drops sharply.
Reverse sawtooth can be obtained by going quickly up from F1 to F2 and back to F1 with some slope.

• Linear triangular sweep
The triangular waveform is a non-sinusoidal waveform and can also be thought of as a sawtooth waveform in which both the rising and falling ramps have a certain slope taking the shape of a triangle.
The various triangles that make up the triangular waveform can be the same or different, both in amplitude and in duration, thus adding a further amplitude and/or frequency modulation.

• Piecewise sweep (with multiple steps).
The piecewise or segmented linear waveform (Figure 4) is a waveform with a graph formed by straight segments.
The frequency range to be swept is divided into a sequence of n ramps (segments).
the linear waveform profile at times can be continuous and in this case the ramps are contiguous (where the ramp n-1 ends the ramp n starts) or stepped (the ramps are spaced in frequency)This allows the programming of different sweeps with different start and finish frequency offsets as part of one ECM technique.

Figure 4: Piecewise sweep

Programmable parameters:
• Noise BW of the pot noise (MHz)
• Sweep law and time of the additional frequency modulation (sec)
This technique allows to rapidly varying the central frequency of the jammer (spot noise) over a wider frequency band thus crossing the band of the victim receiver.

a) If the band of the victim receiver is narrower than the band swept by the jammer, it will appear as blinking.

Figure 5: Slow sawtooth modulation

b) If the sweeping frequency is very high, compared to the passband of the receiver, the effect is similar to barrage noise with an almost uniform power spectral density.

Figure 6: Fast Sawtooth modulation

▪ As an alternative to Barrage Noise, it is possible to wobble a Spot jamming frequency in a wide band.
This technique is useful to maintain a good Noise Quality Index (NQI) because the instantaneous bandwidth of the jammer is “matched” to the radar one and the carrier of the jamming is coherent (obtained through a DRFM that provides detection and memorization of the threat radar signal).

Figure 7: Sawtooth modulation applied to SPOT noise jamming to counter an agile Radar

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