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Thursday, 26 September 2019

Delay Line Canceller Block Diagram

The delay line canceller is used in MTI radar system and it works as a filter. It eliminates the clutters in received signal thereby improving the resolution of target detection. The delay line canceller subtracts echoes from the successive signals.

Delay Line Canceller Block Diagram
The output of the MTI radar is given as input to delay line canceller. The input signal is converted to its equivalent digital value by an analog to digital converter. The signal is delayed, which is achieved by storing the radar output during the pulse transmission. The original signal and delayed version of the signal are then subtracted to produce an output signal which is free from echoes. The absolute value of the difference signal is then taken and is fed to a digital to analog converter to produce the output signal.

Let V1 = KSin[2πfdt - Φ]

V2 = KSin[2πfd(t – Tp) - Φ]

Therefore, V = V2 – V1

V = KSin[2πfd(t – Tp) - Φ] - KSin[2πfdt - Φ]
= 2K { Sin[2πfd(t – Tp) – Φ-2πfdt - Φ]/2 . Cos[2πfd(t – Tp) – Φ + 2πfdt - Φ]/2}

Simplifying we get,

V = 2K Sin [-πfdTp]. Cos [2πfdt - πfdTp - Φ]

Considering only the magnitude of the signal,

V = 2K Sin [πfdTp]. Cos [2πfdt - πfdTp - Φ]

ie, V = H(f) x Cos [2πfdt - πfdTp - Φ]

where, H(f) is the amplitude of the transmitted signal and

H(f) = 2π Sin [πfdTp].

The delay line cancellers have a disadvantage called ‘blind speed’, where the target will not be detected and there will be an uncancelled clutter remaining interfering with target detection process. The Doppler shift frequency in presence of clutter is defined as,

fd = nfp ---------------------- (1)

The general equation for Doppler Shift frequency is,
fd = 2 ftVr/C --------------- (2)

equating equation (1) and (2)
nfp = 2ftVr/C

Therefore, Vr = Cnfp/2ft
= nfpλ/2

Vr = nλ/2Tp

Vr , represents the velocity for calculating blind speed. Four methods for reducing the effects of blind speed are

1. Operate radar at longer wavelengths.
2. Operate radar with high pulse repetition frequency
3. Operate with more than one repetition frequency.
4. Operate with more than one radio frequency.


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