# Radar Questions and Answers – Doppler Filter Banks and Digital MTI Processing

This set of Radar Multiple Choice Questions & Answers (MCQs) focuses on “Doppler Filter Banks and Digital MTI Processing”.

1. A set of contiguous filters for detecting targets is known as a ____________
a) Band pass filter bank
b) High pass filter bank
c) Doppler filter bank
d) PRF filter bank

Explanation: A Doppler filter bank is a set of contiguous filters for detecting targets. A filter bank has several advantages over the single filters like in a filter bank, a measure of the target’s radial velocity can be obtained. It might be ambiguous, but a change in the PRF can resolve the ambiguity in the radial velocity.

2. Why are filter banks preferred in order to separate multiple moving targets from one another?
a) To increase signal to noise ratio
b) To allow transmission of more quantity of echo signal at a time
c) To increase the rate of transmission
d) To separate echo signal and undesired moving clutter

Explanation: Multiple moving targets can be separated from one another in a filter bank. This can be particularly important when one of the echo signals is from undesired moving clutter, such as a rain storm or birds with a nonzero Doppler filters.

3. The narrowband Doppler filters exclude less noise than the MTI delay-line cancelers and provide coherent integration.
a) True
b) False

Explanation: The narrowband Doppler filters exclude more noise than the MTI delay-line cancelers and provide coherent integration. In general, this improvement in signal to noise ratio by means of coherent integration is seldom the major reason for using filter banks in MTI radar.

4. Who indicated that when only a few pulses are available in a coherent processing interval, empirical design procedures can be used since there is little flexibility for more sophisticated design?
b) W.M and G.W. Ewell
c) Galani and R.A. Campbell
d) M. Schmidt and R.B. Hammond

Explanation: It was Shrader and Gregers-Hansen. From their theory, with a large number of pulses available a more systematic approach can be employed based on Chebyshev filter design. The frequency response of the Chebyshev filter has all its sidelobes equal and the width between the nulls of the main response is a minimum for a given sidelobe level.

5. Which of the following delay line canceler is used to reduce the clutter, especially in those filters close to zero frequency or the clutter spectral lines at the harmonics of the PRF?
a) N-pulse non recursive canceler
b) Three pulse canceler
c) N-pulse recursive canceler
d) Five pulse delay line canceler

Explanation: To reduce the clutter, a relatively simple delay line canceler such as a three pulse canceler with sin2πfTp response can be placed ahead of the filter bank. It is expedient to compensate for the inability to reduce the sidelobes of the filter bank as much as might be desired.

6. Why is Doppler filter bank more complex than the simpler delay line cancelers?
a) PRF can resolve range ambiguities
b) Multiple moving targets can be separated
c) Requires lesser signal to noise ratio
d) Requires more pulses for good performance

Explanation: Doppler filter bank is more complex than the simpler delay line cancelers because it generally requires more pulses for good performance, and it requires a larger signal to noise ratio if the true radial velocity is to be extracted when two or more pulse repetition frequency are employed.

7. During MTI processing with a single phase detector and single processing channel, there is a loss when the Doppler shifted signal is not sampled at the peak positive and negative values of the sine wave.
a) True
b) False

Explanation: It is true that whenever the Doppler shifted signal is not sampled, it goes through a loss at both the crest and trough i.e. the peak positive and peak negative. So there is a high probability of a generation of blind speed if PRF’s sampling generates a loss.

8. In an MTI Doppler signal processor, what is the use of sample and hold circuit?
a) Larger SNR
b) Effective power transfer
c) Increase bandwidth
d) Effective digitizing

Explanation: A sample and hold circuit usually is needed ahead of the A/D converter for more effective digitizing. Sample and hold are often on the same chip as the A/D converter. Some A/D converters, such as the flash type, do not require a sample and hold.

9. The magnitude of the Doppler signal is obtained by taking the square root of __________
a) I2 * Q2
b) I2 + Q2
c) I2 – Q2
d) I + Q

Explanation: The magnitude of the Doppler signal is obtained by taking the square root of I2 + Q2. Sometimes, for simplicity, the sum of the magnitudes of the two channels, |I| + |Q|, is taken or the “greater of” of the two channels might be used instead. The I and Q processor has a square law detector characteristic.

10. The number of quantization levels in the A/D is generally given as __________
a) 4N
b) 3N
c) 2N
d) 5N

Explanation: The number of quantization levels in the A/D is generally given as 2N. Since there are two channels, I and Q, sampling in each can be at one half the Nyquist rate, which generally makes the implementation of the A/D converter simpler. D/A converters are usually easy to achieve than A/D converters of the same resolution.

Sanfoundry Global Education & Learning Series – Radar.

To practice all areas of Radar, here is complete set of Multiple Choice Questions and Answers.

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