HAM Modulation and Digital Signals 4

Question 1

What is the Nyquist sampling theorem's requirement for digitizing an analog signal without aliasing?

Accepted Answer

The sample rate must be at least twice the highest frequency component in the signal

Answer

The Nyquist theorem states that a signal must be sampled at a rate of at least twice its highest frequency to reconstruct it accurately from the digital samples.

Question 2

What causes intermodulation distortion in a transmitter's RF power amplifier when handling digital signals?

Accepted Answer

Non-linearity in the amplifier producing unwanted mixing products of the signal's spectral components

Answer

Amplifier non-linearity causes the instantaneous frequency components of a signal to mix with each other, generating intermodulation products that appear as spectral regrowth and adjacent channel interference.

Question 3

In Amateur Radio digital modes, what is the function of a 'waterfall' display?

Accepted Answer

It provides a time-frequency spectrogram that scrolls downward, showing signal activity across a range of frequencies

Answer

A waterfall display is a real-time spectrogram where frequency is on the horizontal axis, time scrolls downward, and signal amplitude is shown by color, making it easy to spot digital signal activity.

Question 4

What is the purpose of differential encoding in digital PSK systems such as DPSK?

Accepted Answer

To allow demodulation without a coherent phase reference by encoding information in phase transitions rather than absolute phase

Answer

Differential encoding represents each bit as a change in phase from the previous symbol, so the receiver needs only to detect phase transitions rather than an absolute phase reference.

Question 5

Which of the following digital modes uses a combination of frequency shift and multiple tones to achieve a low-baud-rate, high-sensitivity signal suited for weak-signal HF work?

Accepted Answer

JT65

Answer

JT65 uses 65 orthogonal tones with very low symbol rate and strong FEC, enabling contacts at signal levels well below what SSB voice can achieve.

HAM - Radio Extra Class Test Practice Test

HAM - Radio Extra Class Test Practice Test

HAM Modulation and Digital Signals 4