Communication Systems (Analog and Digital)
Communication Systems (Analog and Digital) tests how information is placed on a carrier, transmitted, and recovered. Expect modulation index, bandwidth, sampling, PCM bit rate, digital modulation, noise, and multiplexing.
Core rules
- AM bandwidth is 2fm; modulation index m = Vm/Vc. Overmodulation (m > 1) causes envelope distortion.
- FM has constant amplitude; Carson’s rule gives bandwidth ≈ 2(Δf + fm).
- Nyquist sampling requires fs ≥ 2fm to avoid aliasing; practical systems use a higher rate and an anti-alias filter.
- PCM bit rate = fs × n, where n is bits per sample. Quantisation noise falls by about 6 dB per extra bit.
- ASK, FSK, and PSK change amplitude, frequency, and phase respectively; QAM combines amplitude and phase.
- Shannon capacity C = B log2(1 + S/N) sets the error-free limit for a noisy channel.
Worked example
A 4 kHz audio signal is sampled at 8 kHz and coded with 8 bits per sample. Bit rate = 8000 × 8 = 64 kbps. If 16-QAM is used, each symbol carries 4 bits, so symbol rate = 64,000/4 = 16,000 symbols/s. The minimum baseband bandwidth for ideal Nyquist signalling is about 16 kHz; practical RF systems need more for filtering and roll-off.
Traps
Sampling exactly at 2fm is a theoretical limit, not a comfortable design margin. Confusing bit rate with baud rate overcounts or undercounts bandwidth. Assuming FM bandwidth equals 2fm ignores deviation. In AM, forgetting sidebands halves the calculated bandwidth. Noise figure and SNR are not interchangeable.