Signals, Communications and Embedded Systems
Signals, Communications and Embedded Systems tests the chain from sensor or data source through sampling, processing, transmission and embedded control. Expect numerical reasoning on sampling, link budgets and real-time behaviour.
Sampling and signal processing
Sample at more than twice the highest frequency component, and use an anti-alias filter before the converter; Nyquist alone does not remove out-of-band noise. An N-bit ideal converter has SNR ≈ 6.02N + 1.76 dB over the full Nyquist band; ENOB is lower after noise, distortion and jitter. Clock jitter limits high-frequency SNR. For filters, know passband ripple, stopband attenuation, group delay and the effect of component tolerance.
Communications
Compare modulation schemes by spectral efficiency and robustness: ASK/FSK are simple; PSK/QAM carry more bits per symbol but need better SNR and linearity; OFDM handles multipath but has high peak-to-average ratio. Shannon gives the capacity bound C = B log2(1 + S/N). A link budget adds transmit power and antenna gains, subtracts cable, connector and path losses, and compares the result with receiver sensitivity plus fade margin.
Embedded systems
Separate MCU and MPU architectures, memory maps, interrupt latency, DMA and cache behaviour. In an RTOS, priority inversion can occur when a high-priority task waits on a mutex held by a low-priority task; priority inheritance or a ceiling protocol mitigates it. Use a watchdog with a defined recovery state, CRC or hash for firmware integrity, and a bootloader with fail-safe update. For protocols, remember I2C open-drain pull-ups and bus capacitance, SPI mode polarity and phase, CAN differential termination and arbitration, and UART framing and parity limits.
Worked example
A 2.4 GHz link has 10 dBm transmit power, 2 dBi gain at each end and 100 m range. Free-space path loss is about 80 dB at 2.4 GHz for 100 m. Received power ≈ 10 + 2 + 2 − 80 = −66 dBm. With receiver sensitivity −85 dBm, the margin is 19 dB before cable and body losses. If the requirement is 20 dB fade margin, the link fails and needs more gain, lower loss or shorter range.
Traps
Sampling at exactly 2fmax; forgetting the anti-alias filter; mixing dBm and dBi; ignoring cable loss; assuming a 32-bit MCU is automatically real-time; and using a watchdog without a safe recovery path.