Chapter 7 - Frequency Tuning and Simple Synchronization

Tuning

Select a frequency band of interest and capture/receive signals within that band. Center frequency is the local oscillator (LO). Bandwidth set by filtering and setting of sample rate

Synchronization

The need to correct for differences in time/frequency (e.g. frequency difference of the carrier signal)

Channel Effects

Radio signals go through attenuation, noise, interference, fixed or time varying propogation delay (translates into a phase shift of θ degrees), and multipath propogation. If the transmitter/receiver are moving, the propogation delay can increase or decrease over time (translates into a frequency shift ("doppler shift")).

Phase Locked Loops (PLLs)

Are able to recreate an input sinusoid, track deviations in its frequency, and reject noise. Obtains "Phaselock" when the output PLL sine wave converges to the same original TX signal frequency (not necessarily same PHASE, but same FREQUENCY).

Made up of 3 components:

• Phase Detector: generates a varying signal based on the phase difference of rx and locally generated sin wave. Calculates "Phase Error" = θ_e(t). In the diagram θ_i = input phase, θ_o = local oscillator phase. K_p is a scalar phase detector gain value. The phase error approaches 0 as the PLL output signal approaches the original intended TX signal.
• Controllable Oscillator: A VCO (Voltage Controlled Oscillator) (analogue) or NCO (Numerically Controlled Oscillator) (digital) which generates a sin output with phase/frequency based on its input. The filtered phase error is input, and "the larger the error, the larger the adjustment applied"
• Loop Filter: removes high frequency from phase detector and outputs to the VCO/NCO

Carrier Synchronization for transmitted carrier AM (AM-DSB-TC)

Coherent RX extracts the transmitted carrier component and uses it to demodulate to baseband. Done using a PLL with the carrier as the input reference signal.

Carrier Synchronization for supressed carrier AM (AM-DSB-SC)

for SC, the carrier is not explicitly present. Have to recover the carrier from the modulated RX signal, then demodulate. Commonly used way to do this is the "Costas Loop."