FM Generation and Detection

1. FM Generation and Detection Analog and Digital Communications Autumn 2005-2006 CS477: Analog and Digital Communications

2. Observation for falls off quickly for  Consider components only for Significant frequency components lie within Observation for is insignificant for  Consider components only for Significant frequency components lie within FM Bandwidth Consider Tone Modulation CS477: Analog and Digital Communications

3. depends upon the allowable distortion level Usually approximated by For worst case, use: What is the corresponding value of NO! A smaller may result in larger FM Bandwidth: Single Tone Is this the maximum value? CS477: Analog and Digital Communications

4. Use For practical systems : FM Bandwidth: Generic Signal Estimate BW using worst case tone modulation! Define Deviation ratio: Approximations: Carson’s rule: Carson’s rule is applied for extreme values of deviation ratio. It underestimates the BW for practical systems. (Do Example 5.2-1 and Exercise 5.2-1 in text) CS477: Analog and Digital Communications

5. Transmission BW: Summary CS477: Analog and Digital Communications

6. Generating NBFM Signals • Direct method • Use a voltage-controlled oscillator • Output frequency is proportional to input voltage • Use information signal as the input voltage • Restricted to low power applications • Most circuits unstable at carrier frequency • Not suitable for commercial FM broadcast • Require sophisticated feedback control CS477: Analog and Digital Communications

7. (NBPM) 900 NBPM Modulator NBFM Output Input Generating NBFM Signals Indirect method: (NBPM/NBFM) CS477: Analog and Digital Communications

8. NBFM Modulator Frequency Converter Frequency Multiplier Non-linear Device Bandpass Filter Generating WBFM Signals Direct Method  Use a VCO (as in case of NBFM) Indirect Method  Generate NBFM first Frequency converter might be needed if the multiplier produces a carrier at a frequency different from the desired carrier frequency! CS477: Analog and Digital Communications

9. FM Detection • Four primary methods • Differentiator with envelope detector • FM to AM conversion • Phase-shift discriminator • Approximates the differentiator • Zero-crossing detector • Frequency feedback • Phase lock loops (PLL) CS477: Analog and Digital Communications

10. Amplitude Limiter Signal Differentiator Envelope Detector DC Block FM Detection: FM to AM Differentiate the FM signal to get an AM signal! Then use an Envelope Detector! CS477: Analog and Digital Communications

11. Can we produce Amplitude Limiter+BPF LPF Phase-shift Network For small values of FM Detection: Discriminator CS477: Analog and Digital Communications

12. FM Detection: Zero crossing detector Idea: Produce a pulse at each zero crossing; Integrate over a suitable period Higher frequencies produce more pulses; More pulses result in higher output values; Output values are proportional to message Want to track message but not the carrier! CS477: Analog and Digital Communications

13. LPF VCO FM Detection: Phase Lock Loops Basic Task of PLL: Lock the instantaneous angle of a VCO with the instantaneous angle of an external signal. i.e., lock (or track) the phase and frequency. CS477: Analog and Digital Communications