FM Receiver

1. Chapter 5 FM Receivers

2. FIGURE 5.1 Double-conversion FM receiver block diagram Basically - - - > similar to AM receivers Double-Conversion Superheterodyne FM Receivers

3. Prevent mixer saturation when strong RF signals are received 8) AGC Remove info signal from FM wave 7) Detector/demodulator Clipping amplitude varied (noise) 6) Limiter Provide gain & sensitivity 5) IF Amplifier Down converts 1 st IF to 2 nd IF * Normally 2 nd IF low - - > 455 KHz 4) 2 nd mixer/converter Down converts RF to 1 st IF * Normally 1 st IF high - - > 10.7 MHz 3) 1 st mixer/converter Establish SNR & NF 2) RF Amplifier Reject f image 1) Preselector Main Function Main Stage / Block

6. Figure 5.3: Slope detector (a) schematic diagram (b) voltage-versus-frequency curve. Linear portion AM out AM peak detector

8. Balanced slope detector Tuned circuit Balanced peak detector f a > f c f a < f c Figure 5.4: Balanced slope detector (a) schematic diagram (b) voltage-versus-frequency response curve.

11. Figure 5.5: Foster Seeley discriminator (a) schematic diagram (b)vector diagram, f in = f o ; (b) f in > f o ; (c) f in < f o

13. 6. Is (secondary winding T1) always have 90 o phase inversion with V La & V LB Is ∠ θ o <= = > VLb ∠ θ o ± 90 o & VLb ∠ θ o ± 90 o 7. V D1 is the vector sum of V L3 & V La V D2 is the vector sum of V L3 & V Lb Previously, know that V L3 is fed directly by Vin - - - > same phase & value. 8. If input frequency ( f in ) same with resonant freq of the secondary tank circuit (IF centre freq), I s is in the phase with total secondary voltage (V s ) : f in = f o (IF centre frequency) : C 1 & C 2 charge to equal magnitude voltage but opposite polarities : V D1 & V D2 will have equal voltages : V out = V C1 - V C2 = 0 The phase relation can be represented as Figure 5.5 (b) 9. If IF goes above resonance (X L > X C ), tank circuit impedance become inductive & I s lags the V s by some angle, θ ’ o which is proportional to the magnitude of the ∆ f : f in > f o (incoming IF signal freq > IF centre freq) : C 1 charges & C 2 discharges : V D1 > V D2 (sum vector of V D1 > sum vector of V D2 ) : V out = V C1 – V C2 = +ve value The phase relation can be represented as Figure 5.5 (c) V La V Lb Is V D2 V La V Lb Vin V D1 V s I s Θ ’ o

16. Figure 5.7 : Ratio detector (a) schematic diagram; (b) voltage-versus-frequency response curve

19. Figure 5.8 (a) Block diagram for a PLL FM demodulator.

22. Figure 5.8 (b) PLL FM demodulator using the XR-2212 PLL

24. Figure 5.9 Amplitude limiter input and output waveforms: (a) input waveform; (b) output waveform

25. FIGURE 5.10 Limiter output: (a) captured by noise; (b) captured by signal.

27. FIGURE 5.12: Stereo FM transmitter using frequency-division multiplexing

32. FIGURE 5.14

35. FIGURE 5.15 FM stereo and mono receiver