1. Eeng 360 1 Chapter 4 Bandpass Circuits   Limiters   Mixers, Upconverters and Downconverters   Detectors, Envelope Detector, Product Detector   Phase Locked Loops (PLL) Huseyin Bilgekul Eeng360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University

2. Eeng 360 2 Limiters   Limiter is a nonlinear circuit with an output saturation characteristic.   It rejects envelope variations but preserves the phase variations.

3. Eeng 360 3 Mixers   Ideal mixer is a mathematical multiplier of two input signals. One of the signals is sinusoidal generated by a local oscillator. Mixing results in frequency translation. Input signal: Output: SSB mixer

4. Eeng 360 4 Mixers (Choosing LO Freq.)   If (f c - f 0 ) = 0  Low Pass Filter gives baseband spectrum   If (f c - f 0 )> 0  Bandpass filter  Modulation is preserved Filter Output:   If f c >f 0  modulation on the mixer input is preserved   If f c <f 0  ‘  ’ needs to be positive Complex envelope is conjugated ~ sidebands are exchanged -f → Upper & lower sidebands are exchanged * → Phase spectrum is inverted Up-conversionDown-conversion Bandpass Filter Baseband/bandpass Filter (f c -f 0 )

5. Eeng 360 5 Mixers (Up Converter and Down Converter)   Complex envelope of an Up Converter: - Amplitude is scaled by A 0 /2   Complex envelope of a Down Converter: i.e., f 0 <f c  down conversion with low-side injection - Amplitude is scaled by A 0 /2 i.e., f 0 >f c  down conversion with high-side injection - Sidebands are reversed from those on the input - Amplitude is scaled by A 0 /2

6. Eeng 360 6 Mixer Realizations Without Multipliers Multiplication operation needed by mixers can be obtained by using a nonlinear device together with a summer. Multiplication operation needed by mixers can also be obtained by using an analog switch.

7. Eeng 360 7 Frequency Multiplier   Frequency Multipliers consists of a nonlinear device together with a tuned circuit. The frequency of the output is n times the frequency of the input.

8. Eeng 360 8 Detector Circuits Transmission medium (Channel) Carrier circuits Signal processing Carrier circuits Signal processing Information m input   Detectors convert input bandpass waveform into an output baseband waveform.   Detector circuits can be designed to produce R(t), Θ (t), x(t) or y(t). Envelope Detector Product Detector Frequency Modulation Detector

9. Eeng 360 9 Envelope Detector   Ideal envelope detector: Waveform at the output is a real envelope R(t) of its input Bandpass input: K – Proportionality Constant Envelope Detector Output: Diode Envelope Detector Circuit

10. Eeng 360 10 Envelope Detector   The Time Constant RC must be chosen so that the envelope variations can be followed. In AM, detected DC is used for Automatic Gain Control (AGC)

11. Eeng 360 11 Product Detector   Product Detector is a Mixer circuit that down converts input to baseband. Output of the multiplier: LPF passes down conversion component: Where g(t) is the complex envelope of the input and x(t) & y(t) are the quadrature components of the input: f c - Freq. of the oscillator θ 0 - Phase of the oscillator

12. Eeng 360 12 Different Detectors Obtained from Product Detector   Oscillator phase synchronized with the in-phase component We obtain INPHASE DETECTOR. The product detector output isor If the phase difference is small   We obtain QUADRATURE PHASE DETECTOR   We obtain ENVELOPE DETECTOR If the input has no angle modulation and reference phase (θ 0 ) =0   We obtain PHASE DETECTOR If an angle modulated signal is present at the input and reference phase (θ 0 ) =90  The output is proportional to the Phase difference (Sinusoidal phase characteristics)

13. Eeng 360 13 Frequency Modulation Detector The DC output can easily be blocked   A ideal FM Detector is a device that produces an output that is proportional to the instantenous frequency of the input.

14. Eeng 360 14 Frequency Detector Using Freq. to Amplitude Conversion

15. Eeng 360 15 Phase Locked Loop (PLL)   PLL can be used to Track Phase and Frequency of the carrier component of the incoming signal   Three basic components: - Phase Detector : Multiplier (phase comparator) - VCO : Voltage Controlled Oscillator - Loop filter: LPF   Operation is similar to a feedback system

16. Eeng 360 16 PLL, Voltage Controlled Oscillator (VCO) Voltage Controlled Oscillator (VCO):   Oscillator frequency is controlled by external voltage   Oscillation frequency varies linearly with input voltage   If e 0 (t) – VCO input voltage, then its output is a sinusoid of frequency  (t)=  c +ce 0 (t)    c - free-running frequency of the VCO.   The multiplier output is further low-pass-filtered & then input to VCO   This voltage changes the frequency of the oscillator & keeps it locked.

17. Eeng 360 17 Phase Locked Loop (PLL) Let input signal be : Let the VCO output be: The phase detector output v 1 (t) is given by : The sum frequency term is rejected by LPF so the filter output v 2 (t) is:    e (t) is called the Phase Error. The Phase Error voltage characteristics is SINUSOIDAL.   A PLL can track the incoming frequency only over a finite range  Lock/hold-in range   The frequency range over which the input will cause the loop to lock  pull-in/capture range

18. Eeng 360 18   Various types of Phase Detector characteristics used in PLL’s. Phase Locked Loop (PLL)

19. Eeng 360 19   PLL used for coherent detection of AM signals. A synchronized carrier signal is generated by the PLL. extra phase shift is needed VCO locks with 90  phase difference so a - 90  extra phase shift is needed. The generated carrier is used with a product detector to recover the envelope Aplications of PLL

20. Eeng 360 20   PLL used as a frequency synthesizer. Frequency dividers use integer values of M and N. For M=1 frequency synthesizer acts as a frequency multiplier. Aplications of PLL