Chapter 4 Bandpass Circuits Limiters Mixers Upconverters and

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

Limiters Ø Limiter is a nonlinear circuit with an output saturation characteristic. Ø It rejects envelope variations but preserves the phase variations. Ideal limiter characteristic with illustrative input and unfiltered output waveforms. 2

Mixers Ø 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. SSB mixer 3

Mixers 4

Choosing LO Frequency of Mixers Up-conversion Down-conversion Bandpass Filter Baseband/bandpass Filter (fc-f 0) Ø If (fc- f 0) = 0 Low Pass Filter gives baseband spectrum Ø If (fc- f 0 )> 0 Bandpass filter Modulation is preserved Filter Output: Ø If fc>f 0 modulation on the mixer input is preserved Ø If fc<f 0 ‘ ’ needs to be positive Complex envelope is conjugated ~ sidebands are exchanged 5

Mixers (Up Converter and Down Converter) Ø Complex envelope of an Up Converter: - Amplitude is scaled by A 0/2 Ø Complex envelope of a Down Converter: i. e. , f 0<fc down conversion with low-side injection - Amplitude is scaled by A 0/2 i. e. , f 0>fc down conversion with high-side injection - Amplitude is scaled by A 0/2 - Sidebands are reversed from those on the input 6

Mixer Realizations Without Multipliers Ø Multiplication operation needed by mixers can be obtained by using a nonlinear device together with a summer. Nonlinear device used as a mixer. 7

Mixer Realizations Without Multipliers Ø Multiplication operation needed by mixers can also be obtained by using an analog switch. Linear time-varying device used as a mixer. 8

Mixer Realizations Without Multipliers Analysis of a double-balanced mixer circuit. 9

Frequency Multiplier ØFrequency Multipliers consists of a nonlinear device together with a tuned circuit. The frequency of the output is n times the frequency of the input. 10

Detector Circuits Ø Detectors convert input bandpass waveform into an output baseband waveform. Ø Detector circuits can be designed to produce R(t), Θ(t), x(t) or y(t). • Envelope Detector • Product Detector • Frequency Modulation Detector Information Signal input processing m Carrier circuits Transmission medium (Channel) Carrier circuits Signal processing Detector Circuits 11

Envelope Detector Ø Ideal envelope detector: Waveform at the output is a real envelope R(t) of its input Bandpass input: Envelope Detector Output: K – Proportionality Constant Diode Envelope Detector Circuit 12

Envelope Detector Ø 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) 13

Product Detector Ø Product Detector is a Mixer circuit that down converts input to baseband. fc- Freq. of the oscillator θ 0 - Phase of the oscillator 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: 14

Different Detectors Obtained from Product Detector Ø Oscillator phase synchronized with the in-phase component We obtain INPHASE DETECTOR. Ø We obtain QUADRATURE PHASE DETECTOR Ø We obtain ENVELOPE DETECTOR If the input has no angle modulation and reference phase (θ 0) =0 Ø We obtain PHASE DETECTOR If an angle modulated signal is present at the input and reference phase (θ 0) =90 The product detector output is or If the phase difference is small The output is proportional to the Phase difference (Sinusoidal phase characteristics) 15

Frequency Modulation Detector Ø A ideal FM Detector is a device that produces an output that is proportional to the instantenous frequency of the input. Frequency demodulation using slope detection. • The DC output can easily be blocked 16

Frequency Detector Using Freq. to Amplitude Conversion Figure 4– 16 Slope detection using a single-tuned circuit for frequency-to amplitude conversion. 17

Balanced Discriminator 18

Balanced zero-crossing FM detector. 19

Phase Locked Loop (PLL) Ø PLL can be used to Track Phase and Frequency of the carrier component of the incoming signal Ø Three basic components: - Phase Detector : Multiplier (phase comparator) - VCO : Voltage Controlled Oscillator - Loop filter: LPF Ø Operation is similar to a feedback system Basic PLL. 20

PLL, Voltage Controlled Oscillator (VCO): Ø Oscillator frequency is controlled by external voltage Ø Oscillation frequency varies linearly with input voltage Ø If e 0(t) – VCO input voltage, then its output is a sinusoid of frequency (t)= c+ce 0(t) Ø c - free-running frequency of the VCO. Ø The multiplier output is further low-pass-filtered & then input to VCO Ø This voltage changes the frequency of the oscillator & keeps it locked. 21

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: Ø e(t) is called the Phase Error. The Phase Error voltage characteristics is SINUSOIDAL. ØA PLL can track the incoming frequency only over a finite range Lock/hold-in range Ø The frequency range over which the input will cause the loop to lock pull-in/capture range 22

Phase Locked Loop (PLL) Ø Various types of Phase Detector characteristics used in PLL’s. 23

Aplications of PLL Ø PLL used for coherent detection of AM signals. • A synchronized carrier signal is generated by the PLL. • 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 Figure 4– 24 PLL used for coherent detection of AM. 24

Aplications of PLL Ø 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. Figure 4– 25 PLL used in a frequency synthesizer. 25