Eeng 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

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

Eeng 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

Eeng 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 )

Eeng 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

Eeng 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.

Eeng 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.

Eeng 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

Eeng 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

Eeng 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)

Eeng 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

Eeng 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)

Eeng 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.

Eeng Frequency Detector Using Freq. to Amplitude Conversion

Eeng 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

Eeng 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.

Eeng 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

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

Eeng   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

Eeng   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