Analog and Telecommunication Electronics -...
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13/04/2011 - 1 ATLCE - C5 - © 2011 DDC
Politecnico di Torino - ICT School
Analog and Telecommunication Electronics
C5 - Synchronous demodulation
» AM and FM demodulation» Coherent demodulation» Tone decoders
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Lesson C5: synchronous demod.
• FM demodulators
• AM demodulators– Envelope and – Synchronous demodulators
• Digital modulations– FSK, PSK, ASK and others
• Tone decoders– NE 567 block diagram and parameters
• References sect. 3.7.1, 3.7.2
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FM mo-demodulation
• Carrier frequency Fp, modulating signal M(t)– Fp = Fpo + K M(t)– Actual spectrum related with K
• Demodulation:– Techniques derived from AM
» Change FM into AM» Use AM demodulator
– PLL-based technique» Fp must be in the lock range» M(t) proportional to Vc(t)
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Analog FM example
• Red: modulating signal
• White: FM signalAdd spectrum
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FM AM demodulation
• Exploit the transition region of a steep filter– Frequency changes become amplitude changes
f
|V1/V2|
FM
AM
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FM PLL demodulation
• Fp = Fpo + K M(t)
• PLL locked– Full Fp spectrum
in the lock range
– Vc(t) proportional to M(t)
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Lesson B5: synchronous demod.
• FM demodulators
• AM demodulators– Envelope demodulators – Synchronous demodulators
• Digital modulations– FSK, PSK, ASK and others
• Tone decoders– NE 567 block diagram and parameters
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AM signals
• Modulation:– Multiplication of carrier P(Fp) by modulating signal M
» S = Fp x M» Time domain: M is the envelope of S(t)» Frequency domain: M spectrum M(0) translated to M(Fp )
• Demodulation– Obtain envelope:
» Half- or full-wave rectifier + LPF– Move S to baseband (spectral translation)
» Coherent demodulation» Needs reference signal: PLL
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Examples of AM signals
• AM modulation– Modulating signal: A sin t + B
» max: A+B, min: B-A» Modulation index m
m = (max-min)/(max+min) = A/B» m = 1: A = B (DC = Vp)
– Examples with simulator» AM 25% (m = 0,25)» AM 50% (m = 0,5)» AM 100% (m = 1)» AM 200% (PSK-180°, DSB – suppressed carrier)
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AM modulation: m = 0,25
• A = 2,5V, B = 10 V, m = 0,25 (25%)
spectrum
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AM modulation: m = 0,5
• A = 5V, B = 10 V, m = 0,5 (50%)
spectrum
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AM modulation; m = 1
• A = 10V, B = 10V, m = 1 (100%)– the spectrum shows the carrier – Modulation signal envelope (red)
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AM modulation - PSK
• A = 10V, B = 0V (suppressed carrier, PSK/180°)– the carrier does not appear in the spectrum – Modulation signal envelope (red)
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Envelope demodulator
• Peak detectorwith loss
– C charged to+ peaks, discharged through R
• Problems:– Diode threshold– Noise sensitivity– Not for M > 1
• Improvements– Active diode– Full wave rectifier
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Coherent demodulation
• B1 cuts outband noise• The PLL generates the reference signal • The multiplier M moves Vi to baseband• B3 isolates baseband signal
π/2 phase shift
M
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Effect of frequency drift
• Changes in Fi cause phase error in the PLL– To move the VCO: Vc 0– To get Vc 0: θe 0– Output error related with cos θe– Cross-demodulation
(from FM to AM)
• To reduce the error– Increase loop gain– I/Q demodulator
ior
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AM demodulator output vs frequency
VCO control (Vc)
AM demodoutput
i, oor
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I-Q demodulation
tsinVV
tcosVV
S
C
Q(cos)
I(sin)
Vc(Qcomp.)
Vs(I component)
V2
S2
C VVV
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Analog I-Q demodulation
• Independent I-Q amplitude demodulation
• Va independent from Ve – Vo phase relation
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Digital I-Q demodulation
• A/D conversion after the multiplier (baseband)
• Digital processing
ADC
ADC
DSP
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RF I-Q demodulation
• Direct A/D conversion of RF signal • Digital I/Q demodulation• Digital processing
A/D
X
X
sincos
DigitalprocessingAnti-alias filter
(lowpass or bandpass)
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Lesson C5: synchronous demod.
• FM demodulators
• AM demodulators– Envelope demodulators – Synchronous demodulators
• Digital modulations– FSK, PSK, ASK and others
• Tone decoders– NE 567 block diagram – Tone decoder parameters
• Lab experiment 5: tone decoder behavior
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Digital modulations
• ASK, PAM– Amplitude Shift Keyed
Pulse Amplitude Modulation
• FSK– Frequency Shift Keyed, FM to 2, …N frequencies
• PSK– Phase Shift Keyed, PM to 2, …N phases
• Mixed, QAM– …
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FSK signal
• FSK frequency shift keyed
Q
Iω1
ω2
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Digital FM FSK
• Same techniques as analog FM– The transient response depends on loop parameters
• Separate AM channels for each carrier– Tone decoders with WTA
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FSK Demodulators
• Frequency Shift Keyed: FM to 2, …N frequencies
• Demodulation techniques– Single PLL, always locked output = Vc– Multiple coherent AM detector
» For each frequency PLL + AM coherent demodulators» output comparison (WTA)
– Multiple envelope detector» Passband filter bank + envelope demodulators» output comparison (WTA)
• Tradeoff for all circuits (symbol passband filters)– Wideband: fast response, noise-sensitive– Narrowband: slow response (good if Bitrate < carrier/10)
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PAM/ASK demodulators
• Pulse Amplitude Modulation (PAM) or Amplitude Shift Keyed (ASK)
– Discrete AM (2, …N levels)
• Demodulation techniques– Envelope detector– Coherent demodulation (PLL)– + output comparison (WTA)
• Tradeoff for all circuits: post-detection filter– Wideband : fast response, noise-sensitive– Narrowband: slow response
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ASK signal
• ASK (PAM) discrete amplitude change
Q
I
S1S2
2-level ASK
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PSK signal
• PSK phase shift
Q
I
S1 S2
Q
I
2-phase(rotation π)
8-phase(rotation π/4)
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Complex signal (ASK+PSK)
• Amplitude and phase discrete changes– QAM– Signal constellation
• Example with 16 symbols– In the I/Q plane can be seen
as combinations of – 4 possible I values– 4 possible Q values
I
Q
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Complex signals demodulation
• Example 2– Phase shift: п/2
(4 states)– Amplitude levels 2– 3 bit/symbol
• In both cases:
• I/Q demodulation• The I/Q components
are obtained by coherent demodulation
I
Q
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I/Q demodulation: A/P components
tsinVV
tcosVV
S
C
C
S
2S
2C
VVarctgV
VVV
-
:
Artg(Ve)
X/Y
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I/Q demodulation: I/Q components
-
: ADC
ADC
I1I0
Q1Q0
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Complex signal (ASK+PSK)
• Amplitude and phase discrete changes– QAM– Signal constellation
• Example with 16 symbols– In the I/Q plane can be seen
as combinations of – 4 possible I values– 4 possible Q values
I
Q
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The ADPLL
• PLL based on digital circuits– Wired logic– Programmable logic– Processor + SW DSPLL
• Programmable parameters, with better control
• Digital Vi ADC on Vd– Digital loop filter + Numeric Controlled Oscillator
• Analog Vi ADC on Vi– Digital PD + Digital loop filter + NCO
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Example of DSP-PLL: Si550
• Variable frequency clock generator– Analog PD, then ADC – 10 MHz – 1,4 GHz range (Si550)
From https://www.silabs.com
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Lesson C5: synchronous demod.
• FM demodulators
• AM demodulators– Envelope demodulators – Synchronous demodulators
• Digital modulations– FSK, PSK, ASK and others
• Tone decoders– NE 567 block diagram and parameters
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Integrated Tone Decoder
• NE567, LM576, …: bipolar / LMC567: CMOS– Same function, sligthly different structures
• Tone decoder operation: – Recognize a carrier (tone) within a frequency interval, even with
strong interferences by noise and other signals– Used to recognize commands, numbers (phone), signalling
overlapped with other signals
• Structure– PLL for analog signals + AM synchronous demodulator
• Features – Very narrow bandwidth– ON/OFF output + intermediate analog output
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Tone decoder: specs
• Frequency range– Allowed values for the tome to be recognized
• Bandwidth – Frequency window for deconding of the tone
• Input amplitude range– Maximum in-band input signal which is not recognized– Minimum in-band input signal which is recognized
• Noise and interferers – Allowed wideband noise – Allowes narroband interferers (other tones)
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Tone decoder: bandwidth
• The device id designed to recognize a specific frequency, with minimum change
– The PLL can lock only to a narrow frequency range– The possible shift of the VCO is minimum
• Detection bandwidth: about 15% of central frequency
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Tone decoder: input range
• Amplitude range for tone recognition
• Guaranteed minimum input level which is definitely recognized by the tone decoder (max of the min levels among several devices)
• Guaranteed maximum input level which is NOT recognized by the tone decoder (min of max levels)
• Same kind of specs as Vih/Vil for logic circuits
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Tone decoder: noise and interferers
• Noise immunity
• Tone detection capability with noise or interferer stronger than the tone itself
– (Outband signal)/(inband tone): + 6 dB– signal(tone)/(wideband noise): - 6 dB
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Tone decoder: bandwidth and F(s)
• The next diagram plots the capture range vs VI and filter parameters
• Detection bandwidth (capture range) depends on:– Input level– Loop filter F(s)
• Linear area– Vi < 100 mV– Capture range depends on Vi
• Saturation area– Vi > 200 mV– Capture range does not depend on Vi
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Linear and saturating input levels
saturation
Linear operation
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Tone decoder A: block diagram
1
2
3
4
5
6
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Tone decoder functional units
1. Phase detector2. Amplitude demodulator
– Gilbert cell multiplier
3. Loop filter4. AM filter
– Single pole (RC), Internal R, external capacitor
5. VCO and I/Q outputs– C charge/discharge, threshold control– Third comparator or double frequency VCO
6. Output stage– open collector, high current capability (150 mA)
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PD and AM demodulators
• Gilbert cell multipliers
Input Vx
Input Vy
Output Iz (current)
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Linearization of differential stage
Vx input(Vo from VCO)
Vy input(Vi signal)
Input biasnetwork
R26, R27: emitter feedback
Vy input to AM demodulator
output Iz(current)
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I-C fixed τ VCO
• Unique time constant (τ = RC)• Frequency control by variable thresholds (Vs1, Vs2)
– Limited frequency change
Vs1
Vs2
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I/Q VCO outputs
• Additional comparator on the capacitor voltage– Almost triangular waveform on C– Intermediate threshold Vs3
Va
VoVs3 Vo
Va
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VCO waveforms
• Additional comparator on the capacitor voltage– Almost triangular waveform
Vs1
Vs2
Vs3
VoVa
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Tone decoder: VCO circuit
C charge/dischargeswitches
RC timinggroup
VCO Vs1comparator
VCO Vs2comparator
Q-oputputComparator(Vs3)
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Tone decoder B: block diagram
• CMOS features– Higher R/Z– Lower current– Better I/Q VCO
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Double VCO frequency I/Q
• VCO operates at double frequency– I/Q signals by Mod2 dividers on complementary edges
VoVa
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Lab experiment 4
• Design a circuit using the NE567 IC, to obtain a tone decoder with detection range centered on 25 kHz, and the maximum bandwidth allowed by the device.
• Build the circuit
• Verify the tone decoder parameters– Measure capture and lock ranges– Find the amplitude threshold (at central frequency) – Plot the VCO characteristic: Fo(Vc)– Plot the Va(Fi) and Va(Vi) characteristic– Verify the behaviour with noise and interferers at the input
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NE 576 – PD and AM demodulator
• PD and AM demodulator outputs vs input signal frequency.
VCO control voltage(butterfly plot)
Coherent AM demodulatoroutput(cosine shape)
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Lesson C5
• Describe the techniques for FM demodulation, with and without PLL.
• Define the criteria to select R and C in the AM envelope detector.
• Which are the benefits of coherent AM demodulation vs envelope demodulation?
• How to reduce the FM sensitivity of an AM coherent demodulator ?
• List the parameters which describe the performance of a tone decoder.
• Propose some circuits to generate I/Q signals from a single VCO.