gnss receiver front-ends ii: componentskom.aau.dk/~dpl/courses/mm08_slides.pdf · danish gps center...

DANISH GPS CENTER

GNSS Receiver Front-ends II: ComponentsGPS Receiver Technology MM8Darius Plauš[email protected]

Based on original slides from Ragnar V. Reynisson

DANISH GPS CENTERAgenda

• Receiver Component Overview• Filters• Low Noise Amplifiers (LNAs)• Mixers• Frequency Synthesizers• ADCs

2008 Danish GPS Center 2

DANISH GPS CENTERA Simple Receiver Architecture


I

QRF DetIFFRQ

Conv

Filter unwantedsignals and amplify

LO

Filter LNA outputConvert to IF freq

Filter mixer outputAmplify signal

90°

I

Q

Convert IF signalto baseband I/Q

An RF front-end of a GNSS

receiver

In GNSS this is usually a part of the

digital signal processing

DANISH GPS CENTER

Receiver ComponentsFilters

DANISH GPS CENTERFilters

• Filters are used to extract the desired frequency band from the input while rejecting the remainder:– Low pass: Reject high frequencies– High pass: Reject low frequencies– Band pass: Reject frequencies higher and lower

than passband


s (t)in s (t)in s (t)ins (t)out s (t)out s (t)out

Low-Pass High-Pass Bandpass

DANISH GPS CENTERFilter Properties


-3dB-1dB0dB

Insertion loss

Ideal filter

Less frequency selective

More frequencyselective

fcenter

DANISH GPS CENTERFilter Key Parameters

• Passband attenuation/gain• Stopband attenuation• Bandwidth

– 3 dB bandwidth– Cutoff frequencies

• Filter order • Filter type (Chebyshev, Butterworth,…)

– Passband/Stopband gain ripple– Phase shift

• Active/Passive filters


DANISH GPS CENTERFilter Parameters (Example)

• Sample lowpass filter (Chebyshev I, 2nd order)


10-1

100

101

-70

-60

-50

-40

-30

-20

-10

0

ω [rad/s]

H( ω

) [dB

]

Passband

Transitionalband

Stopband

DANISH GPS CENTERFilters (Example 2)

Bode Diagram

Frequency (rad/sec)

Pha

se (d

eg)

Mag

nitu

de (d

B)

-70

-60

-50

-40

-30

-20

-10

0

10-0.09

10-0.07

10-0.05

10-0.03

10-0.01

100.01

100.03

100.05

100.07

-180

-135

-90

-45

0

45

90

135

180

• Same filter converted to bandpass– Center frequency 1 rad/s– Bandwidth 1.33e-2 rad/s Corresponds to 20 MHz @ 1.5 GHz (GPS)



• For physical filter construction, there is a tradeoff between the filter specifications and the complexity of the filter

• At RF, active filters are possible, but even state-of-the-art active filters are noisy



• Lumped element (Capacitor/Inductor) filters are limited by component quality factor (loss) Lossy components mean less sharp filters.– Possible to obtain some selectivity, but never enough

• For sharp filtering at RF, Surface Acoustic Wave filters are used– Standard filters available for GPS– Impossible to integrate on IC– Tradeoff between selectivity and filter loss

• Filter noise figure = filter loss!• High attenuation also detrimental to selectivity, specially at input

of receiver


DANISH GPS CENTER

Receiver ComponentsLow Noise Amplifiers

DANISH GPS CENTERLow Noise Amplifiers

• Placed early in the receiver chain (typically: immediately after antenna filter)

• Purpose: Amplify the signal while adding a minimum of noise– Later stages (mixers, baseband amplifiers) often

are very noisy– High gain/Low noise figure early in the receive

chain improves overall receiver noise figure• Design problems:

– Gain/Noise/Distortion tradeoff– Power consumption


DANISH GPS CENTERLNA Key Parameters

• Gain (Power/Voltage)• Noise Figure• Linearity (Intercept points/Compression)• Power Consumption (Noise/Gain/Linearity tradeoff)• Input/Output impedance matching

– LNA between antenna filter/image rejection filter– Passive filters depend on some nominal impedance at output– Important to present correct impedance to input/output– LNA source/load impedance imperative to noise/gain/linearity

performance tradeoff


DANISH GPS CENTER

Receiver ComponentsMixers

DANISH GPS CENTERMixers

• Used for frequency conversion of information

• Mixers used at different places in the receiver chain– Mixer functional description

• Multiplication/Non-linearity• Switching/Clipping

– Image problem– Single Balanced / Double Balanced– Image Rejection Mixer


DANISH GPS CENTERMixers: Mixer function

• Functional description of a mixer: Analog multiplier– Multiplies input signal and local oscillator (LO) signal

• Multiplication in time domain Convolution in frequency domain (Fourier)

• Information in the input signal converted both up and down in frequency sum and difference of carrier/LO frequencies


DANISH GPS CENTER

Mixers: Frequency Domain View


*

DANISH GPS CENTER

Mixers: Frequency Up-conversion

0 1 2 3 4 5 6 7 8 9 10

-1.5

-1

-0.5

0

0.5

1

1.5


0 1 2 3 4 5 6 7 8 9 10-1.5

-1

-0.5

0

0.5

1

1.5

Modulation Carrier

DANISH GPS CENTER

Mixers: Frequency Down-conversion


0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 10-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7 8 9 100

0.5

1

1.5

DANISH GPS CENTERMixers: Key Parameters

• Conversion gain: Ratio of output signal power (at IF) to input signal power (at RF)

• Isolation: The ratio between LO input power and the fraction of the LO that appears at input/output ports

• Noise figure• Balanced/Unbalanced• Intercept points• Power consumption


DANISH GPS CENTER

Mixers: Balanced / Single Ended• Simple mixers: large unwanted frequency content at

mixer output– Local oscillator signal (+ harmonics)– RF signal (+harmonics)– Intermodulation products

• Using more than one mixer, this can be alleviated


DANISH GPS CENTERMixers: Image Signal

• Consider the down conversion alone:– The distance from the RF signal and the LO signal

corresponds to the Intermediate Frequency (IF)– A signal that has the same distance from the LO

signal (at opposite side) will also be converted to the IF Image signal


DANISH GPS CENTERMixers: Image Rejection

• Steps must be taken to ensure that any signal in the image band is eliminated before down-conversion– Image reject filter– Image rejection mixer

• Image rejection filter must pass the wanted (RF) frequency band while reject the image frequency choice of IF… must be sufficiently high to aid in filtering

• Image rejection mixers use phase properties of wanted/image signals to reject the image signal– Uses 4 mixers along with phase shifts Increased power

consumption– Matching of components important


DANISH GPS CENTER

Receiver ComponentsOscillators & Synthesizers

DANISH GPS CENTEROscillators & Synthesizers

• Oscillators are sine-wave (or square-wave) generators (local oscillators)

• Simple oscillators provide signal at a single frequency

• Oscillator consists of amplifier and a resonating feedback circuit (tank)

• Oscillator frequency can be tuned by varying feedback circuit– Old days: Mechanical tuning of LC circuits– Now: Electrical tuning of tank circuit


DANISH GPS CENTER

Oscillators & Synthesizers: Key Parameters• Output amplitude/power• Harmonic distortion• Phase noise• Power consumption• Clock (frequency) stability• For Frequency Synthesizers

– VCO frequency range– Reference frequency/feedback division


DANISH GPS CENTEROscillators

• An ideal oscillator supplies a single sine-wave• An actual oscillator includes phase noise, thermal

noise and signal harmonics (signal is sine-wave like but not quite)

• RF oscillators are either fixed or voltage controlled


Ideal Oscillator Actual Oscillator

Phase noiseHarmonics

DANISH GPS CENTERCrystal Oscillators

• A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency

• Crystal oscillators have good short term stability, low phase noise

• React to temperature changes and vibrations• Various types exist, but the most popular are:

– TCXO – temperature-compensated crystal oscillator

– OCXO – oven-controlled crystal oscillator


DANISH GPS CENTERFrequency Synthesizer

• A frequency synthesizer is basically a Phase Locked Loop with a fixed reference frequency at the input and frequency division in the feedback

• Usually used for generating channel frequencies with a certain spacing (no need for channels in GNSS)

• Has noise shaping properties comparedto free-running OSC


φ

PLL

DANISH GPS CENTEROscillators & Synthesizers• Synthesizers shape the noise function• Close to center frequency: less noise than free-

running oscillator• Outside loop bandwidth: more noise


DANISH GPS CENTER

Receiver ComponentsDemodulators

DANISH GPS CENTERDemodulators

• I/Q demodulator • Correlates the input RF signal with the In-

phase and Quadrature• Resulting signal is baseband information

signal


DANISH GPS CENTER

Receiver componentsAnalog to Digital Conversion

DANISH GPS CENTERAnalog to Digital Conversion

• After the I/Q demodulator and baseband filter/amplifier, signal is converted to digital domain (traditional receiver)

• Already the IF signal is converted to digital domain in a GNSS receiver

• Typically IF signal sampling requires a higher sampling rate then baseband signal sampling



• Quantization of signal adds noise to the signal– Noise power density assumed flat

• Important that the signal exploits the full range of ADC– Quantization noise stays the same reduction in

SNR• Oversampling improves SNR but costs more

in terms of power



• Sampling frequency selection depends on number of factors– Sampling also can be used for frequency

translation (like mixers)– Sampling frequency must be bigger than 2*f of the

highest frequency component of the sampled baseband signal

– Sampling frequency must be bigger than 2*B (B –two-sided signal bandwidth) of the sampled IF (bandpass) signal

• Signals must be filtered prior sampling to avoid signal aliasing



• Figures show steps (in frequency domain) of a signal sampling


DANISH GPS CENTER

Aliasing Due To Incorrect Sampling Frequency


DANISH GPS CENTERFrequency Translation


DANISH GPS CENTER

GPS Receiver TechnologyReceivers III: Receiver Architectures

DANISH GPS CENTERAgenda

• GPS Receiver requirements• The Superheterodyne Receiver• Direct Conversion Receivers• A GPS receiver IC


DANISH GPS CENTERGPS Receiver Requirements

• Input signal– Typical input signal around –130dBm (0.1fW)– Signal bandwidth of interest: 2MHz Noise floor of –110dBm

(Signal power < Noise power!)• Large gain through receiver

– -130dBm corresponds to an RMS amplitude of 70nV at the antenna

– To exploit the ADC to the fullest, need p-p voltage of 1-2 volts at the input

– Difference: 137 dB (voltage)– Noise floor 20 dB over signal level around 117 dB

difference– Typical voltage gain for GPS receiver: 106-107 dB

• Code correlation provides 43 dB of processing gain


DANISH GPS CENTERSuperheterodyne Receiver

• Invented by Armstrong (1918)• Good sensitivity performance• Good dynamic range • Gain/Selectivity spread along the chain

– Gradual attenuation of interfering signals• Not very flexible (i.e. usably for more than one

system)


0°

90°

I

Q

RF processing IF processing Baseband processing

LO1 ( )f - f = fLO RF IF±

fIF

DANISH GPS CENTER

Superheterodyne Receiver: RF Processing• Antenna filter removes unwanted input frequencies

– Low loss: High loss at the start of the receiver chain kills sensitivity

• LNA amplifies input signal while adding as little noise as possible– Goal: Amplify signal above noise floor of subsequent circuits,

most notably mixer• Image rejection filter (IRF) attenuates signal at mixer

image frequency– Higher selectivity than Ant.Filter possible as higher loss

allowed


Antenna filter Image rej. filter

LNA

DANISH GPS CENTER

Superheterodyne Receiver: IF Processing• Mixer converts the signal to the IF frequency

using the LO signal as a reference

• IF filter removes unwanted mixer output (harmonics/IM).

• IF amplifier amplifies resulting signal again• Design dilemma: Choice of IF frequency


Local Oscillator IF filterIF amplifierMixer

DANISH GPS CENTER

Superheterodyne Receiver: Baseband Processing• IQ demodulator converts IF signal into I and Q baseband

components• IFLO runs at the IF frequency

• BB filter and amplifier take care of the last gain and selectivity• Design dilemma: BBF/BBA noise performance

• Baseband circuits are influenced by flicker (1/f) noise• Gain is cheap at baseband compared to IF (power) but placing

too much gain at BB can hurt selectivity


0°

90°

I

Q

BB filter

BB amplifier

I/Q demodulator

IF LO

DANISH GPS CENTER

Superheterodyne Receiver: Choice of IF• The choice of IF presents a tradeoff situation• High IF:

– Relaxed requirements to image rejection filter attenuation

– More harsh requirement to IF filtering– Lower selectivity

• Low IF:– Mixer post filtering and gain easier – Harsh requirements to IRF attenuation more loss

in IRF– Lower sensitivity


DANISH GPS CENTER

Superheterodyne Receiver: Dual Conversion• Solution to IF frequency selection: Choose both• First IF is high to ease requirements to Image Reject

Filter• Second IF is low to provide cheap gain/selectivity• Cost: Higher power consumption/Higher

complexity/Less flexibility


0°

90°

I

Q

RF processing Second IF Baseband processing

LO2

fIF

First IF

LO1

DANISH GPS CENTER

Superheterodyne Receiver Advantages/Drawbacks• Advantages

– High Dynamic Range (Requirements spread over many blocks, many possibilities for variable gain)

– Low Noise • Disadvantages

– Harsh requirements to filters, especially at RF Not IC-friendly!

– Complex– Low flexibility

• Very hard to re-use components for other systems• Important for GPS GPS used in more and more peripheral

applications cost must be as low as possible to promote proliferation


DANISH GPS CENTERDirect Conversion (Homodyne)

• One way of addressing the Image Rejection/IF problem is to choose an IF of zero

• Removes image problem completely• Very flexible: Possible to re-use components for other

systems• IC-friendly• Not good for a traditional GNSS receiver (due Doppler

of all signals), but could be used for an SDR


0°

90°

RF processing Baseband processing

fRF

I

Q

I

Q

DANISH GPS CENTER

Direct Conversion Receivers -Problems• Unfortunately, there is no such thing as a free lunch• Gain placed in two places: RF and Baseband

– Stability problems– More harsh requirements to each block

• Local oscillator at same frequency as signal– LO signal impervious to filtering– Leakage problems

• LNA gain vs. Mixer noise performance– LNA gain limited Low noise for Mixer (1/f + thermal)


DANISH GPS CENTER

Direct Conversion Receiver -Problems• Second order distortion produces a DC

(constant) term at the mixer output– Strong Interfering signals– LO leakage signal mixes with itself– The high gain in the BB amplifier can cause a

significant DC offset at the ADC– Extremely high iIP2 requirement for the receiver

• Depending on the signal, some of the offset may be high-pass filtered before digitization


DANISH GPS CENTER

Direct Conversion Receiver -Problems


DANISH GPS CENTER

Direct Conversion Receiver Advantages/Drawbacks• Advantages:

– Highly integrateable (no sharp RF filters)– Simple

• Disadvantages– Strict requirements for Gain/Linearity/Noise– Not very sensitive– Problems with LO leakage


DANISH GPS CENTERAn Example Of a Front-end


Digital SamplesTo Signal

ProcessingAlgorithms

IF = 9.548 MHz

ADCBPF

Active AntennaGain ≈ 30 dB

Noise Figure ≈ 2.5 dB

BPF BPFBandpass Filter

FCENTER = 1575.42 MHz3db BW ≈ 50 MHz

Bandpass FilterFCENTER : 47.74 MHz3db BW ≈ 18 MHz

Amplifier(s)Gain ≈ 50 dB


PLL

Bandpass FilterFC : 47.74 MHz

3db BW ≈ 6.0 MHz

Amplifier(s)Gain: ≈ 50 dB


PLL Output1527.68 MHz

@ 7 dBm

Analog-to-DigitalConverter

FSAMPLING ≈ 38.192 MHz4 Bit Samples

Rooftop CableLoss ≈ 8.0 dB

TCXOF = 10.00MHz

40Sampling

Clock

DANISH GPS CENTERIntegrated GPS Front-end


DANISH GPS CENTERSummary

• Receivers can be implemented in many ways• Superheterodyne has good performance

regarding noise/dynamic range– Needs sharp filters– Not very flexible/adapts poorly to integrated

circuits• Direct conversion bypasses the image

problem– New problems arise: flicker noise, gain spread over

few blocks


DANISH GPS CENTER

Questions and Exercises

http://gps.aau.dk

DANISH GPS CENTER

2008 59Danish GPS Center

gnss receiver front-ends ii: componentskom.aau.dk/~dpl/courses/mm08_slides.pdf · danish gps center...

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