Lecture Slide 4
Lecture on Low-Noise Amplifier (LNA) Function & Measurement
Lecturer: Ts. Dr. Khairul Najmy Abdul Rani
Chapter Outline:
Usage of Low-Noise Amplifier
Transmission Measurement: Gain, Isolation, Group Delay, P1dB
Reflection Measurement: Return Loss, Impedance, SWR
Distortion Measurement: OIP3, IIP3
Noise Figure Measurement: Definitions, Y-factor Method, Calibration and Measurement, Impact of Losses, Cold Source Method
Example of LNA Specifications
Appendix: NF in Cascaded System
Low-Noise Amplifier (LNA) determines overall system noise level
LNA has low noise figure (e.g., 2 dB) and high gain (e.g., 25 dB)
LNA is the first active device in the receiver, reducing the noise of subsequent stages
LNA's noise is injected directly into the received signal
LNA is a special type of amplifier used in communication systems to amplify weak signals captured by an antenna
LNA boosts desired signal power while adding minimal noise and distortion
LNA is often located close to the antenna to minimize losses in the feedline
LNA Measurement includes:
Return loss
Impedance
Isolation
Gain
Group delay
P1dB (gain compression)
Noise figure
Intermodulations (OIP3, IIP3)
Measurement categories: Reflection, Transmission, Distortion
Gain is the ratio of an amplifier's output power to input power at a particular frequency
Small signal gain is the difference in dB between output and input power levels
Small signal gain (dB) = Pout (dBm) - Pin (dBm)
Small Signal Gain Measurement:
Transmission measurements using S21 in magnitude or logarithmic (dB)
Calibration (SOLT, TRM, TRL) required to remove systematic errors
Input power level set to minimum to avoid damage and compression
Receiver port attenuators can be used if necessary
Isolation is a measure of transmission from output to input
Isolation measurement is similar to small signal gain measurement, but stimulus is applied to the amplifier's output
Good reverse isolation means the signal from the output is prevented from reaching the input
Isolation (dB) = P2 (dBm) - P1 (dBm)
Reverse Isolation Measurement:
Measure of transmission from output to input using S12 in magnitude or dB
Measurement similar to small signal gain, but stimulus is applied to the amplifier's output
Calibration (SOLT, TRM, TRL) required to remove systematic errors
Noise floor of the analyzer can be lowered for amplifiers with high isolation
Group delay is a measure of the transit time through an amplifier at a particular frequency
Group delay is also a measure of amplifier distortion
Group delay can be viewed in delay format in the network analyzer
P1dB is the input power level where the amplifier gain drops 1 dB relative to the small signal gain
P1dB indicates the amplifier's output capability
P1dB is typically specified as an output power level (e.g., 20 dBm)
Gain Compression Measurement with Network Analyzer:
Transmission measurements using S21 in magnitude or logarithmic (dB)
Calibration (SOLT, TRM, TRL) required to remove systematic errors
Input power source calibrated with power meter
Optional receiver calibration depending on the instrument used
Swept Power Gain Compression with Vector Network Analyzer (VNA):
Swept power test done at a CW frequency
Input power increased with a step sweep to observe 1 dB gain reduction
Input power at P1dB is recorded
Swept Power Gain Compression with Spectrum Analyzer (SA):
Requires good RF source spectral purity
Scalar offset normalization required for accuracy
Same setup as gain test with manual input power sweep and readout
Pin vs. Pout plotted manually to determine P1dB
Reflection Measurements:
Input/Output Return Loss/SWR measures the match quality of the amplifier's input and output
Reflection coefficient includes magnitude and phase information of reflected signals
Return loss and SWR examine the magnitude portion of the reflection coefficient
Input/Output Impedance can be displayed in complex format mapped onto the Smith Chart
Reflection measurements use the same setup and full two-ports calibration as transmission measurements
Return loss and SWR are usually specified for the amplifier's input and output ports
Input and output complex impedances can be viewed in Smith Chart format in the analyzer
Rule of thumb: 20 dB difference between noise level and reflected signal
Distortion caused by interaction of input signals inside the DUT (Device Under Test)
Extra tones at the output besides the two input tones are undesirable distortion terms
3rd order terms (2f1 - f2) and (2f2 - f1) are the most significant and hard to filter, hence OIP3 is measured
Plotting the 3rd Order Response:
3rd order product versus input power predicts a 3:1 response
OIP3 = Gain + IIP3
OIP3 typically ranges from 10 to 20 dB for every dB increase in input power
OIP3 Calculation for the amplifier
TOI point is never obtained due to compression
OIP3(dBm) = S(dBm) + D(dB)/2
OIP3 Measurement setup
Signal sources, LPF, attenuators, power combiner, DUT, LPF
Additional attenuation improves isolation and minimizes interaction
Low pass filters minimize generation of harmonics
Attenuators and isolators improve matching to DUT
Three sources of error: interaction between test equipment, dynamic range of spectrum analyzer, quality of test equipment
Power levels should be set such that 3rd order products are at least 10 dB over the noise
OIP3 = (S + loss at output) + D/2
Definition of noise
Undesirable signal present with wanted signal
Important as system has limited bandwidth
Types of noise
External noise (interference) and internal noise (produced by internal components)
Reduction methods for external noise: reduce external source or increase screening
Reduction methods for internal noise: improve design or better components
Internal noise types: thermal/Johnson/Nyquist noise, shot noise, flicker noise
Thermal noise arises from random motion of electrons
Shot noise due to current flowing across potential barrier in PN junction
Flicker noise significant at audio frequencies, amplitude inversely proportional to frequency
Thermal noise calculation
Max available noise power, PN = V^2 / R = kTB
Voltage (rms) = 4kTB/R
Noise figure definition
Measure of noise added by device, no concern for external noise
Noise figure quantifies degradation in signal-to-noise ratio (SNR)
Lowest possible value of NF is 0 dB
Noise figure IEEE definition
Noise figure measurement equation
N = G + N_in + N_a + N_o
G = DUT associated gain, N_in = input noise, N_a = added noise, N_o = output noise
Noise figure measurement methods
Y-Factor Method with Noise Figure Meter/Spectrum Analyzer
Cold Source (or Direct Source) Method using Vector Network Analyzer with Built-in Noise Figure Receiver
Y-Factor (Hot/Cold Source) Method Measurement
Using Spectrum Analyzer with Personality Software Option or Noise Figure Meter/Analyzer
Require low noise preamplifier to improve sensitivity
Y-Factor Method of Computation
Temperature of Source, Impedance, Noise Power Output
ENR table defined
Measured by NF meter/analyzer
Using NF Meter/Analyzer or SA
Noise source, DUT, LNA, low-noise power supply, Faraday cage
NF Instrument Calibration and Measurement
Noise source, DUT, NF instrument
2-stage NF measurement
Impact of Losses on Noise Figure
Attenuation degrades NF dB-for-dB if placed before an LNA
Example of LNA Specification
Frequency range, small signal gain, gain flatness, 1 dB compression, 3rd order intercept, noise figure, input match, group delay
Gain and Flatness
Gain Compression
Phase
Group Delay
Isolation
Return Loss/SWR
Input Impedance
Output Impedance
Output Power
Noise Figure
Intermod Distortion
Scalar Network Analyzer
Noise Figure Meter/Analyzer
Spectrum Analyzer
Power Meter
Vector Network Analyzer
Low Noise Amplifier (LNA) is the first active device in a receiver
LNA has low noise and high gain characteristics
The NF of the entire receiver is determined by the NF of only the LNA
OIP3 or TOI is a 2-tone linearity test of the LNA measured for in-band operation
Higher TOI value indicates a more linear LNA
Noise is an undesirable addition to an ideally pure signal
Different types of noise: thermal noise, shot noise, and flicker noise
Thermal noise density: kTo = -174 dBm/Hz (in a 1 Hz bandwidth)
NF is the degradation in SNR in dB as signal-and-noise pass through a DUT
Popular method of measuring NF is the Y-factor method using Hot/Cold source
NF of an attenuator by itself equals its losses in dB
Losses degrade NF dB-for-dB if placed before an LNA
10 dB of attenuation before an LNA will degrade overall NF by 10 dB, but not at all if placed after the LNA
Keysight Technologies, “PNA Microwave Network Analyzers - Amplifier Linear and Gain Compression Measurements”, AN 1408-7
Keysight Technologies, “PNA Microwave Network Analyzers - Amplifier and CW Swept Intermodulation-Distortion Measurements”, AN 1408-9
Keysight Technologies, “A Seminar on RF Measurement”, 2001, Spectrum Analysis Basics
Keysight Technologies, “Fundamentals of RF and Microwave Noise Figure Measurements”, AN 57-1
Keysight Technologies, “Noise Figure Measurement Accuracy – The Y-Factor Method”, AN 57-2
Rohde & Schwarz, “Performing Amplifier Measurements with the Vector Network Analyzer ZVB”, ZVB VNA AN
Anritsu, “Fast, Flexible, and Accurate IMD Measurements Using a Vector Network Measurement System”, IMD Scorpion Option 13 AN
David Ballo, “Making Source-Corrected Noise Figure Measurement ”, September 2007, Microwave & RF Magazine
F1,G1 F3 ,G3 F2 ,G2 Noiseless amplifiers
Amp added noise referred back to input
G N (F 1)N 1 N G N F N G N N G F a in in a in a in
in 1 in 1 2 in N (F 1)N G (F 1)N
in 1 2 1 2 1 in 1 2 in 1 2 1 2 in sys F G (F 1 ) (F 1 ) 1 G (F 1 ) N (G G ) (G G ) (F 1)N (G G ) N (G G ) G (F 1)N F
LNA is usually used as the 1st stage amplifier for a receiving circuit
LNA amplifies the weak signal from the antenna without contributing too much noise
Larger signal is then fed to the mixer, which generally has a higher NF
This improves the overall NF at the IF output
If the power gain of the 1st stage is around 10 or more, the signal will be sufficiently large at the output of the 1st stage
Additional noise contributed by the following amplifier stages or mixer will have a small degrading effect on the overall SNR
Minimum noise requirement is more important than the maximum power gain or VSWR in the design of the 1st stage
Architecture with high NF of the mixer suffers from lower sensitivity
Designing a mixer with low noise and sufficient conversion gain is generally avoided
LNA provides isolation against the leakage of the local oscillator (LO) signal
LNA has a small |S12|
Prevents the power from the LO going into the antenna and radiate out, causing unwanted radiation
Lecture on Low-Noise Amplifier (LNA) Function & Measurement
Lecturer: Ts. Dr. Khairul Najmy Abdul Rani
Chapter Outline:
Usage of Low-Noise Amplifier
Transmission Measurement: Gain, Isolation, Group Delay, P1dB
Reflection Measurement: Return Loss, Impedance, SWR
Distortion Measurement: OIP3, IIP3
Noise Figure Measurement: Definitions, Y-factor Method, Calibration and Measurement, Impact of Losses, Cold Source Method
Example of LNA Specifications
Appendix: NF in Cascaded System
Low-Noise Amplifier (LNA) determines overall system noise level
LNA has low noise figure (e.g., 2 dB) and high gain (e.g., 25 dB)
LNA is the first active device in the receiver, reducing the noise of subsequent stages
LNA's noise is injected directly into the received signal
LNA is a special type of amplifier used in communication systems to amplify weak signals captured by an antenna
LNA boosts desired signal power while adding minimal noise and distortion
LNA is often located close to the antenna to minimize losses in the feedline
LNA Measurement includes:
Return loss
Impedance
Isolation
Gain
Group delay
P1dB (gain compression)
Noise figure
Intermodulations (OIP3, IIP3)
Measurement categories: Reflection, Transmission, Distortion
Gain is the ratio of an amplifier's output power to input power at a particular frequency
Small signal gain is the difference in dB between output and input power levels
Small signal gain (dB) = Pout (dBm) - Pin (dBm)
Small Signal Gain Measurement:
Transmission measurements using S21 in magnitude or logarithmic (dB)
Calibration (SOLT, TRM, TRL) required to remove systematic errors
Input power level set to minimum to avoid damage and compression
Receiver port attenuators can be used if necessary
Isolation is a measure of transmission from output to input
Isolation measurement is similar to small signal gain measurement, but stimulus is applied to the amplifier's output
Good reverse isolation means the signal from the output is prevented from reaching the input
Isolation (dB) = P2 (dBm) - P1 (dBm)
Reverse Isolation Measurement:
Measure of transmission from output to input using S12 in magnitude or dB
Measurement similar to small signal gain, but stimulus is applied to the amplifier's output
Calibration (SOLT, TRM, TRL) required to remove systematic errors
Noise floor of the analyzer can be lowered for amplifiers with high isolation
Group delay is a measure of the transit time through an amplifier at a particular frequency
Group delay is also a measure of amplifier distortion
Group delay can be viewed in delay format in the network analyzer
P1dB is the input power level where the amplifier gain drops 1 dB relative to the small signal gain
P1dB indicates the amplifier's output capability
P1dB is typically specified as an output power level (e.g., 20 dBm)
Gain Compression Measurement with Network Analyzer:
Transmission measurements using S21 in magnitude or logarithmic (dB)
Calibration (SOLT, TRM, TRL) required to remove systematic errors
Input power source calibrated with power meter
Optional receiver calibration depending on the instrument used
Swept Power Gain Compression with Vector Network Analyzer (VNA):
Swept power test done at a CW frequency
Input power increased with a step sweep to observe 1 dB gain reduction
Input power at P1dB is recorded
Swept Power Gain Compression with Spectrum Analyzer (SA):
Requires good RF source spectral purity
Scalar offset normalization required for accuracy
Same setup as gain test with manual input power sweep and readout
Pin vs. Pout plotted manually to determine P1dB
Reflection Measurements:
Input/Output Return Loss/SWR measures the match quality of the amplifier's input and output
Reflection coefficient includes magnitude and phase information of reflected signals
Return loss and SWR examine the magnitude portion of the reflection coefficient
Input/Output Impedance can be displayed in complex format mapped onto the Smith Chart
Reflection measurements use the same setup and full two-ports calibration as transmission measurements
Return loss and SWR are usually specified for the amplifier's input and output ports
Input and output complex impedances can be viewed in Smith Chart format in the analyzer
Rule of thumb: 20 dB difference between noise level and reflected signal
Distortion caused by interaction of input signals inside the DUT (Device Under Test)
Extra tones at the output besides the two input tones are undesirable distortion terms
3rd order terms (2f1 - f2) and (2f2 - f1) are the most significant and hard to filter, hence OIP3 is measured
Plotting the 3rd Order Response:
3rd order product versus input power predicts a 3:1 response
OIP3 = Gain + IIP3
OIP3 typically ranges from 10 to 20 dB for every dB increase in input power
OIP3 Calculation for the amplifier
TOI point is never obtained due to compression
OIP3(dBm) = S(dBm) + D(dB)/2
OIP3 Measurement setup
Signal sources, LPF, attenuators, power combiner, DUT, LPF
Additional attenuation improves isolation and minimizes interaction
Low pass filters minimize generation of harmonics
Attenuators and isolators improve matching to DUT
Three sources of error: interaction between test equipment, dynamic range of spectrum analyzer, quality of test equipment
Power levels should be set such that 3rd order products are at least 10 dB over the noise
OIP3 = (S + loss at output) + D/2
Definition of noise
Undesirable signal present with wanted signal
Important as system has limited bandwidth
Types of noise
External noise (interference) and internal noise (produced by internal components)
Reduction methods for external noise: reduce external source or increase screening
Reduction methods for internal noise: improve design or better components
Internal noise types: thermal/Johnson/Nyquist noise, shot noise, flicker noise
Thermal noise arises from random motion of electrons
Shot noise due to current flowing across potential barrier in PN junction
Flicker noise significant at audio frequencies, amplitude inversely proportional to frequency
Thermal noise calculation
Max available noise power, PN = V^2 / R = kTB
Voltage (rms) = 4kTB/R
Noise figure definition
Measure of noise added by device, no concern for external noise
Noise figure quantifies degradation in signal-to-noise ratio (SNR)
Lowest possible value of NF is 0 dB
Noise figure IEEE definition
Noise figure measurement equation
N = G + N_in + N_a + N_o
G = DUT associated gain, N_in = input noise, N_a = added noise, N_o = output noise
Noise figure measurement methods
Y-Factor Method with Noise Figure Meter/Spectrum Analyzer
Cold Source (or Direct Source) Method using Vector Network Analyzer with Built-in Noise Figure Receiver
Y-Factor (Hot/Cold Source) Method Measurement
Using Spectrum Analyzer with Personality Software Option or Noise Figure Meter/Analyzer
Require low noise preamplifier to improve sensitivity
Y-Factor Method of Computation
Temperature of Source, Impedance, Noise Power Output
ENR table defined
Measured by NF meter/analyzer
Using NF Meter/Analyzer or SA
Noise source, DUT, LNA, low-noise power supply, Faraday cage
NF Instrument Calibration and Measurement
Noise source, DUT, NF instrument
2-stage NF measurement
Impact of Losses on Noise Figure
Attenuation degrades NF dB-for-dB if placed before an LNA
Example of LNA Specification
Frequency range, small signal gain, gain flatness, 1 dB compression, 3rd order intercept, noise figure, input match, group delay
Gain and Flatness
Gain Compression
Phase
Group Delay
Isolation
Return Loss/SWR
Input Impedance
Output Impedance
Output Power
Noise Figure
Intermod Distortion
Scalar Network Analyzer
Noise Figure Meter/Analyzer
Spectrum Analyzer
Power Meter
Vector Network Analyzer
Low Noise Amplifier (LNA) is the first active device in a receiver
LNA has low noise and high gain characteristics
The NF of the entire receiver is determined by the NF of only the LNA
OIP3 or TOI is a 2-tone linearity test of the LNA measured for in-band operation
Higher TOI value indicates a more linear LNA
Noise is an undesirable addition to an ideally pure signal
Different types of noise: thermal noise, shot noise, and flicker noise
Thermal noise density: kTo = -174 dBm/Hz (in a 1 Hz bandwidth)
NF is the degradation in SNR in dB as signal-and-noise pass through a DUT
Popular method of measuring NF is the Y-factor method using Hot/Cold source
NF of an attenuator by itself equals its losses in dB
Losses degrade NF dB-for-dB if placed before an LNA
10 dB of attenuation before an LNA will degrade overall NF by 10 dB, but not at all if placed after the LNA
Keysight Technologies, “PNA Microwave Network Analyzers - Amplifier Linear and Gain Compression Measurements”, AN 1408-7
Keysight Technologies, “PNA Microwave Network Analyzers - Amplifier and CW Swept Intermodulation-Distortion Measurements”, AN 1408-9
Keysight Technologies, “A Seminar on RF Measurement”, 2001, Spectrum Analysis Basics
Keysight Technologies, “Fundamentals of RF and Microwave Noise Figure Measurements”, AN 57-1
Keysight Technologies, “Noise Figure Measurement Accuracy – The Y-Factor Method”, AN 57-2
Rohde & Schwarz, “Performing Amplifier Measurements with the Vector Network Analyzer ZVB”, ZVB VNA AN
Anritsu, “Fast, Flexible, and Accurate IMD Measurements Using a Vector Network Measurement System”, IMD Scorpion Option 13 AN
David Ballo, “Making Source-Corrected Noise Figure Measurement ”, September 2007, Microwave & RF Magazine
F1,G1 F3 ,G3 F2 ,G2 Noiseless amplifiers
Amp added noise referred back to input
G N (F 1)N 1 N G N F N G N N G F a in in a in a in
in 1 in 1 2 in N (F 1)N G (F 1)N
in 1 2 1 2 1 in 1 2 in 1 2 1 2 in sys F G (F 1 ) (F 1 ) 1 G (F 1 ) N (G G ) (G G ) (F 1)N (G G ) N (G G ) G (F 1)N F
LNA is usually used as the 1st stage amplifier for a receiving circuit
LNA amplifies the weak signal from the antenna without contributing too much noise
Larger signal is then fed to the mixer, which generally has a higher NF
This improves the overall NF at the IF output
If the power gain of the 1st stage is around 10 or more, the signal will be sufficiently large at the output of the 1st stage
Additional noise contributed by the following amplifier stages or mixer will have a small degrading effect on the overall SNR
Minimum noise requirement is more important than the maximum power gain or VSWR in the design of the 1st stage
Architecture with high NF of the mixer suffers from lower sensitivity
Designing a mixer with low noise and sufficient conversion gain is generally avoided
LNA provides isolation against the leakage of the local oscillator (LO) signal
LNA has a small |S12|
Prevents the power from the LO going into the antenna and radiate out, causing unwanted radiation