Page 1:

• Lecture on Low-Noise Amplifier (LNA) Function & Measurement

• Lecturer: Ts. Dr. Khairul Najmy Abdul Rani

Page 2:

• 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

Page 3:

• 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

Page 4:

• 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

Page 5:

• LNA Measurement includes:

  • Return loss

  • Impedance

  • Isolation

  • Gain

  • Group delay

  • P1dB (gain compression)

  • Noise figure

  • Intermodulations (OIP3, IIP3)

• Measurement categories: Reflection, Transmission, Distortion

Page 6:

• 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)

Page 7:

• 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

Page 8:

• 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)

Page 9:

• 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

Page 10:

• 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

Page 11:

• 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)

Page 12:

• 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

Page 13:

• 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

Page 14:

• 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

Page 15:

• 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

Page 16:

• 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

Page 17:

• 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

Page 18:

• 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

Page 19:

• OIP3 Calculation for the amplifier

• TOI point is never obtained due to compression

• OIP3(dBm) = S(dBm) + D(dB)/2

Page 20:

• 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

Page 21:

• Definition of noise

• Undesirable signal present with wanted signal

• Important as system has limited bandwidth

Page 22:

• 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

Page 23:

• 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

Page 24:

• Thermal noise calculation

• Max available noise power, PN = V^2 / R = kTB

• Voltage (rms) = 4kTB/R

Page 25:

• 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

Page 26:

• 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

Page 27:

• 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

Page 28:

• 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

Page 29:

• Y-Factor Method of Computation

• Temperature of Source, Impedance, Noise Power Output

• ENR table defined

• Measured by NF meter/analyzer

Page 30:

• Using NF Meter/Analyzer or SA

• Noise source, DUT, LNA, low-noise power supply, Faraday cage

Page 31:

• NF Instrument Calibration and Measurement

• Noise source, DUT, NF instrument

• 2-stage NF measurement

Page 32:

• Impact of Losses on Noise Figure

• Attenuation degrades NF dB-for-dB if placed before an LNA

Page 33:

• Example of LNA Specification

• Frequency range, small signal gain, gain flatness, 1 dB compression, 3rd order intercept, noise figure, input match, group delay

Page 34: Summary of Measurement Solutions

• 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

Page 35: Summary

• 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

Page 36: Reference

• 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

Page 37: Appendix: NF in Cascaded System

• 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

Page 39: Use of a Low-Noise Amplifier (LNA)

• 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

Page 40: Use of Low-Noise Amplifier (LNA) (cont’d)

• 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

Page 41: Use of Low-Noise Amplifier (LNA) (cont’d)

• Architecture with high NF of the mixer suffers from lower sensitivity

• Designing a mixer with low noise and sufficient conversion gain is generally avoided

Page 42: Another Reason for Including a LNA in the Receiver Stage

• 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