Radio frequency Fundamentals
Radio frequency (RF) basic terms, principle of operations, practical design and testings are discussed.
Basic RF Terms:
Radio Frequency (RF)
A signal frequency with greater than 300 MHz is considered RF.
In modern cellular standard LTE, the low band roughly starts at 600 MHz and extends to 6000 MHz (refer https://www.sqimway.com/lte_band.php) B46 is the last LTE band in frequency.
It's a high frequency signal that "carries" the lower bandwidth of "real" information signal such voice and data
The reason is that due to miniaturization of consumer electronics, high RF carrier signal is needed for efficient electromagnetic radiation using small antenna in transmission and reception of wireless network.
It's a low bandwidth analog signal containing data the such as voice and data typically found after downconverting from RF to DC before analog to digital converter (ADC) in radio receiver or right after digital to analog converter (DAC) in radio transmitter.
Two main reasons for using low BW baseband signal are:
Come within sampling range of ADC/DAC: sampling rate are limited on ADC/DAC mixed signal hardware which is needed for pre/post digital signal processing.
Improved selectivity: easier filter design with sharper rolloff can be constructed at lower frequencies.
Quadrature: Orthogonal relationship (i.e 90 degrees) between two vectors.
Quadrature I/Q Components: two components I (in phase signal) and Q (out of phase signal) are 90 degrees out of phase. IQ signals is the building block of modern digital modulation and demodulation scheme.
Modulation: Modulation means encoding a message. In the context of RF, one can think it in two ways:
Encoding a baseband message on a carrier frequency signal.
Frequency shift the baseband signal to radio frequency signal.
It's is used to encode analog baseband signal onto a RF carrier signal for long range transmission with low data bandwidth such as voice, small data packets.
Typical analog modulations are:
Amplitude Modulation (AM): analog baseband signal is encoded on a RF carrier signal in the form of varying RF carrier signal amplitude.
Frequency Modulation (FM) analog baseband signal is encoded on a RF carrier signal in the form of varying RF carrier signal frequency
It's is used to encode digital base band signal (i.e binary bits 0 and 1) onto a of RF carrier signal for short to mid range transmission with high data bandwidth such as large file download and video streaming.
Typical digital modulations are:
Amplitude Shift Keying: 1 bit binary signal modulation scheme where 1 and 0 is represented by turning on and off RF carrier transmission.
Frequency Shift Keying: 1 bit binary signal modulation scheme where 1 and 0 is represented by change RF carrier signal from high to low frequency.
Phase Shift keying: 1 bit symbol binary signal modulation scheme where 1 and 0 is represented by change RF carrier signal phase relationship.
Quadrature Amplitude modulation (QAM): a base band signal is encoded by summing two carrier signals 90 degrees out of phase (i.e I & Q components) resulting a single signal that is controlled by phase and amplitude variation of two I/Q carriers.
Digital symbols are reconstructed by combination of amplitude and phase relationship of I & Q component. This is very clever and powerful because by varying amplitude and phase relationships of I & Q carriers, a range of unique symbols can be created resulting in an increase of bits/symbol; Hence higher data throughput is achieved.
This technique increases spectral efficiency (i.e more bits/symbol transmitted per fixed channel bandwidth).
Quadrature Phase shift Keying (QPSK): It's a 2 bit modulation technique with 4 unique symbols, 00, 01, 10, 11 are represented by phase change (45,135, 225, or 315 degrees) of the carrier signal using I and Q components in quadrature amplitude modulation (QAM).
Note: QPSK is a special type of 4-QAM where resultant signal has equal amplitude with varying phase.
Demodulation is analogous to decoding a RF signal which involves removing carrier frequency component from the RF signal leaving the baseband signal behind for further processing.
Basic Principles of Operations
RF system is responsible for transmission and reception of wireless signals.
RF Transmission: A RF signal is created by upshifting a low bandwidth (DC like) signal to radio frequency by a radio transmitter.
RF reception: A RF signal is picked up by the antenna and downshifted to low bandwidth (DC like) signal by a radio receiver.
Simplified Wireless Communication Block Diagram
The main RF system design criteria is to minimize path loss and maximize power transfer using lumped matching network.
Transmitter: It is an integrated RF hardware containing DAC for generating analog baseband signal, mixer for modulation, and power amplifier for over the air transmission.
Its main job is to up-shift baseband signal to RF signal and increases RF output power for transmission.
Receiver: It is an integrated RF hardware containing low noise amplifier (LNA), mixer for demodulation ,and ADC for converting analog baseband signal to digital domain for digital signal processing.
Its main job is to amplify received RF signal before down-shift to baseband signal.
Matching Network: Matching network is made up of inductors and capacitors that is used to match source output impedance (Transmitter Power Amplifier/ Receiver) to the load impedance (Antenna Impedance).
Its main job is to maximize power transfer.
In practice, matching to 50 ohm is used due to characteristic impedance of transmission line in RF system in designed as 50 ohms.
Types of Matching Network
Two element matching: L matching network
No flexibility of quality factor Q: highly dependent on input and load impedance
Forbidden region exists on the smith chart for load impedance
This means that 2 element matching network is limited to certain load impedance type.
Application: Antenna uses L network because it covers a wider range/broadband signals (i.e higher bandwidth and lower quality factor of the matching network)
Three element matching: PI and T matching network
More flexibility on quality factor for Q
No forbidden region on the smith chart for load impedance
This means that any arbitrary source and load impedance can be matched using 3-element matching network.
In practice, 3-element is used for RF system design due to stringent wireless channel bandwidth and roll offs to meet the spectrum mask and adjacent channel leakage of RF emission standards.
Quality factor Q: Q value of a matching network reports the oscillatory resonance performance of the matching network.
A high Q matching means that the equivalent lossy component of the network is low. This means that the resonance peak of the circuit (assuming matching network is modeled as series RLC) is higher because series R is smaller due to lower equivalent lossy element.
Note: Resonance curve is a current vs frequency response where current is measured as the series current going through the RLC matching network model.
Another important relationship between Q to signal bandwidth is that Q = Fo/(3dB bandwidth), which means that higher Q means narrower 3-dB bandwidth which is needed for limited and fixed bandwidth standards used today such as 20 MHz bandwidth for WiFi channels.
Smith Chart: it's an impedance visualization tool that is used to characterize impedance of a lumped passive circuit.
For two element matching network, two circles are used, constant conductance circuit and resistance circle, to move the load impedance ZL to the origin of the smith chart (i.e 50 ohms).
For 3 element matching network, 3 circles are used to the move load impedance ZL to the origin of the smith chart (i.e 50 ohms).
RF Performance Characterization
Efficiency, output dynamic range.
Gain compression: it's a measures of nonlinearity usually characterized by input power level that reaches 1 dB compression (i.e 1 dB point away from the perfect straight linear line).
Sensitivity: minimal power level needed for required SNR
Selectivity: only select desired signal (i.e specify attenuation of out of band signals)
Link Budget: it is used to estimate line of sight range of wireless communication link by calculate received power at receiver.
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)
Gains are amplifers and antenna.
Losses are free-space path loss, pcb path and component loss.
Link Margin: it is used to calculate much margin is needed for a communication system accounting signal attenuation due to multi-path fading during the transmission channel.
Link Margin = Received Power − Receive Sensitivity
Adjacent Channels Power Ratio (ACPR): It is a measurement of the amount of interference in the adjacent frequency channel usually as the ratio of average power adjacent outband channel to the average power in the in-band channel,
Phase Noise: Random frequency fluctuations of the main RF carrier frequency cause by imperfect clock generator source such as the local oscillator used for generating RF carrier frequency.
Noise Figure: A measure of degradation of signal SNR caused by RF hardware placed in the transmission and reception path.
The reason for the added noise is due to intrinsic RF component noise that is added to the signal path.
S parameter: it is used to measure traveling waves for RF signals on printed circuit boards.
Insertion loss (S21/S12)
It measures the loss of signal power by an insertion element such as a RF filter measured in dB.
Application: lower insertion loss of an RF element means lower power loss in the RF element resulting in high power available power transfer to the load.
Return loss (S11/S22)
It measures the loss of signal power reflected by the transmission medium such as impedance discontinuity measured in dB.
Application: Antenna radiation power is generally characterized by return loss (S11). A -10 dB S11 means that power return is 10% of the original signal, which infers that 90% of signal is radiated.
Digital Modulation Scheme Representation
Constellation Diagram: It plots each signal symbol after demodulation on a complex plane represented by I (lies on real horizontal axis) and Q (lies in imaginary Y axis) components.
Application: Due to noise such as Gaussian noise and phase noise introduced by the communication channel, the received symbols does not lie exactly on the ideal reference points on the chart, hence constellation aggregates a period of multiple received signals as a visual indicator for engineers to evaluate amount of signal deviation error also known as error vector magnitude (EVM)
Carrier to Noise Ratio (C/N) and Signal to Noise Ratio (SNR)
They used to characterize RF signal quality in an noisy environment. C/N is analogous to SNR with the difference that digital modulation is used for C/N analysis and analog modulation is used for SNR analysis. In digital modulation and transmission,RF signal quality is entirely depended on carrier signal (constant frequency and amplitude) with respect to noisy environment.,
Bit Error Rate (BER)/ Packet Error Rate (PER)
As constellation points packed more closely used in digital modulation scheme such as QAM for higher data throughput, lowering C/N increases bit error rate.
BER/PER is an important test metric to evaluate communication system robustness against noise environment.
Common Wireless System Level Test items
Average power: It measures the radio transmitter output power performance.
Spectrum Mask: wireless standards limits transmitter spectrum shape from exceeding certain limits.
Error Vector Magnitude: it measures how far the received symbol points away from ideal location on the constellation Diagram.
Frequency error/offset: it's the frequency error of the carrier frequency from its ideal carrier frequency measured in HZ PPM.
Minimum Detectable Signal (Sensitivity), Selectivity (ability to reject out-of band signals), Linearity, Bit error rate (BER).
Practical Design Overview
The main goal of RF system design is to maximize power transfer within allowed radiation limit and acceptable signal integrity.
Lowering the insertion loss of the matching network increases amount of power available to the Antenna.
RF testing covers generally lowest, mid, and highest channel withing an RF band (e.g 2.4 GHz and 5.0 GHz WLAN band).
Limiting power amplifier output on lowest and highest RF channel are typical configuration of WLAN transmiter to limit adjacent channel interference as required by wireless standards and regulatory communication compliance.
Summary & Conclusion.
Modulation means encoding a baseband signal on top of a RF carrier signal.
RF carrier signal is need to efficient antenna radiation with small antenna.
Digital Modulation is widely used modern wireless network standards
Matching Network is used to maximize power transfer between source and load
2 element is used for broad band signal matching
3 element is used for narrow band signal matching
Quality factor determines bandwidths of signal matching.
Constellation Diagram is used to visualize received signal integrity on the radio receiver for digital modulation schemes.
Wireless testing generally involves measurements in: transmit power, transmit spectral performance, transmit frequency ,and transmit modulation accuracy.
Wireless communication system involves understanding principles of operation, practical design trade-offs, and performance measurements. As an electronics system design engineer, understanding the basics of wireless connectivity shortens design, testing, and debug time.
Reference and Further Reading
"Back to Basics Impedance Matching Part 3", https://www.electronicdesign.com/technologies/communications/article/21801154/back-to-basics-impedance-matching-part-3
"Lumped Matching Network", http://www.clivepoole.com/wp-content/uploads/2016/07/Lecture-9-Lumped-Element-Matching-Networks.pdf
"Carrier to Noise Ratio",https://en.wikipedia.org/wiki/Carrier-to-noise_ratio
"Analysis and Design-RF and Digital Systems Using PathWave System Design (SystemVue) ",https://literature.cdn.keysight.com/litweb/pdf/5992-0197EN.pdf