Digital Signal Analysis
Objectives - Provide a digital modulation overview - Review common digital radio impairments
Digital Modulation Overview
Signal Characteristics to Modify
Polar Display / IQ Relationship
IQ Modulator 1 0 0,1 0,0 1,1 1,0 0 symbol 1
IQ Demodulation
Filtering The digital signal has theorically an infinite spectrum. It is therefore necessary to filter the transmitter in order to match an allocated spectrum. There is also filtering at the receiver in order to limit noise and interferers at the input. There are different type of filters such as Gaussian, Raised Cosine or Root Raised Cosine. The Raised cosine filter has the advantage of not creating any Inter Symbol Interference. For this reason, it is often used. In this case, the transmit filter and the receive filter will be Root Raised Cosine so that the full chain filter is a Raised Cosine. On top of the type of filter, there is an associated coefficient that makes it more or less selective. The bandwidth of the filtered signal is the symbol rate multiplied by the sum of 1 and the filter coefficient.
Quadrature Amplitude Modulation QPSK 2 bits /symbol Symbol Rate = 1/2 bit rate ROBUST 64 QAM 6 bits /symbol Symbol Rate = 1/6 bit rate EFFICIENT 16 QAM 4 bits /symbol Symbol Rate = 1/4 bit rate 32 QAM 5 bits /symbol Symbol Rate = 1/5 bit rate 128 QAM 7 bits /symbol Symbol Rate = 1/7 bit rate
Transmitter Overview
Receiver Overview
Signal Analysis
Error Vector Concept
How EVM is calculated
Vector Signal Analysis on one page
Digital Radio Impairments and Measurements
Common Radio Impairments Transmitter I IQ Imbalance I/Q Modulator Compression Symbol Encoder BB Filter DAC 0 deg IF LO 90 deg + IF Filter PA Q DAC/DSP Error Phase Noise Spurious RF LO Incorrect Coefficients I/Q Demodulator I Receiver Interferers RF Filter LNA IF Amp IF Filter 0 deg IF LO 90 deg ADC BB Filter Symbol Decoder Thermal Noise RF LO Ripple & Tilt Q
Wrong filter coefficients I I/Q Modulator Symbol Encoder BB Filter DAC 0 deg IF LO 90 deg + IF Filter PA Q Baseband Filter Impulse Response RF LO Overshoot from incorrect alpha Baseband Filter Freq. Response For this example, the alphas for the Nyquist filter were 0.2 in the transmitter and 0.35 in the receiver.
Amplifier Nonlinearity Transmitter I I/Q Modulator Compression Symbol Encoder BB Filter DAC 0 deg IF LO 90 deg + IF Filter PA Q RF LO Traditional Two-Tone Test Testing a Digitally Modulated Signal IMD With compression f 1 f 2 Without compression
Amplifier Nonlinearity - CCDF What are the other observable effects of Compression? CCDF Measurements AWGN Signal as a reference Compressed QPSK Signal probability Non-compressed QPSK Signal db above the average
Amplifier Nonlinearity - Demodulation What are the other observable effects of Compression? Vector (IQ) Diagram Without compression Increased EVM With compression
I/Q Impairments Imbalances I I/Q Modulator 0 deg IF Filter Symbol Encoder BB Filter DAC IF LO 90 deg + PA Q Gain Imbalance Quadrature Error DC Offset Path Difference - Delay RF LO
I/Q Impairments Gain Imbalance IQ Constellation QPSK Summary Table Gain Imb. = 1.02 db (Ideally 0 db) Ideal (square) Measured (rectangle)
I/Q Impairments Quadrature Error IQ Constellation QPSK Summary Table Quad. Error = 5.9 deg. (Ideally 0 deg*) Ideal (square) Measured (parallelogram) * meaning that I and Q are ideally 90 deg. apart
What is causing this problem? I/Q Demodulator I Receiver Interferers RF Filter LNA IF Amp IF Filter 0 deg IF LO 90 deg ADC BB Filter Symbol Decoder Q RF LO Spectrum A Spectrum B No Spurious Signals ACP Looks Good
Signal A: Demodulation is Good Spectrum A Demod Points randomly distributed EVM Low (0.4%)
Signal B: Demodulation Spectrum B Demod Points are not randomly distributed EVM High (3.6%) Mag & Phase Errors High and comparable
Signal B: EVM Spectrum Shows Spur EVM Spectrum Spurious Signal -36dBc spur was buried under the modulated carrier Spectrum B
Infamous V-shape RF Filter Receiver LNA IF Amp IF Filter I/Q Demodulator 0 deg IF LO I ADC BB Filter Symbol Decoder 90 deg RF LO Q Symbol Rate
Digital Modulation Measurements Constellation Errors Ideal Symbol Point Random Noise 64 QAM Constellation Q Phase Noise AM Distortion I PM Distortion Delay Distortion/ISI Interference
Demonstration
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