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Transcript
RF Power Measurement Basics Why bandwidth and speed matters by Wolfgang Damm, WTG Agenda • Significance of power measurements • RF signal theory • Vital system characteristics • Sampling techniques • Product specification examples • Questions - Answers Understanding Important Parameters Technologies effected by Power Measurements Telecommunication Mobile Devices, LNA, High Power Amplifier, Antennas, Receiver, Filter, EMV Life sciences MRI, Telemetry, Skin / Deep Muscle Treatment Aerospace / Defense Radar, Communication, Telemetry, EMV Automotive EMV, Radar (Collision Avoidance Systems, Parking Aids, Speed Control), EMV, Communication, Telemetry Consumer Electronics Communication, CATV, WLAN, Microwave Measuring Power: Essential for Circuit Design This webinar discusses vital factors supporting better product design and maintenance. • Component and system output signals are critical in the design and performance of almost all RF and microwave equipment. • Signal measurement levels are critical at every system level – from the single component to the overall system. • In a system, each component must receive a proper signal level from the previous component. Why Measure RF Signal Levels A component output signal level is often the critical factor in the design of RF and MW equipment. Signal content gets lost in noise, causing high BER • Signal too low – information gets lost in noise Signal information gets lost, due to clipping • Signal too high – Signal is clipped – Component can be destroyed Too much power destroys circuitry RF Signal Theory Why Not Just Measure Voltage? DC and low-frequency measurements can be calculated by the formula: P=U2/R. RL V I Depending in the match between RF or MW source impedance and load impedance, parts of the signal energy is reflected. A waveguide setup makes it very difficult to measure voltage. Low frequency allow easy power measurements Forward Power RI Effective Power RL ~ Reflected Power Specifics of RF Power? V I Amplitude RF and MW systems show different behavior: Voltage and current vary depending on the position measured. Power stays the same at every point. t In RF Domain “power” refers usually to average power [Pavr] Pavr V*I cos(φ) = phase angle between V and I Amplitude PRF = Pavr = V * I * cos(φ) t V Note: Frequency of power is double the AC frequency I Power Units (W, dBm) Power is energy transferred per unit of time. Basic power unit is Watt (W): AC component of Power DC Component of Power A logarithmic (decibel) scale is often used to compare power levels: Relative Power P(dB) = 10 log (P/Pref) Absolute Power P(dBm) = 10 log (P/1mW) Amplitude 1W = 1 joule / sec t Power Meters Measure Power Envelope All RF power measurements relate to the power envelope. RF Power Sensors Types of Sensors Thermocouple: True RMS measurement Relatively insensitive to temporary overpowering. Moderate dynamic range (typically 40dB) Slow Diode (CW): True RMS measurement in the square root area. Very sensitive to temporary overpowering. Very high dynamic range (typically 90dB) Faster Diode (Peak): True RMS measurement in the square root area. Very sensitive to temporary overpowering. Moderate to high dynamic range (typically 40 – 80dB) Very Fast Sensors Diodes are the most commonly used sensors. The matching resistor is the termination for the RF signal. RF voltage is turned into DC voltage at the diode. The capacitor C smoothes the rippled output signal and serves as a low pass filter. Ccpl Rsource RL Vsource ~ 50 Ohms C Vout Dual-Diode Sensors Boonton uses dual-diode sensors. Why? Static effects: Double the output voltage (Villard Principle) Dynamic effects: Better suppression of harmonic content Ccpl Rsource RL Vsource C ~ Vout C Fast Diode Sensors Ccpl Ri Rsource RL Vsource C RC ~ Vout C RC Ri Influence of Ri, C and Rc on pulse measurements: traise = Ri * C tfall = Rc * C Vital System Characteristics Choosing Power Meters / Sensors • VSWR • Maximum power level • Dynamic power range • Rise / fall time • Frequency • Bandwidth • Sample rate / Effective sample rate Fourier Transformation (FFT) • Every non-sinusoid signal consists of sinusoids of equal and higher frequencies. • Amplitude of the base frequency and its multiples depends on the form of the original signal. • If any components of the test setup elements have insufficient capability to handle these frequencies, the measurement will be inaccurate and the signal trace will be shown degraded. FFT (2) Nyquist The Nyquist–Shannon Sampling Theorem states: the minimum sampling frequency of a limited bandwidth, time-continuous analog signal may be no less than twice the maximum signal frequency in order to fully reconstruct an signal from the acquired discrete data. Nyquist Nyquist’s Theorem applies also to nonbaseband signal frequencies with limited bandwidth. The required sample frequency depends on the signal’s bandwidth. The sampling rate must then be higher than twice the occupied bandwidth. Harry Nyquist 1889-1976 Example: a 1900 MHz signal with a bandwidth of 5 MHz would require a sampling frequency of just above 10 MHz to provide a sufficient number of data points to fully reconstruct the signal. Sampling Techniques Sampling • Sampling gathers slices of a signal envelope at specific time intervals. • Minimum requirements: samples fulfill Nyquist requirements. Measurements (dots) are mathematically and graphically connected to rebuild the original signal. Sample points are interpolated with a sin(x)/x function. The sampling rate in this example is about 1.5 times the Nyquist frequency. Due to the rectangular form of the original signal the representation comes with a high harmonic content. Repetitive Random Sampling Power meters take continuous samples independent of trigger event. Although taken sequentially in time, they are always completely random. Additional data points are added with every sweep. As a result, the waveform is completely reconstructed. Repetitive Random Sampling Detailed Pulses Analysis 1) 2) 3) Product Specification Examples Boonton Power Meters (excerpt) • Maximum Power Level: 47 dBm* • Dynamic Power Range: 90dB * • Frequency: 40 GHz* • Bandwidth (peak): up to 50MHz • Rise / Fall Time: down to 3ns • Time Resolution: down to 100ps • Effective Sample Rate: up to 10GSa/s * Sensor dependent For more information visit: http://boonton.com/Products/Power Meters.aspx Boonton Systems – Sensors (excerpt) • Maximum Power Level: up to 47dBm • Dynamic Power Range: up to 90dB • Frequency: up to 40 GHz • Bandwidth (peak): up to 65 MHz • Rise / Fall Time: from < 7ns For more information visit: http://boonton.com/Products/Sensors.aspx Conclusion • Significance of power measurement • RF signal theory • Vital system characteristics • Sampling techniques • Product specification examples Questions – Answers THANK YOU ! Join us for our next Webinars: In Building DAS Systems by Rand Skopas, WTG Date: 10/20/2010