Introduction to power
Power is an important parameter that characterizes the characteristics of electrical signals. A power meter is a meter that measures the active power of an electrical signal.
In the DC and low frequency range, the power can be calculated by measuring the voltage and current. The instantaneous value of the power can be expressed by the following formula:
For a periodic signal with a period of T, the instantaneous power in a period The average value of is called active power. The active power is calculated as follows:
For a sine circuit, the following formula holds:
In the above formula, U and I are the effective values of sinusoidal alternating current, and φ is the phase difference between the voltage and current signals.
For non-sinusoidal circuits, suppose U and I are the effective values of the n-th harmonic component of the AC power, φ is the phase difference between the n-th harmonic component of the voltage and the n-th harmonic component of the current, and P is n The above formula still holds true for the active power of the sub-harmonic component. When n=1, P is the fundamental active power.
In the UHF and microwave frequency bands, there are TEM waves and non-TEM waves. In the TEM wave coaxial system, although voltage and current have exact meaning, it is difficult to measure their absolute value. In the waveguide system, voltage and current lose their uniqueness because of different electromagnetic modes. In each frequency band and each transmission system, power is an important method for single value characterization of signal strength. Direct measurement of power in the radio frequency range replaces the measurement of voltage and current.
Unit of measurement
Power is defined as the work done per unit time. The basic unit is watt (W), and 1W is equivalent to doing 1 joule of work in 1 second. Commonly used power units include megawatts (1MW=10^6W), kilowatts (1KW=10^3W), milliwatts (1mW=10-3W), microwatts (1μW=10-6W), picowatts (1Pw= 10-12W).
Another common power unit is expressed in decibel milliwatts (dBm). It takes 1 milliwatt as the reference level P0=1mW, and the actual power value P (mW) is compared with P0 and then taken logarithm. This is the absolute unit of power.
Decibel watts (dBW) can also be used as the power unit, at this time P0=1W, that is, 1dBW=30dBm.
The power meter consists of two parts, a power sensor and a power indicator. The power sensor is also called the power meter probe, which converts the high-frequency electric signal through energy into an electric signal that can be directly detected. The power indicator includes signal amplification, conversion and display. The display directly displays the power value. Use a cable to connect the power sensor and the power indicator. In order to meet the needs of different frequencies, different power levels and different transmission line structures, a power meter should be equipped with several power meter probes with different functions.
According to the measured signal frequency, the power meter can be divided into: DC power meter, industrial frequency power meter, variable frequency power meter, RF power Meter and microwave power meter.
Since DC power is equal to the simple product of voltage and current, in actual measurement, voltmeter and ammeter are generally used instead. Power frequency power meter is a commonly used power meter, and the power meter often referred to generally refers to a power frequency power meter. The frequency conversion power meter is the product of the rapid development of frequency conversion speed regulation technology in the 21st century. The measurement object is variable frequency power, which refers to the AC power used to transmit power and meets one of the following conditions:
Common variable frequency power waveforms and frequency spectra (5 Zhang)1. The signal spectrum contains only one frequency component, and the frequency is not limited to the power frequency AC signal.
2. The signal spectrum contains electrical signals with two or more frequency components of interest.
Variable frequency power includes active power, reactive power, apparent power, active power, and reactive power derived from voltage, current, and voltage and current.
In addition to the PWM wave output by the inverter, the current waveform input by the diode-rectified inverter, the voltage waveform output by the DC chopper, the input current waveform of the transformer without load, etc., all contain larger harmonics. , The figure on the right shows the waveforms and related spectrograms of common variable frequency power.
Due to the complex frequency components of variable frequency power, the measurement of variable frequency power meters generally includes fundamental active power (referred to as fundamental power), harmonic active power (referred to as harmonic power), total active power, etc. Compared with the industrial frequency power meter, it has more functions and more complicated technology. It is generally called a variable frequency power analyzer or a broadband power analyzer. Some high-precision power analyzers are also suitable for variable frequency power measurement.
The frequency conversion power analyzer can be used as an industrial frequency power analyzer. In addition, it generally needs to meet the following requirements:
1, meet the necessary bandwidth requirements, and sample The frequency should be higher than twice the bandwidth of the instrument.
2. The accuracy of the analyzer is required to meet certain requirements within a wide frequency range.
3. With Fourier transform function, it can separate the fundamental wave and harmonic of the signal.
According to the different connection methods in the test system, radio frequency or microwave power meters can be divided into:
There are two types of terminal type and through type. The terminal power meter uses the power meter probe as the terminal load of the test system, the power meter absorbs all the power to be measured, and the power indicator directly reads the power value. Through-type power meter, it uses some coupling devices, such as directional couplers, coupling loops, probes, etc., to couple a part of the power from the transmitted power according to a certain ratio, and send it to the power meter for measurement. The total transmitted power is equal to the power. Multiply the indicated value by the scale factor.
Radio frequency or microwave power meters are classified by sensitivity and measurement range
Thermistor type power meters use thermal resistance as power sensing elements. The temperature coefficient of the thermal resistance value is relatively large. The power of the signal to be measured is absorbed by the thermal resistance to generate heat, which causes its own temperature to rise, and the resistance value changes significantly. The resistance bridge is used to measure the resistance value change and display the power value.
The thermocouple junction in the thermocouple power meter directly absorbs the high-frequency signal power, the temperature of the junction rises, and the temperature difference electric potential is generated. The magnitude of the electric potential is proportional to the absorbed high-frequency power value .
The calorimetric power meter is a typical thermal effect power meter. It uses a heat-insulated load to absorb high-frequency signal power to increase the temperature of the load, and then uses a thermocouple element to measure the temperature change of the load. Heat calculation high frequency power value.
Crystal detector type power meter crystal diode detector transforms high frequency signal into low frequency or direct current signal. Properly choose the operating point so that the amplitude of the detector output signal is proportional to the power of the high-frequency signal.
Classified by measured signal
There are continuous wave power meters and pulse peak power meters.
Power meter for frequency conversion power measurement (3 photos)The following is frequency conversion Typical technical indicators of power analyzers
Sampling frequency: more than twice the bandwidth;
Voltage and current accuracy levels: 0.02, 0.05 , 0.1 level, 0.2 level, 0.5 level;
Power accuracy level: 0.05 level, 0.1 level, 0.2 level, 0.5 level, 1 level;
Accuracy level applies to the fundamental frequency range ：DC, 0.1Hz~400Hz;
Applicable voltage range for accuracy level: 0.75%Un~150%Un;
Applicable current range for accuracy level: 1%In~200%In;
Applicable power factor range for accuracy level: 0.05~1.
The following are the typical technical indicators of the RF power meter
Guarantee measurement accuracy The maximum and minimum range of measurable power. The power range of the power meter is determined by the power probe.
Maximum allowable power
The maximum input power value at which the probe is not damaged, usually refers to the average power. When measuring high-power peak signals, pay attention that the peak voltage cannot exceed a certain value, otherwise it will cause voltage breakdown. The power meter must not be used to measure signals greater than the allowable power value, otherwise the power probe will be burned.
The frequency range of the measured signal that can guarantee the measurement accuracy and performance indicators.
refers to the accuracy of the power probe after calibration and correction. Does not include the mismatch error of the test system.
The stability of the power meter depends on the stability of the power probe and the zero drift of the indicator and noise interference.
Also known as the time constant of the power sensor element. Usually refers to the time required for the power indicator to rise to 64% of its stable value.
Model and impedance of the probe
When selecting a power meter probe, the frequency and power range of the power probe must be consistent with the signal under test. The structure and impedance should match with the transmission line under test.
Frequency range 9KHz～110GHz (depending on sensor)
Power range -70～+44dBm
Absolute accuracy of measurement accuracy: (logarithm) ±0.02dB; (linearity) ±0.5%
Relative accuracy: (logarithm) ±0.04dB; (linearity) ±1.0%
Resolution logarithmic method: 1.0; 0.1; 0.01 and 0.001dB (default setting: 0.01dB)
Linear method: 1 to 4 digits (default setting: 3 digits)p>
SWR1.06 maximum value (option 003 can reach 1.08)
Applicable sensors: E series sensors, 8480 series sensors
Common application field average power measurement ( Such as on-site maintenance service)
Warranty and calibration standard configuration: global 3-year warranty and 2-year calibration cycle
Connect the probe to the host via a cable
Connect the probe to the host calibration source after power-on and warm up, press the calibration key to calibrate
After calibration, remove the probe and set the test point frequency Take measurements
Before use, note that the power meter and the measured signal share the same ground
Pay attention to the selection of the probe direction and range
Don’t connect the power meter’s antenna-connected port to the RF transmitter of the device. It is easy to burn the power meter.
Be sure to use the calibration source to calibrate before measuring.
The used power timing frequency and the measured frequency should be the same
When the measured power is less than -50dBm, the zero should be zeroed before the test
Optical power measurement
is used to measure absolute optical power or relative loss of optical power through a section of optical fiber. In fiber optic systems, measuring optical power is the most basic, much like a multimeter in electronics. In optical fiber measurement, the optical power meter is a heavy-duty commonly used meter. By measuring the absolute power of the transmitter or optical network, an optical power meter can evaluate the performance of the optical device. Using an optical power meter in combination with a stable light source can measure connection loss, check continuity, and help evaluate the transmission quality of optical fiber links. To select a suitable optical power meter for the specific application of the user, the following points should be paid attention to:
1, select the best probe type and interface type
2, evaluate the calibration accuracy And manufacturing calibration procedures to match the scope of your fiber and connector requirements.
3. Make sure these models are consistent with your measurement range and display resolution.
4. With the dB function of direct insertion loss measurement.
Application of frequency conversion power analyzer (7 photos)The frequency conversion power analyzer is suitable for electric propulsion, motor, fan, and water pump Product inspection and testing, energy efficiency evaluation and power quality analysis in the fields of, wind power generation, rail transit, electric vehicles, inverters, special transformers, fluorescent lamps, LED lighting, etc.