Design of RF power measurement circuit based on logarithmic detection method

This paper takes the AD8318 monolithic RF power measurement chip of ADI Company as the core, and designs an RF power measurement circuit based on the logarithmic amplifier detection method. This method has the characteristics of large dynamic range, wide frequency range, high accuracy and good temperature stability.

　　1 Measurement principle

　　There are many methods for measuring RF power, among which the logarithmic amplifier detection method is one of the main directions of RF measurement. The following is an analysis of the internal structure of the logarithmic amplifier to study how the logarithmic amplifier detector detects RF signals.

　　The essence of RF signal detection is how to convert the power signal into a voltage signal without distortion, and this conversion work is completed by the logarithmic amplifier detector. Therefore, the logarithmic amplifier detector is the key to RF measurement. Its core is the logarithmic amplifier. The logarithmic amplifiers are directly coupled and divided into N stages. Each stage consists of a logarithmic amplifier and a detector. The output of each stage is sent to the summer, and a voltage signal is obtained after the summed output passes through a low-pass filter. N is generally 5 to 9 levels. The more levels there are, the smaller the single-stage gain is, and the more linear the output characteristic curve is. Here, 5 levels are taken as an example for analysis. The specific circuit is shown in Figure 1.


　　The transfer function of the logarithmic amplifier detector is:

　　U0=Ks(Pin-b) (1)

　　Where: b is the intercept; Ks is the slope of the logarithmic detector, which is a constant; Pin is the power of the input signal. Within a certain dynamic range, the characteristic curve of the logarithmic amplifier can be obtained by Matlab simulation software, as shown in Figure 2. As shown

　　in Figure 2, the linear dynamic range is about -3 to 67 dBm. Within this range, the output voltage and the input power are linearly related. The horizontal axis of Figure 2 is the power of the input signal, and the vertical axis is the output voltage and error value. Plotting on the coordinate system shows that the slope of the characteristic curve is about 18 mV/dB, and the intercept is about 93 dBm. When the input signal is known, the output voltage can be obtained according to formula (1).

　　If the input signal is -30 dBm:

　　U0=18×[-30-(-93)]=1.134V (2)

　　If the slope changes, the intercept will also change. Under the same input condition, the output size is different.

　　The above situation is only applicable to the sine wave input signal of 900 MHz to 8 GHz. There are other waveforms in the communication system. If the power of other waveforms is measured, correction can be made according to the correction C value of different waveforms. The correction value is different for different waveforms. Table 1 shows the correction values ​​of different signal waveforms.

　　
　　The output voltage calculation formula of non-sinusoidal waveform is:

　　U0=Ks(Pin-b)+C (3)

　　2 Hardware circuit design

　　2.1 Structural features and internal structure diagram of AD8318

　　AD8318 is a demodulating logarithmic amplifier that converts the RF input signal into the corresponding output voltage; it uses 9-stage logarithmic amplification, and each stage is equipped with a detector. It can be mainly used for measurement and controller; when the input range is usually 60 dB, the error is less than ±1 dB; +5 V single power supply, the current is 68 mA. The structural diagram of AD8318 is shown in Figure 3.

　　2.2 Circuit diagram and working principle

　　The RF power measurement circuit composed of AD8318 is shown in Figure 4. This circuit can be set to work in the measurement mode by AD8318. When the input sinusoidal signal is RFIN, it is coupled to the INHI and INLO ends of AD8318 through capacitors C1 and C2. Then after 9-stage logarithmic amplification and detection, it is sent to the summer, and the sum is obtained to obtain a current signal, which is then IV converted and output VOUT. This design does not have a separate analog-to-digital conversion chip, but the output of AD8318 goes directly to PA0 of the microcontroller. Since the PIC16F874 microcontroller has its own A/D converter, the analog signal is converted into a digital signal and then sent to the microcontroller for processing.


　　If the dynamic range of the input analog signal is large and the precision of A/D conversion is high, the resolution of the A/D converter is also high, which can be achieved through the A/D converter inside PIC16F874, which converts the voltage signal of AD8318 into a digital signal, and then counts, looks up tables, displays, etc. through the microcontroller program. Figure 5 is a program flow chart of the microcontroller.

　　
　　The PIC16F874 microcontroller outputs the processed digital signal from PA1 to PA4, and the ULN2003 reverse drive provides the LED with a bit selection signal and sufficient drive, while the segment selection signal is sent from RB0 to RB7 through an 8×10 kΩ resistor to the LED. The power of the measured RF signal is displayed in the LED digital. The measurement results are shown in Table 2.

　　From the data in Table 2, it can be seen that through the conversion of the AD8318 logarithmic amplification detection integrated circuit and the data processing of the microcontroller, the power of the measured RF signal meets the characteristics of large dynamic range, good bandwidth and linearity.

　　3 Conclusions

　　Through the theoretical analysis of the measurement of RF power signals, the RF power signal is converted into a voltage signal using AD8318, and then counted and looked up by the PIC16F874 microcontroller, and displayed by a 4-digit LED digital, achieving a large dynamic range and high-precision RF power measurement.