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Using Cheap Asian Electronic Modules Part 14: by Jim Rowe Banggood’s RF Detector This nifty RF Detector module from Banggood can measure the power of RF signals from 1MHz to 8GHz, over a range of 60dB. It is on a tiny PCB measuring 33 x 24.5mm and has an SMA RF input connector attached to one end. It’s based on the Analog Devices AD8318 chip, which is an enhanced version of the AD8307. A s a matter of interest, we used the Analog Devices AD8307 chip in the RF Level and Power Meter project of October 2008 (siliconchip.com.au/ Article/1971) and also in the Arduino Multifunction Measuring Shield of April-May 2016 (siliconchip.com. au/Series/299). Both the AD8318 and AD8307 are logarithmic amplifier/detectors which provide a DC output voltage proportional to the RF input power level. But the AD8318 has a much wider bandwidth of 1MHz to 8GHz, compared with the DC-500MHz range of the AD8307. While the AD8307 has a range of just over 90dB, the AD8318 has a smaller dynamic range of about 60dB (necessary to get the improved frequency range). Unlike the AD8307, which operates from a nominal supply voltage of 3V, the AD8318 is designed to operate from 5V. It also has a typical supply current of 68mA, compared with the 7.5mA drawn by the AD8307. But perhaps the most important functional difference between the two devices is in terms of the output circuitry. The AD8307 has a current mirror in the output circuit which provides a positive slope to the DC output voltage. So the output voltage is directly proportional to the RF input level, with a slope of 25mV/dB. In contrast, the AD8318 has a different output circuit designed to alsiliconchip.com.au low it to be used for power amplifier gain control. As a result, it provides an output voltage which is inversely proportional to the RF input, with a slope of -25mV/dB. Is this a problem? Not when you are going to use it in conjunction with an Arduino or other microcontroller. Fig.1 shows a simplified version of the circuitry inside the AD8318. It has nine detector stages, interspersed with eight cascaded gain stages. The nine detector outputs are fed to an adder which drives a current-to-voltage converter to produce the output voltage, Vout. The V-I (voltage-to-current) converter at upper right allows adjustment of the slope of Vout in measurement mode. For example, the output slope of -25mV/dB is achieved when the Vset pin and the Vout pin are tied together. Higher output slopes can be obtained by connecting a voltage divider between the Vout pin and ground, and feeding a fraction of Vout back to the Vset pin. So if the voltage fed back to Vset is Vout ÷ 2, this changes the output slope to -50mV/dB. However, the output voltage is always in the range of 0.5-4.6V, so beyond -55mV/dB, the dynamic range will be reduced as the output at lower RF levels will be pegged at 4.6V. Note that the AD8318 includes an internal temperature sensor as well as bias stabilisation circuitry for the cascaded gain stages so that changes Fig.1: simplified block diagram of the AD8318 logarithmic detector/controller. It has nine detector stages interspersed with eight gain stages. Celebrating 30 Years March 2018  73 Fig.2: circuit diagram of the log detector module. Clpf and Cobp are optional capacitors used to filter ripple from IC1’s output. Suitable values are 1nF for Clpf and 10nF for Cobp with pads provided for mounting on the PCB. in ambient temperature do not unduly affect accuracy. All this is squeezed into a tiny 4 x 4mm 16-lead LFCSP (SMD) package; much smaller than the 8-pin SOIC/ PDIP packages used for the aforementioned AD8307. Now have a look at the circuit for the Banggood log detector module shown in Fig.2. Apart from the AD8318 chip itself (IC1), there is not much to it. The only other IC is REG1, a 78L05 regulator in a SOT-89 3-pin package with tab. This provides a regulated 5V rail for IC1. But the 78L05 has a nominal dropout voltage of 2V, so the module needs a power supply (Vcc) of at least 7.5V. As with many modules, there’s one LED to indicate when power is applied. LED1 is connected directly be- tween the Vcc input and ground with a 10kW series resistor. CON1 is the RF input, an SMA edgemount socket. This is terminated via a 51W resistor and then coupled to the INhi input (pin 14) of IC1 via a 1nF capacitor, with a second 1nF cap coupling the INlo pin of IC1 (pin 15) to ground. As the input resistance of IC1 between pins 14 and 15 is close to 1200W, this gives the input circuit a low-frequency cutoff of around 300kHz. The effective input resistance at frequencies below about 100MHz is around 49W (51W || 1200W). Pin 16 of IC1 is the enable input, which can be used to switch the device into a low-current standby mode if desired, by pulling it to ground. However, in the Banggood module, it’s connected to the +5V line, so the chip always functions while the module is powered up. But what’s the purpose of that 510W resistor connected between pin 10 (Tadj) of IC1 and ground? It allows adjustment of the chip’s internal temperature compensation, to optimise its operation at different frequencies. A value of 510W apparently gives very close to optimum compensation at frequencies up to 2.2GHz, and also at 8GHz, while optimum operation at 3.6GHz and 5.8GHz can be achieved by changing RTadj to 51W or 1kW, respectively. Even so, a value of 510W apparently gives acceptable performance over the whole range. The two capacitors shown in red, Clpf and Cobp, are used for filtering any ripple in the output from IC1. If both capacitors are omitted, the nominal output video bandwidth of the AD8318 is around 45MHz, making it suitable for demodulating pulse signals. But if you’re using it purely for measuring unmodulated RF, this wide bandwidth can allow significant second-harmonic ripple to appear in the output for input signals below 22MHz. Since this ripple can cause measurement jitter, the simplest way to reduce its effect is to add either Clpf or Cobp, or both. A suitable value for Clpf is 1nF, while that for Cobp is around 10nF and these values give an output bandwidth of around 100kHz. By the way, neither of these capacitors are fitted to the module board (even though pads are provided for fitting them as 0603 SMD components) Banggood’s logarithmic RF Detector module detector module is based on the Analog Devices AD8318 chip. It has an RF bandwidth of 1MHz to 8GHz with a range of -65dBm to +5dBm and an input impedance of 50W. These photos are almost twice actual size. 74 Silicon Chip Celebrating 30 Years siliconchip.com.au which is why we’ve shown them in red in Fig.2. Trying it out To check out this module, I hooked it up to a suitable 9V DC power supply and connected its RF input up to a VHF/UHF signal generator. Then I monitored its output using a 4.5-digit bench DMM while varying the RF input level over the range from +10dBm to -70dBm, for four different frequencies: 100MHz, 1GHz, 2GHz and 4GHz. I wasn’t able to go above 4GHz because that’s the highest frequency my signal generator provides. These measurement runs were used to plot the module’s transfer characteristic at each of the four sample frequencies and the results are shown in Fig.3. The four plots are very close to linear between RF input levels from -5dBm down to -60dBm and only curve gently away at the upper and lower extremes. Although the truly linear part of the module’s transfer characteristic only covers about 55dB, the curved sections at each end give it a useful range of about 70dB as claimed in the data sheet. The linear sections of all four plots are well within ±1dB of each other and have a slope of -24.33mV/dB; very close to the expected -25mV/dB. Note that we’ve mentioned a 60dB range before, as this is the practical range over which you can expect to get an accurate result. Connecting to an Arduino or Micromite Interfacing this module to a micro is straightforward. Just feed the module with 7.5-9V DC and connect its Vout to either one of the micro’s own ADC inputs directly, or to a higher-resolution ADC coupled to the micro via an SPI or I2C interface. Then it’s just a matter of writing a firmware sketch or MMBasic program to read the analog Vout signal and convert it into an RF power level. So this module should be suitable for use as the sensor section of a homebrew VHF/UHF level and power meter. You could even use it as an RF sensor head for our Arduino Multifunction Measuring Shield (MFM), although its negative-slope transfer characteristic would require some changes to the MFM’s firmware sketch. Other uses would be in an RSSI (received signal strength indicator) for UHF base station receivers and WLAN routers. In short, it seems to represent good value at around $16.50. SC Fig.3: plot of the transfer characteristic for the AD8318 at four different input frequencies. Note the excellent linearity. siliconchip.com.au Celebrating 30 Years March 2018  75