1MHz to 8GHz RF Power Meter
Created: Feb 24, 2017
No description available.
This reference design features a circuit that can be used for measuring power on RF signals. It uses an AD8318 demodulating logarithmic amplifier that can convert the RF input signal into an analog voltage that is scalable to the power of the RF signal. The AD8318 can accurately measure the power of RF signals from 1MHz up to 6GHz, but it can still measure up to 8GHz. It has a 60dB input range with error less than +/-1dB. The device operates with a 5V supply with a current consumption of 68mA. Its power consumption decreases to less than 1.5mW when the device is in disabled mode.
The AD8318 device can operate in measurement or controller mode. In this design, the device is set into measurement mode since the output voltage is fed back to VSET pin. The RF input signal can be connected to J1 which is connected to the RF input pin of AD8318 through an ac-coupling network. The RF signal passes through the 9-stage internal demodulating logarithmic amplifier of the device to be measured. The equivalent analog voltage of the RF signal's power is sent to the MCP3201 analog-to-digital converter through the VOUT pin. The MCP3201 converts the analog voltage into digital and the digital output can be read through its SPI compatible interface.
The AD8318 device also includes an internal temperature sensor. This sensor internally outputs a voltage that is proportional to absolute temperature. The voltage is multiplied by a factor of five. It can be measured across the TEMP pin of the device. For every increase in temperature, there is a 2mV increase in the TEMP pin output voltage. The output of the MCP3201 and the temperature output of the AD8318 are connected to the J2 header. The AD8318 supply is 5V while the supply for MCP3201 and AP7331-ADJ can be switched between 3.3V and 5V by shorting the pin 5 of J2 to pin 7 or pin 3, respectively.