Power meter head for RF power Peak detector dBm proportional outputDesigned by vitor_market in Portugal
Based on the Analog Devices AD8307AR chip, this module is a complete RF Power meter head for 50 ohm systems working from very low frequencies to more than 500 MHZ. The RF input SMA is terminated to...Read More…
Based on the Analog Devices AD8307AR chip, this module is a complete RF Power meter head for 50 ohm systems working from very low frequencies to more than 500 MHZ. The RF input SMA is terminated to a 52.3 - 0.1% Ohms resistor (R1) to match the AD8307 input resistance to 50 Ohms.
It draws under 12 mA at 12 VDC and has two internal voltage regulators.
For low noise operation, a special ultra low-noise 3.3 V voltage regulator is used. The TPS79333 is made by Texas Instruments and is optimized for RF applications. This one powers the AD8307 chip. Its input is connected to an on-board 5 V voltage regulator (LM78L05) to facilitate wide range for the input voltage.
The AD8307 output voltage is routed to the output connector pin 5 for 25mV/dB measurements (O3). On that route, two 100 nF capacitors are connected to the ground and can be removed or modified. Space for installing optional scaling resistors (R3 and R4 of 1206 size) is provided.
Two op-amp DC amplifiers (LM358) multiply the output voltage to 2x (50 mV/dB) routed to pin 4 (O2) and to 4x (100 mV/dB) routed to pin 3 (O1). If used, the op-amp chip should be powered by a higher voltage supply (say 12V) in order to be able to deliver higher voltages at higher RF levels. Again, the op-amp gain scaling resistors are of 1206 size for easier handling if should be changed for any reason.
The PCB dimensions are 30 x 45 mm and a special gold-plated band is provided on the edges to help soldering any shielding material (copper clad board, tin etc).
Please note that as described in the datasheet,
"Note that while the AD8307 can operate down to supply voltages of 2.7 V, the output voltage limit is reduced when the supply drops below 4 V. This characteristic is the result of necessary headroom requirements, approximately two VBE drops, in the design of the output stage."
The chip will not be able to produce its full range output voltage for large RF input if supply is low. -10 dBm at 3.3V supply or maximum input 17dBm when power supply is 5V.
The 3.3V ultra-low-noise voltage regulator really helps on AD8307 performance with low-level RF signals. Of course if you would like to use +5V DC for its operation voltage you can place a wire link from the 5V voltage regulator directly to R2.
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