This project will explain a digital wattmeter from 1pW to 10mW up to 500MHz.The wattmeter can work alone or together with a computer for collecting data. Information will be displayed in dBm, Watt, and Voltage into 50 ohm. With proper attenuator, the measure range can go up to 1kW. The unit is very accurate, fast and easy to build.
In all your RF application you will have great advantage if you can measure the signal or the power. To measure the signal you will probably use an expensive oscilloscope or some other extremely expensive RF instrument. A wattmeter use a 50 ohm dummy load and will present to you the energy into this load. The most widely used reference in RF systems is decibels above 1 mW in 50 ohm, written dBm. There is a circuit AD8307specially made for this purpose and it has a very wide working range and is very easy to use.
The power is measured by the circuit AD8307 over a 50 ohm dummy load. An A/D converter of 12 bit convert the analog output from the AD8307 to a digital number. Since the AD is a12 bit A/D you will have 4096 combinations. The output values will be from 0 to 4095. To calculate the power into dBm you need some mathematical calculations. Since it is a bit difficult to implement mathematic into the PIC. the author use a look up table. It is memory bank containing all the display text. He had pre-calculated all the display text for each A/D value. The output values from the AD (0 to 4095) points to a direction in the memory where the actual display text value is stored. The text will be fetched and presented on the display.
The advantage of using a memory map is that it will be easy to build, you can implement prefix in the text, It will be fast and it will work! The EEPROM is cheap but there is one tricky thing to do. The memory table must be programmed into the EEPROM before it can be used.
It is very important to hand program each byte manually to get it working.
If you don’t have a “manually programming unit” you can simply connect a RS232 cable to a computer and use the program that the author will support you with. This program will handle everything, just start the program and go make yourself a cup of coffee.
Every memory cell will take 10ms to burn so it will take 12-15min. Once the EEPROM has been programmed you will not have to do this again.
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