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Hi,
I'm trying to measuring RF power with a directional coupler,followed by a schotky detection diode.
It gives me a voltage proportional to the power
On my LCD I would like to display "Power" in Watt
A 3dB increase in power will give a 6dB increase in voltage
so double power is not double voltage
(remember 10log Pout for Power, 20log Pout for voltage)
How can i calculate the decimal value in watt's out of my 10bit ADC value
Hi, RFQuote:
Originally Posted by RFsolution
the question will be which range for the power output ...
a direct use of ADC + calculations ( or retrieve table ! ) will show ~ 30 dB Range ...
use a CA 3089 / NE 604 / NE 605 highly recommanded if broader level range required.
Alain
RFsolution,
The power gain is expressed in dBs and is proportional to twice the voltage gain. Now, the power in Watts is proportional to the square of the voltage.
Robert
You have a couple of options (that I can think of):Quote:
How can i calculate the decimal value in watt's out of my 10bit ADC value
- Direct Math
- Lookup Table
In the direct math, you would follow the equation: Pwr = (Vrms ^ 2 / R). Where Vrms is the voltage read from the ADC; meaning you need to convert the ADC reading to voltage, and then input into equation. You can optimize a lot of this by pre-calculating many of the terms. For example Vrms would be ADC_count * Vref / 2^bits), where bits is the number of bits on your ADC (note that any gain/loss between the detector and ADC must be accounted for on the equation as well). In any case, pre-calculate all the known values beforehand so the PIC has less computation to do.
In the lookup table method, you would build up a table which relates ADC_count to Pwr (again, this is all pre-calculated beforehand - Excel is great for this). You may need to interpolate between values if you need more precision.
In either case you still have to account for any gains/losses (e.g. coupler, and conditioning OPAMP / divider - if used) between the signal you are attempting to measure and the ADC input.
The following link provides some good examples on math techniques using the STAMP (pretty close to PBP) :http://www.emesystems.com/BS2math3.htm
Hi,
i suggest using a logarhitmic power detector. For example the HMC600 series from Hittite, it delivers 19mV/dB in a range of 70dB. Or you must put an logarithmic amplifier behind your diode detector, in both cases you will get a
better dynamic range.
Regards,
Ralf