Logaritmic ADC representation

[RFsolution]

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

[Acetronics2]

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, RF

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

[rsocor01]

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

[languer]

How can i calculate the decimal value in watt's out of my 10bit ADC value

You have a couple of options (that I can think of):

• 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

[ralfmayr]

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