# Building an RF Power Meter

Here is yet another AD8307 based RF power meter which adds an opamp to produce a DC output voltage of 1mV/dBm (e.g., 0mV = 0dBm, -10mV = -10dBm, etc). The RF power in dBm can be read directly with a multimeter, and I’ve found it particularly useful for measuring RF filter response when combined with a sweep generator and an oscilloscope:

Visualizing a 6MHz Low Pass Filter Response over 1-11MHz (10dBm/div)

The AD8307’s high dynamic range and good accuracy over a wide bandwidth make it especially useful if, like me, you don’t own a spectrum analyzer which would otherwise be ideal for filter measurements.

### Circuit Design

The AD8307 circuit is pretty much lifted directly from the datasheet; it only requires a few coupling/shunt capacitors and a 52.3 ohm 1% resistor to ensure a 50 ohm input:

The output voltage slope of the AD8307 is specified to be ~25mV/dBm, which was verified at 10MHz using an Agilent 33120A function generator and an HP 3490A multimeter:

Input Power (dBm) Output Voltage (V)
-20 1.6784
-10 1.9237
-5 2.0574
-1 2.1562
0 2.1808

Knowing the 0dBm output voltage (2.1808V, in my case) and the rate of change (25mV/dBm), we can easily calculate the input power in dBm for any given output voltage from the AD8307:

\begin{aligned} \mathbf{P}_{dbm} & = \frac{(\mathbf{V}_{8307} – 2.1808)}{.025} \\ \end{aligned}

Although useful, it would be much nicer if we didn’t have to perform the voltage-to-dBm conversion ourselves. The above equation can be easily solved with an opamp, but not without a slight adaptation.

Consider what the output voltage requirements would be for an opamp using the previous equation: for any given input power, the opamp will output an equivalent voltage (e.g., 1V = 1dBm). So, for a 1dBm input signal, the opamp would need to output 1V; no problem there. But what about an input signal of -50dBm? The opamp would need to output -50V! Clearly, the output needs to be scaled down; a factor of 1,000 (e.g., 1mV = 1dBm) works nicely:

\begin{aligned} \mathbf{P}_{dbm} & = \frac{\frac{(\mathbf{V}_{8307} – 2.1808)}{.025}}{1000} = \frac{(\mathbf{V}_{8307} – 2.1808)}{25}\\ \end{aligned}

With this scaled down equation, a differential amplifier can be used to solve for Pdbm:

Differential Amplifier

If R1=R3 and R2=R4, then the output of the differential amplifier is simply:

\begin{aligned} \mathbf{V}_{out} & = (\mathbf{V}_2 – \mathbf{V}_1) \times \frac{\mathbf{R}_2}{\mathbf{R}_1}\\ \end{aligned}

This is essentially the same as our previous Pdbm equation, we just have to plug in the right values for V and R. We know that V2 will be the output from the AD8307, and V1 will be a fixed 2.1808V; and, we know that the ratio of R2 to R1 must be 1/25. This makes picking the resistor values easy:

\begin{aligned} \mathbf{V}_{out} & = (\mathbf{V}_{8307} – 2.1808) \times \frac{10k}{250k}\\ \end{aligned}

Since 250k isn’t a standard resistor value, four 100k resistors can be combined to form the R1 and R3 values:

Differential Opamp Resistor Values

The 2.1808V input voltage can be realized with a voltage divider (10k potentiometer, shown below), and the output of the AD8307 should be buffered with a opamp follower to prevent loading down the AD8307’s output voltage:

For best results, the 10k and 250k opamp resistors should be .1% or better, but good accuracy was also achieved with 5% resistors.

### Circuit Performance

I’m quite happy with the power meter’s performance. It shows less than 1dB error from 1MHz to over 150MHz (I don’t have the means to test up to the full 500MHz bandwidth), and a dynamic range of -73dBm to +16dBm, almost identical to the AD8307’s specifications.

With the function generator set to perform a 10mS sweep from 1MHz to 11MHz, and the oscilloscope set to 1mS/div (i.e., 1MHz/div) and 10mV/div (i.e., 10db/div), a 6MHz Coilcraft low pass filter (P3LP-605L) was inserted between the function generator and the power meter:

6MHz Low Pass Filter (Left) Under Test

As expected, the filter’s response over the 1-11MHz range mimics that of the filter’s specifications, dropping to -3db at 6MHz and -10db at 9MHz:

P3LP-605L Filter Response from 1-11MHz at 10db/div

### Full Schematic

The full power meter schematic, with a simple rail-splitter circuit for providing the necessary +5V and -5V rails, is shown below; note the addition of a 1mH inductor to remove noise from the output when viewed on a ‘scope:

Power Meter Full Schematic

## 2 thoughts on “Building an RF Power Meter”

1. val says:

Hello
My name is Valeriu Apostu YO4 SAZ-ROMANIA.
I built Digitall watt by 8307 AD and please answer me how could I improve sensitivity? I’ve seen other builders that range from -70db to -80 db or even so that can measure very low levels.
The picture that I sent it, I managed nearly -60 dB, respectivelyschema date, but I would like to measure the gain of anantenna amplifier, and then you should start with the lowest level.